Section 3 Site Index Book Index

Appendix 1: Mouse Software Interfaces

The two operating systems supplied with your PC1512, MS-DOS 3.2 and DOS Plus, both support a set of Mouse functions which allow a program to access the mouse and control the cursor. These functions can be called from within your application programs by using the software interfaces described below.

The mouse functions described in this appendix are as follows.

Number Function
0** Mouse Initialisation
1 Show Cursor
2 Hide Cursor
3 Get Mouse Position and Button Status
4** Set Mouse Cursor Position
5 Get Button Press Information
6 Get Button Release Information
7** Set Minimum and Maximum X-Cursor Position
8** Set Minimum and Maximum Y-Cursor Position
9 Set Graphics Cursor Block
10 Set Text Cursor
11 Read Mouse Motion Counters
12** Set User Defined Subroutine Input Mask
13 Light Pen Emulation Mode On
14 Light Pen Emulation Mode Off
15** Set Mickey/Pixel Ratio
16 Conditional Off
19 Set Double Speed Threshold

The DOS Plus operating system supports only the subset of mouse functions marked with the double asterisks built into the DOS Plus input/output system.

For the MS-DOS operating system, the program MOUSE.COM must be loaded by either typing "MOUSE" at the keyboard or by having a "MOUSE" line in your AUTOEXEC.BAT.

When MOUSE.COM is loaded, it performs an initialisation process and installs the mouse driver software into the system. Once installed, the mouse driver remains permanently resident until the next time you bootstrap your computer. After successfully completing initialisation, the following message is output to the display:

"--- Installing Mouse Device Driver V5.00 ---"

If the mouse driver fails to load you will get one of the two following messages:

1. "MOUSE: Mouse Driver already installed."
Because either you've previously installed MOUSE.COM or you've attempted to install it under DOS Plus which has its own built in mouse driver.
2. "MOUSE: Amstrad Mouse not found."
Because either there's a hardware fault or your hardware isn't an Amstrad PC.

During initialisation, the following actions take place:

  1. The hardware ticker routine residing at software interrupt 8 which is invoked every 54ms is replaced by a Mouse Ticker routine that is invoked every 18ms.
  2. Counter 0 of the 8253 interrupt controller is re-programmed so that it produces an interrupt every 18ms rather than every 54ms.
  3. The Mouse Buttons Interrupt routine is inserted into the Mouse Buttons Interrupt vector (software interrupt 6).
  4. The Amstrad PC Mouse X and Y movement registers are initialised to zero. The Mouse Buttons are both marked as being released.
  5. The Non Volatile RAM is read to determine the X & Y Scaling factors, which are to be used during cursor key generation for mouse movement in text mode.
  6. The Mouse driver is initialised to be in Text Mode.

The above initialisation applies only to MOUSE.COM in the MS-DOS environment. For DOS Plus which has a built in mouse driver, a similar initialisaiton takes place during system bootstrap. The basic ticker rate for DOS Plus is also 18ms (or about a 54Hz rate).

The general procedure for making an assembly language program call to the mouse function driver program is:

  1. Load the specified register parameters.
  2. Execute software interrupt 51 (033h).

The cursor coordinates required for the various function calls are in the form of X-Cursor (horizontal) and Y-Cursor (vertical) values. The range of the X-Cursor is always the full 0 to 639 points of the high resolution graphics screen and the Y-Cursor ranges from 0 to 199. This coordinate system defines the "virtual" screen and when in modes with less resolution than 640 points then the least significant bits of the X-Cursor are ignored. In 4-Colour (320 x 200) graphics only even values are significant while in 80 column text mode only every eighth position is valid and in 40 column text modes only every 16th position is valid. Supplied values are rounded to the nearest values permitted for the current screen mode.

The standard unit of mouse motion is called the "mickey" and is equal to approximately 1/200 of an inch. See Mouse function 15 which sets the mickey to pixel ratios.

In Text Mode, mouse movement will cause cursor key tokens to be inserted into the keyboard buffer. The scaling factors read from the NVR during initialisation are used to determine how many units of mouse movement are to be sensed before a single cursor key token is inserted into the keyboard buffer. Invoking any Mouse Function except Function 0 or Function 2 will disable this extra mode (i.e. cursor movement tokens are not generated). Invoking Function 0 (Initialisation) enables this extra mode (i.e. cursor tokens are generated) and invoking Function 2 (Hide Cursor) does not change the current mode.

In Text Mode, the mouse buttons interrupt routine (interrupt 6) translates the Left and Right mouse buttons into the appropriate scan codes which are held in NVR bytes 29-30 (for Left) and 31-32 (for Right). The default NVR value for these scan codes is the ignore code (all F's). The NVRPATCH programs can be used to set the mouse button codes to handy values such as CR and ESC.

In Graphics Mode, the mouse buttons interrupt routine translates the Right mouse button to the shift key token, and the Left button is passed through as a mouse event to the user defined subroutine. (See mouse function 12.)

Function 0: Mouse Initialisation.

This function initialises the mouse driver and returns the current status of the mouse hardware and software.

CPU registers are used as follows:

Entry:
AX = 0
Exit:
AX = Mouse Status
BX = Number of Buttons.

All Flags and other registers preserved.

Since the mouse hardware is verified by power-up testing the driver always returns a mouse status of true (-1). If the mouse driver is not resident then AX is returned as false (0).

Both MS-DOS and DOS Plus return the number of buttons as 1.

The mouse driver parameters are reset to the following values:

ParameterValue
Cursor FlagHidden (-1)
Cursor PositionCenter Screen
Graphics CursorArrow
Hot Spot-1, -1
Text CursorInverting box
User Defined Call MaskZeros
Light Pen Emulation ModeEnabled
Mickey to X-Pixel Ratio8
Mickey to Y-Pixel Ratio16 (8 for DOS Plus)
Min/Max X-Cursor Position0/639
Min/Max Y-Cursor Position0/199

The mouse X and Y hardware counters are reset and a number of internal software counters are zeroed.

The mouse driver is initialised to be in Text Mode (and cursor tokens are generated in response to mouse motion).

Function 1: Show Cursor.

This function increments the Cursor Flag and, if the flag is zero, the cursor display is enabled.

CPU registers are used as follows:

Entry:
AX = 1.
Exit:
All flags and registers preserved.

Function 2: Hide Cursor.

This function decrements the Cursor Flag.

CPU registers are used as follows:

Entry:
AX = 2.
Exit:
All flags and registers preserved.

Function 3: Get Mouse Position and Button Status.

This function returns the state of the Left and Right buttons and the current cursor position.

CPU registers are used as follows:

Entry:
AX = 3.
Exit:
BX = Button status.
CX = X-Cursor Position.
DX = Y-Cursor Position.
All flags and other registers preserved.

The Button Status word returned in BX is a single integer value. Bits 0 and 1 represent the Left and Right buttons, respectively. A bit is set if a button is down and clear if it is up.

Function 4: Set Mouse Cursor.

This function sets the cursor to the specified X-Cursor and Y-Cursor positions. The values must in range of the virtual screen. If the screen is not in high resolution mode, the values are rounded to the nearest values permitted for the current screen mode.

CPU registers are used as follows:

Entry:
AX = 4.
CX = X-Cursor position.
DX = Y-Cursor position.
Exit:
All flags and registers preserved.

Function 5: Get Button Press Information.

This function returns the current button status, a count of button presses since last call to this function, and the X-Cursor and Y-Cursor positions at the last button press.

CPU registers are used as follows:

Entry:
AX = 5
BX = Button Number (0=Left/1=Right).
Exit:
AX = Button status.
BX = Count of Presses since last call (0-32k).
CX = X-Cursor at last press.
DX = Y-Cursor at last press.
All flags and other registers preserved.

Function 6: Get Button Release Information.

This function returns the current button status, a count of button releases since last call to this function, and the X-Cursor and Y-Cursor positions at the last button release.

CPU registers are used as follows:

Entry:
AX = 5.
BX = Button Number (0=Left/1=Right).
Exit:
AX = Button status.
BX = Count of releases since last call (0-32k).
CX = X-Cursor at last release.
DX = Y-Cursor at last release.
All flags and other registers preserved.

Function 7: Set Minimum and Maximum X-Cursor Position.

This function sets the minimum and maximum X-Cursor position. Subsequent cursor motion is restricted to the specified range.

CPU registers are used as follows:

Entry:
AX = 7.
CX = Minimum X-Cursor Position.
DX = Maximum X-Cursor Position.
Exit:
All flags and registers preserved.

If the cursor is outside the area when the call is made, it is set to just inside the area. If Minimum is greater than Maximum, the two values are exchanged.

Function 8: Set Minimum and Maximum Y-Cursor Position.

This function sets the minimum and maximum Y-Cursor position. Subsequent cursor motion is restricted to the specified range.

CPU registers are used as follows:

Entry:
AX = 8.
CX = Minimum Y-Cursor Position.
DX = Maximum Y-Cursor Position.
Exit:
All flags and registers preserved.

If the cursor is outside the area when the call is made, it is set to just inside the area. if Minimum is greater than Maximum, the two values are exchanged.

Function 9: Set Graphics Cursor Block.

This function defines the shape, colour, and center of the cursor for graphics mode.

CPU registers are used as follows:

Entry:
AX = 9.
BX = X-Cursor Hot Spot.
CX = Y-Cursor Hot Spot.
DX = Pointer to Screen and Cursor Masks.
ES = Segment Address of Screen and Cursor Masks.
Exit:
All flags and registers preserved.

The Hot Spot is a point relative to the upper left corner of the cursor block used to determine the cursor coordinates. Both coordinates must be in the range of -16 to +16.

The values in the screen mask and cursor mask are used to build the cursor shape and colour. The ES register contains the segment address of the screen and cursor mask array and DX is the offset to be applied to the ES register.

The screen and cursor masks are two 16- by 16-bit arrays arranged contiguously in memory. The screen mask determines whether the cursor pixel is part of the shape or part of the background. The cursor mask determines how the pixel under the cursor contributes to the colour of the cursor. To create the cursor, the mouse software first logically ANDs the screen mask with the 256 bits of data that define the pixels under the cursor. Then, it logically XORs the cursor mask with the result of the AND operation. The following truth table shows the relationship between the screen mask, the cursor mask, and the resultant screen memory.

The values in the screen mask and cursor mask are used to build the cursor shape and colour. The ES register contains the segment address of the screen and cursor mask array and DX is the offset to be applied to the ES register.

The screen and cursor masks are two 16- by 16-bit arrays arranged contiguously in memory. The screen mask determines whether the cursor pixel is part of the shape or part of the background. The cursor mask determines how the pixel under the cursor contributes to the colour of the cursor. To create the cursor, the mouse software first logically ANDs the screen mask with the 256 bits of data that define the pixels under the cursor. Then, it logically XORs the cursor mask with the result of the AND operation. The following truth table shows the relationship between the screen mask, the cursor mask, and the resultant screen memory.

The values in the screen mask and cursor mask are used to build the cursor shape and colour. The ES register contains the segment address of the screen and cursor mask array and DX is the offset to be applied to the ES register.

The screen and cursor masks are two 16- by 16-bit arrays arranged contiguously in memory. The screen mask determines whether the cursor pixel is part of the shape or part of the background. The cursor mask determines how the pixel under the cursor contributes to the colour of the cursor. To create the cursor, the mouse software first logically ANDs the screen mask with the 256 bits of data that define the pixels under the cursor. Then, it logically XORs the cursor mask with the result of the AND operation. The following truth table shows the relationship between the screen mask, the cursor mask, and the resultant screen memory.

The values in the screen mask and cursor mask are used to build the cursor shape and colour. The ES register contains the segment address of the screen and cursor mask array and DX is the offset to be applied to the ES register.

The screen and cursor masks are two 16- by 16-bit arrays arranged contiguously in memory. The screen mask determines whether the cursor pixel is part of the shape or part of the background. The cursor mask determines how the pixel under the cursor contributes to the colour of the cursor. To create the cursor, the mouse software first logically ANDs the screen mask with the 256 bits of data that define the pixels under the cursor. Then, it logically XORs the cursor mask with the result of the AND operation. The following truth table shows the relationship between the screen mask, the cursor mask, and the resultant screen memory.

Screen Mask BitCursor Mask Bit Resultant Screen Bit
000
011
10Unchanged
11Inverted

In high resolution (640 by 200) graphics mode each bit in the screen and cursor masks logically maps to one bit on the screen. In four colour (320 by 200) graphics mode each pair of bits correspond to one pixel.

Function 10: Set Text Cursor.

This function selects the software or hardware text cursor and defines the attributes of the selected cursor.

CPU registers are used as follows:

Entry:
AX = 10.
BX = Cursor Type (0=Software/1=Hardware).
CX = Screen Mask/Scan Line Start.
DX = Cursor Mask/Scan Line Stop.
Exit:
All flags and registers preserved.

If the software text cursor is selected CX & DX contain Screen and Cursor masks. The 16-bit masks are ANDed and XORed in the same manner as the graphics cursor operation and operate upon the character and attributes bytes of the character position of the cursor.

In both 40-column and 80-column text modes the 16-bits of screen data for each character take the following form:

Bits: 15 14..12 11 10..8 7........0
Contents: B Backgnd I Foregnd Character

Where:

B
Blink Bit.
Backgnd
Three bit (RGB) pattern specifying background colour.
I
Intensity bit.
Foregnd
Three bit (RGB) pattern specifying foreground colour.

Refer to section 1.11 for the attribute byte details.

The screen and cursor masks are divided into the same fields as shown above so that the value of the masks defines the new attributes of the character when the cursor is over it. For example a screen mask of 077FFh and a cursor mask of 07700h would invert the foreground and background colours.

If the hardware cursor is selected CX & DX define the first and last scan line in the cursor shown on the screen.

Function 11: Read Mouse Motion Counters.

This function returns the horizontal and vertical mickey count since the last call to this function. CPU registers are used as follows:

Entry:
AX = 11.
Exit:
CX = X-Count.
DX = Y-Count.
All flags and other registers preserved.

The "mickey" is the standard unit mouse motion equal to approximately 1/200 of an inch. See Mouse function 15 which sets the mickey to pixel ratios.

Function 12: Set User Defined Subroutine Input Mask.

This function sets the call mask and subroutine address for the mouse interrupts. CPU registers are used as follows:

Entry:
AX = 12.
CX = Call Event Mask.
DX = Address Offset to Subroutine.
ES = Segment address of Subroutine.

All flags and other registers preserved.

The mouse driver uses the ticker interrupt to poll the mouse hardware (at approx. 55 times per second) and when one of the events specified in the call event mask is noted your subroutine is called. The layout of the call event mask is:

BitEvent (1=Enabled)
15-5Unused.
4Right Button Released. (will never occur).
3Right Button Pressed. (will never occur).
2Left Button Released.
1Left Button Pressed.
0Cursor Position Changed.

Note that calling mouse function zero (Initialisation) disables all events so that function 12 must be called again.

When calling your subroutine the CPU registers are loaded as follows:

Function 13: Light Pen Emulation Mode On.

This function enables the light pen emulation by the mouse. CPU registers are used as follows:

Entry:
AX = 13.
Exit:
All flags and registers preserved.

With the Light Pen Emulation on, the VDU I/O software interrupt (Int 16) returns mouse identification vice the normal light pen address information (See VDU Sub-Function 4).

Function 14: Light Pen Emulation Mode Off.

This function disables the light pen emulation by the mouse. CPU registers are used as follows:

Entry:
AX = 14.
Exit:
All flags and registers preserved.

Function 15: Set Mickey/Pixel Ratio.

This function sets the mickey to pixel ratio for mouse motion. CPU registers are used as follows:

Entry:
AX = 15.
CX = X-Mickey/Pixel Ratio.
DX = Y-Mickey/Pixel Ratio.
Exit:
All flags and registers preserved.

The X- and Y- ratios specify a number of mickeys per 8 pixels. The values must be in the range of 1 to 32767.

With a setting of 16 mickeys per 8 pixels horizontally it takes about 6.4 inches of mouse movement to move the cursor across the screen (640 pixels). With the same 16 mickeys per 8 pixels vertically it takes about 2 inches of travel to move the cursor the full vertical deflection (200 pixels).

Function 16: Conditional Off.

This function defines a region on the screen for updating. CPU registers are used as follows:

Entry:
AX = 16.
CX = Upper X-Screen Coordinate.
DX = Upper Y-Screen Coordinate.
SI = Lower X-Screen Coordinate.
DI = Lower Y-Screen Coordinate.
Exit:
All flags and registers preserved.

The mouse cursor is hidden while the screen is being updated and a call to function 1 is needed to show the cursor again.

Function 16 is similar to function 2 (Hide Cursor) bit is for advanced applications which require quicker screen updates.

Function 19: Set Double Speed Threshold.

This function defines the threshold speed for doubling the cursor's motion on the screen. CPU registers are used as follows:

Entry:
AX = 19.
DX = Threshold Speed in Mickeys/Second.

This function makes it easier to point at images widely separate on the screen.

A threshold value of zero sets a value of 64 mickets.second. Setting a large value (such as 32767) disables the double speed threshold.

Appendix 2 MS-DOS System Configuration

The MS-DOS operating system allows for a number of installation specific configuration options during the system startup progress through the use a file called CONFIG.SYS when it is found in the root directory of the startup disk. These configuration options include the following commands:

BREAK
Extended BREAK checking (Ctrl-C).
BUFFERS
Number of sector buffers.
COUNTRY
Country Specific parameter selection.
DEVICE
Device driver installations.
DRIVPARM
Override the drive parameters for a logical drive.
FCBS
Number of files open by file control blocks.
FILES
Maximum number of file handles open concurrently.
LASTDRIVE
Maximum drive letter allowable.
SHELL
Top level command processor specification.

The CONFIG.SYS can be created with any text editor and the simple screen editor RPED is ideal for this purpose.

2.1 BREAK Command

This command enables the MS-DOS extended break checking to be either set or reset. Normally, MS-DOS checks to see if CTRL-C has been typed while it is reading from the keyboard, writing to the screen or a printer. Setting Break to 'on' allows CTRL-C checking to be extended to other functions such as disk reads or writes. The syntax of the BREAK command is:

If no field is specified then OFF is assumed (as the default value).

2.2 BUFFERS Command

This command allows you to specify the number of buffers that MS-DOS allocates when it starts up. A disk buffer is a block of memory where MS-DOS holds data being read from or written to a disk when the amount of data is not an exact multiple of sector size.

The syntax of the BUFFERS command is:

Where 'n' is a number between 1 and 99. If the BUFFERS command is not used then MS-DOS defaults to 2 buffers. The number of buffers remains in effect after bootstrap until the machine is switched off or bootstrapped again.

2.3 Country Command

The country command is used to select the country dependent information as shown in appendix 3.

The syntax of the country command is:

Where 'nnn' is the 3-digit country (Num) code from Appendix 3. Note that only the information in the table is affected and other country dependent factors such as the language links, N-Utility setup, KEYBxx, and national variant disks affect the total country dependent environment.

2.4 DEVICE Command

This command installs the device driver in the specified pathname to the system list.

The syntax of the DEVICE command is:

The file specified is loaded and given control. The driver may then perform the necessary steps to configure itself and the system for its operation. See the MS-DOS Technical Reference Manual for information on how to create your own device driver.

Your MS-DOS disk (Disk 1) contains two installable device drivers, DRIVER.SYS, and RAMDRIVE.SYS which can be used for variable device configurations.

If you plan to use the ANSI escape sequences described in the PC1512 users manual, you would need to include the following command in your CONFIG.SYS file:

This command causes MS-DOS to replace all keyboard input and screen output support with the ANSI escape sequences.

Note that the Amstrad disc is supplied with a version of ANSI.SYS that conforms with the NVR value (byte 36) for screen colouring and which refrains from blanking the screen unnecessarily when scrolling.

2.4.1 DRIVER.SYS

DRIVER.SYS is an installable device driver that supports external drives. To install DRIVER.SYS, include the following command in your CONFIG.SYS file:

Where:

and optionally:

2.4.2 RAMDRIVE.SYS

RAMDRIVE.SYS is an installable device driver which enables the usage of a portion of the computer's memory as though it were a disk drive. This area of memory is referred to as a RAM disk or a virtual disk.

If you have extended memory installed starting at the 1MB boundary or if you have an extended memory which meets the LIM [Lotus(R)/Intel(R)/Microsoft(R)] Expanded Memory Specification, you can use this memory for one or more RAM disks. Otherwise RAMDRIVE.SYS locates RAM drives in low memory.

To install RAMDRIVE.SYS, include the following command in your CONFIG.SYS file:

Where:

2.5 DRIVPARM Command

The DRIVPARM command allows overriding of the device parameters for a specific logical drive.

The syntax is:

Where:

and optionally:

This command allows the overriding of default system parameters for a particular logical drive. This information would be used by the commands which create new diskettes (such as FORMAT and COPY) when writing out the directory and FAT (File Allocation Table) information. For any physical device which is read the information in the FAT ID is used when determining device characteristics.

2.6 FCBS Command

The FCBS command allows you to specify the number of file control blocks available to the system and consequently the number of files which can be opened at any one time.

The syntax of the FCBs is:

Where <x> is the number of FCBs (in the range of 1 to 255) to allocate and <y> is the number of FCBs protected from closure when a program tries to open more than <x> files. The first <y> files opened will be protected. MS-DOS selects the least recently used (non-protected) FCB when it must automatically close a file.

If the FCBS command is not used MS-DOS defaults <x> and <y> to 4 and 0 respectively. It is an error to set <y> greater than <x>

2.7 FILES Command

The FILES command specifies the maximum number of file handles that can concurrently be opened. When a program opens a file or a device it is assigned an identifier or "handle" which can be used by that program in referring to the file.

The syntax of the FILES command is:

Where 'n' is the number of handles in the range of 8 to 255. When no FILES command is used MS-DOS assumes a default value of 8. Any value higher than 20 serves no useful function.

2.8 LASTDRIVE Command

The LASTDRIVE command is used to set the maximum drive letter which MS-DOS will accept.

The syntax of the LASTDRIVE command is:

Where 'd' is any letter from A to Z (and is case insensitive). When the drive letter is lower than the actual physical drives then MS-DOS ignores the LASTDRIVE specification and uses the default value which is the letter 'E'.

2.9 SHELL Command

The SHELL command is used to specify an alternate top-level command processor in place of the standard COMMAND.COM file.

The syntax of the SHELL command is:

This command is used in conjunction with major software packages which furnish their own command processors. The MS-DOS technical manual contains information on developing command processors.

2.10 KEYBUK Command

When this command is invoked, a resident keyboard interrupt process is installed which replaces the ROS keyboard interrupt process. UK specific characters £, #, @, &, " are mapped to their respective keys. The system available memory will decrease by the resident size of keybuk. Pressing [CTRL] + [ALT] + [F1] restores the ROS keyboard processing and pressing [CTRL] + [ALT] + [F2] resets back to KEYBUK keyboard input.

Note that the PC1512 is supplied with a special version of keybuk which correctly handles the extra key codes generated by the [DEL->] & [ENTER] keys, the joystick port and the two mouse buttons. Using any other version of KEYBUK will render all these keys inoperative.

Appendix 3: Country Dependent Information for MS-DOS 3.2

Country Num DtF DtS TmS TmF CSm CFt CSd ThS DeS DlS
Australia0611-:1 $02,.,
Belgium0321/:1 F32,;
Canada0022-:1 $32,;
Denmark0451/.1 DKR32.,;
Finland3581-:1 MK32,;
France0331/:1 F32,;
Germany0491..1 DM02.,;
Italy0391/:1 Lit10.,;
Israel9721/:1 22,.,
Middle East7851/:1 $33.,;
Netherlands0311-:1 ƒ02.,;
Norway0471/.1 KR22.,;
Portugal3511/:1 $42.,;
Spain0341/:1 Pt32.,;
Sweden0462-.1 SEK22.,;
Switzerland0411..1 Fr22,.,
United Kingdom0441-:1 £02,.,
United States0010-:0 $02,.,

Table Columns:

Num= Country Number Code.
DtF= Date Format. (0 = U.S. M/D/Y, 1=EURO D/M/Y, 2 = JAPAN Y/M/D)
DtS= Date Separator.
TmS= Time Separator.
TmF= Time Format. (0=12-hour clock, 1=24-hour clock)
CSm= Currency Symbol.
CFt= Currency Format. (Bit 0: 0 = Currency symbol Precedes/1=Follows Field, Bits 1 & 2: Number of spaces between Value & Symbol)
CSd= Number of significant decimal digits in currency.
ThS= Thousands Separator.
DeS= Decimal Separator.
DlS= Data List Separator.

Appendix 4: RS232C Connections

For a complete understanding of the connections required between the RS232C and the outside world, it is important to realize that all devices with a serial interface can be classified as either a modem or as a terminal. Modems are merely a way of extending the length of the connection (often via a terminal wire) between two terminals. Fig 1 (below) shows a simplified, idealised terminal to terminal connection through modems.

IDEALISED TERMINAL TO TERMINAL CONNECTION

Fig 1

The standard connector used for serial interfaces has 25 pins although only up to seven are required in most cases. When connecting to a modem a 'one-to-one' cable is used, i.e. pin 1 to pin 1, pin 2 to pin 2, ... pin 25 to pin 25. Assuming such cables are in use, data is transferred as follows:

Following the signal path from left to right (in Fig 2), characters from the keyboard are sent as serial data patterns out of pin 2 of the left-hand terminal, to pin 2 of the modem (the connection marked 'transmit data'). The left-hand modem sends the characters via the telephone line, to the right hand modem. The characters are received pin 3 of the right hand modem (the connection marked 'receive data') which sends them to pin 3 of the right-hand terminal. On receipt of the characters, the right-hand terminal displays them on the screen.

Notice how the names of the connections 'transmit data' and 'receive data' are expressed from the view point of the terminals and not the modems.

The data path from left to right just described, is exactly matched by a data path from right to left which uses the same numbered connections, i.e. pin 2 from the right-hand terminal it its modem (transmitting), and then to pin 3 of the left-hand (receiving) modem to the terminal. This arrangement is perfectly symmetrical, and there is no confusion over who is using which pin number and for what direction of data transfer.

Fig 2

Problems of definition arise, howeve, when we wish to connect two terminals together locally, without the intervening pair of modems. We cannot connect pin 2 to pin 2 because both keyboard will be transmitting head-on and neither screen is connected to anyone who is sending. The obvious solution is to cross over pins 2 and 3 so that the transmit pin of each terminal is connected to the receive pin of the other. A cable containing such a cross-over connection is known as a 'Null-modem' cable because of the way in which in replaces the pair of back to back modems.

The earth pin (pin 7) is still common to both terminals using this arrangement.

Fig 3

Naturally, the Amstrad PC1512 with its RS232C interface is considered a terminal, and therefore to connect to a modem (for example, to dial-up a database) requires a simple one-to-one cable.

The Null-modem cable is required for connecting to other terminals. The sort of equipment we mean by terminals is: a second Amstrad computer plus RS232C, a conventional Visual Display Unit (VDU), a printer with a serial interface, or any other serial interface device.

Fig 4

There is a point to be noted here: many manufacturers of devices such as desk-top computers wire up their serial interface (for VDU or a Printer) as if it were a modem, not a terminal. This is in the belief that life will therefore be simpler because VDU's and printers can connected to that computer with one-to-one cables.

Fig 5

In a perfect world, it would be possible to identify which serial devices behave like modems and which ones behave like terminals by examining the 'sex' of the 25-way connector - terminals should have a 'male' connector, and modems a 'female' connector. This is not, unfortunately, as reliable a guide as it should be, as many manufacturers of terminals and printers equip them with 'female' connectors, mostly for reasons of electrical safety.

If in doubt, the ultimate test is to examine the user manual and determine the function of PIN 2 - if the description includes the word 'TRANSMIT' then the equipment is wired as a terminal, and if it includes the word 'RECEIVE' then the equipment is wired as a modem.

Hardware Flow Control

The simplified connection described so far does not allow any control of the data flow. In practice, we often with the receiving device to have control over the transmitting device, thus preventing the receiving device from being overwhelmed (where it is slower in using the input than the rate at which the input is arriving). In addition, if the transmitting device has reason to mistrust the data which it is sending, there should be some provision for it to disable the receiving device.

In the case of modem to terminal connection; when the terminal is able to transmit it activates pin 4 - the RTS pin (Request To Send). When the modem is ready to receive input it activates pin 5 - the CTS pin (Clear To Send). The terminal will only send when CTS is activated. Thus the modem can control the flow rate using CTS.

When the modem considers that the data which it is about to send is suitable, it activates pin 8 - the DCD pin (Data Carrier Detect). When the terminal is ready to receive input it activates pin 20 - the DTR pin (Data Terminal Ready). The modem will only transmit when DTR is activated. Thus the terminal can control flow rate using DTR.

There are two further signals which must be introduced here. One is on pin 22 - the Ring indicator, which simply allows the modem to tell the terminal that the phone is ringing (at which point software in the terminal might be expected to wake up). The other signal is on pin 6 - DSR (Data Set Ready). This signal is ignored by the receiving side of the RS232C; the modem will activate this signal at much the same time that it activates DCD, and therefore no functionality is lost by ignoring DSR.

Fig 6

In the case of terminal-to-terminal conenctions, the Null-modem cable must be used with the additional connections to pins 2, 3, and 7 as already discussed. The full Null-modem cable swaps pins 4 and 8 - the RTS/DCD 'I am happy to send' signals, and pins 20 and 5 - the DTR/CTS 'busy' signals. To be on the safe side, pin 6 (DSR) is connected to pin 8 (DCD) in case that end of the cable is ever connected to a terminal which is fussy and requires DSR as well as DCD.

Fig 7

There is a school of thought which says that a Null-modem cable, unlike the pair of modems it replaces, is ALWAYS 'happy to send'. Therefore it is quite in order to generate DCD (and DSR) permanently. This is achieved by connecting them to the RTS at the same end of the cable, rather than to the RTS at the other end of the cable.

Fig 8

Finally, if the transmission rate from one of the two terminals is known to be unstoppable (e.g. a person typing at the keyboard), or is so slow and infrequent (e.g. the software handshake characters 'XON, XOFF' sent by the printer) that there is no danger of over-running the receiving end, then it is permissible to permanently enable the transmission by linking pin 5 (CTS) to pin 4 (RTS), i.e. to always send if ready (at the transmitting end of the cable). It may well be facilitated in any case, for the transmitting terminals to ignore the state of CTS under these circumstances.

THE RECOMMENDED TERMINAL CABLE

Fig 9

Appendix 5: Printer Lead (PL-2) Wiring Specification

Connectors.

  1. Computer Centronics Parallel Interface connector is a 25-way, D-plug.
  2. Printer Input connector is a 36-way, IEEE-488 plug.

Cable Wiring.

Line NameComputer Connector Printer Connector
Strobe 11
Data Bit 022
Data Bit 133
Data Bit 244
Data Bit 355
Data Bit 466
Data Bit 577
Data Bit 688
Data Bit 799
Ack 1010
Busy1111
PO1212
Slct Out1313
AutoFd 1414
Error 1532
Reset 1631
Slct In 1736
GND1819
GND1920
GND2021
GND2122
GND2223
GND2324
GND2425
GND2526
GND27
GND28
GND29
GND30
GND33
GND15
GND16
GND17
GND18
GND34
GND35

Appendix 6: Power Supply Requirements

The following is a summary of the power supply requirements for the Amstrad PC 1512.

Main Electronics Board (Including Keyboard)

Expansion Slots Allowance (Three Slots)

Floppy Disc Drive (Single)

Hard Disc Drive

This results in the following total for a worst case configuration:

System with 2 floppy drives and Hard Disc

Appendix 7: 512K to 640K conversion

Use 18 pcs 64K - 150 ns Drams plugged into 18 I.C. sockets provided, numbered IC153 to IC170 on main pcb. A link switch must then be moved into the 640K position. (see link diagram). It is important that this switch position is changed before unit is switched on. Please note if 2nd drive has been fitted, it needs to be removed while adding the extra memory.

[Diagram showing motherboard]

Appendix 8: ROM Character Set

[Character set]

Appendix 9: Keyboard Keycodes

[Keycodes diagram]

Appendix 10: Keyboard Layouts

[USA and UK layouts]
[French and German layouts]
[Spanish and Italian layouts]
[Swedish and Danish layouts]

Section 3 Site Index Book Index