One Per Desk 

Overview

Storage Solutions

Technical Info

..Hardware
..Firmware
..Problems
..Appendices

Printers

This is an extract from the article "A Comprehensive Technical Information Document" written by Murray McCabe in 1990.

INTRODUCTION
The OPD computer was manufactured by ICL in late 1984/early 1985. A version for North America was produced in small quantity. The computer resulted from collaborative development by ICL, Sinclair and BT, with PSION providing the XCHANGE programs. The hardware skeleton is based on the Sinclair QL. This includes the 68008 CPU, the two main QL ULAs (Uncommitted Logic Arrays), the method of dynamic RAM (DRAM) and screen management, the real time clock and the microdrives.

1. OPD HARDWARE SUMMARY

1.1 CPU
The main processor is a Motorola 68008P8, 8X16 bit device with a 7.5 Mhz clock and a 1MB memory address capability. A second processor, an 8051 in the modem, controls the keyboard and the communications functions

1.2 MEMORY
The memory map is shown below. The main components of the map are:-

RAM
128K DRAM (2 banks of 64K 8 bit, using 4164-15 ICs)
2K Battery Backed CMOS RAM. (The PERMANENT STORE: a TC5516AP-2 IC).
32K of RAM is allocated to the screen. This, plus standard demands, leaves approximately 75K of work space (150 blocks of 512 bytes).
128K is reserved for expansion RAM, allowing a total of 256k of DRAM.

ROM, INTERNAL
128K of Operating System (4 x 32K 8 bit ROMs) (16K of Speech synthesiser ROM is controlled directly by the speech synthesizer IC and is not included in the 68008's memory map).
64K is reserved for expansion of the Operating System.

ROMPACK, OPTIONAL
160K of ROM (144K XCHANGE + 14K Operating System, five 32K 8 bit ROMs).
64k maximum in ROM capsules in a 2-slot ROMPACK.
128K maximum in ROM capsules in a 4-slot ROMPACK.

The ROMPACK detail above refers to the OPD as released in 1984, the following changes subsequently occurred:

i) In 1985 ICL introduced the 4-slot ROMPACK. Space for the 2, additional, 'slots' was obtained by squeezing XCHANGE into 2 ROMs. Version 2.5 of XCHANGE plus the ICL firmware in the ROMPACK (CALCULATOR etc.) was contained in a 128K x 8 bit PROM (i.e. a 1MB PROM) and a 32K x 8 bit PROM (27256 equivalent). These devices were the XFAA01 (ICL/PSION - 1M) and XBAJ02 (PSION 256K).

ii) ICL then developed their 128K memory expansion unit (MEU). This had XCHANGE 2.5 in the two PROMs (as above) and a further 8K 8 bit EPROM (27C64) containing the test/initialisation routines for the new memory and firmware for the MEU'S slow RS232 port.

iii) PCML produced their smaller, neater 128k memory expansion, TELESTORE in a ROMPACK case. This had the test and patch code for the expanded memory, XCHANGE 2.5 and the ICL firmware (CALCULATOR etc.) contained in two PROMs. These were the XFAA01 PROM and a 27C512 EPROM.

The capsule addresses of the 2-slot ROMPACK are different from those of the 4-slot at ROMPACK, 192K is reserved for the 6 capsule addresses of the 2 ROMPACK types. As a results there is potential to operate the OPD with 6 capsules.

PCML exploited this in the last version of their TELESTORE which accommodated 6 capsules. However, when using six capsules, if some are multi-program can be too much for the standard Operating System to accommodate. Details appear below on how to alter the OS to accept a greater number of ROM programs if the INTERFILE ROMCAP is available.

There is no 'spare' space in the memory map. Space not listed was 'reserved' by ICL for Input/Output (I/O) and possible future applications. There are, however, 3 'reserved' 32K slots, that may be available if the address de-coding of ULA3 is improved and other changes made.

The OPD DRAM REFRESH method is the same as that on the Sinclair QL i.e. there is no separate REFRESH circuitry. REFRESH is achieved by accessing the DRAM routinely to generate the VDU screen and for other regular, repetitive procedures. This results in delays and a loss of potential memory speed. The OPD has advantage over the QL because much of the OPD memory is allocated to ROM which is not subject to the same delays as DRAM.

1.3 MODEM
The modem is a plug-in, BT approved (and designed), dual line, auto-dial/auto-answer unit based on the ADM7910 IC. The following facilities are provided:-

1200 bps half duplex, (Bell 202, CCITT V23 mode 2)
600 bps half duplex, (CCITT V23 mode 1)
75/1200 bps full duplex, (Viewdata)
300 bps full duplex, (Bell 103, CCITT V21)

Different modems are available for different operating locations e.g. USA and Australia. Australian modems have distinctive 4 pin line plugs with the APT logo. Pulse or DTFM (tone) dialling can be selected by DIL switches in the modem. The keyboard is inoperable if the modem is absent. Transmission format is configurable from internal firmware e.g. parity, stop bits, data bits and start bits.

There is 1 (or 2 depending on the model) ceramic 'sounders' in the modem. The Ring Equivalent Number (REN) of the OPD (modem plus telephone) is 1 per line. The maximum line loading, allowed by BT, to ensure reliable ringing, is REN 4 e.g. 4 OPDs or 1 OPD plus 3 normal phones of REN 1 etc.

A computer access directory with 'search-and-browse', short code dialling and a profile store is provided in similar format to the telephone directory.

1.4 TELEPHONE
The telephone handset is a standard BT 'Sceptre' type. There is an 8 ohm 250mW moving coil loudspeaker in the control unit but no in-built microphone. Facilities include 'hands-free' call initiation, loudspeaker call monitoring, auto-dial from memory or manual dial from the keyboard and store plus re-call/re-dial of the last 6 numbers used.

The telephone directory on the One Per Desk is capable of holding 500+ entries but at this level there is little memory left for other functions. The telephone directory has 'browse-and-search' facilities plus short code dialling. A single screen priority directory can be displayed by the LIST key and there is a facility to record the cost of calls.

The basic telephone functions for manual dialling are battery backed, allowing one line to make and receive voice calls if mains power is lost.

1.5 SPEECH SYNTHESISER
This consists of a Texas TMP5220C IC with a vocabulary in a separate, custom ROM of 152 words, plus letters and numbers. Facilities include limited text-to-voice conversion from keyboard entry and playback-to-test. Up to 16 messages can be pre-assembled and 2 used, automatically, by time-of-day selection, for auto-answer of incoming voice calls. Incoming voice messages cannot be recorded.

l.6 MICRODRIVES
There are 2 microdrives each of 100K nominal capacity (95K minimum). They are not identical to the Sinclair QL drives (or those released for use with the ZX Spectrum) in mechanical construction, or operational protocol, but blank ZX microdrive cartridges can be used in either machine. An adjustable 'end-of-life' warning is provided for tape data quality/total number of seconds used.

The OPD micro drives have greater data packing density than the Sinclair QL but more space between data blocks resulting in comparable overall capacity. Certain metal components are replaced with plastic and the amount of lubrication on the tape has been changed 

1.7 REAL TIME CLOCK
The clock is the same as the QL except that the start date is l/l/1970 while the QL start date is l/l/1961. The clock output is displayed in the Notice Board at the bottom of the screen.

1.8 VDU (Monitor)
The OPD control unit is designed to be permanently powered and to blank the VDU screen if the keyboard is not used for 10 minutes. Pressing any key recovers the display.

The ON/OFF switch removes full voltage from the monitor, but maintains a reduced supply to prewarm the tube heater for fast start-up This switch does not affect supplies to the control unit. The intent was that the monitor should be switched OFF between sessions but the control unit should remain powered for MESSAGING and other, unattended, functions.

The VDU case contains the mains power supply which is an ASTEC 30 Watt SMPS (Switched Mode Power Supply) card (types AA12630, AA12635, or similar). The colour monitor has its own power supply but the mono monitor is powered from the ASTEC card which has the following rating:-

+5 volts 3 to 3.5 Amps
+12 volts 1.5 Amps
-5 volts 0.1 Amps

The power supply can be connected for 115 or 240 volts, both +/- 10% and draws a nominal mains current at 0.75 Amps.

The 9 inch mono monitor uses a Philip's Chassis and contains an ICL composite video board. Colour and mono monitors have LED (or neon) indicators for 'Mains ON' and 'Monitor Power ON' plus a contrast control.

Power supplies and video signals are coupled from the monitor, via a multi-core cable, to a 15 way D plug on the control unit.

2 VDU options were available when the OPD was introduced: a 9" white screen unit with levels of contrast matching the number of colours in a particular screen mode, or a 14" Microvitec colour monitor available in medium resolution with high resolution as a later option.

1.9 SCREEN FORMAT
The screen format is similar to that of the Sinclair QL. It is bit mapped and has 2 modes selectable from software:-

256x256 pixels, 40 characters per line, 24 lines plus a 2 line 'Notice Board' at the bottom of the screen. Green, red and blue can be displayed on a colour monitor, giving 8 shades (contrast levelled plus flashing, on a monochrome monitor).

512x256 pixels, 40 or 80 characters per line, 24 lines plus a 2 line 'Notice Board'. Red and green only are available on a colour monitor giving 4 shades on a monochrome monitor.

In BASIC the screen size available as display for the running program is less than the actual figures quoted above e.g. in the 512x256 mode the BASIC program display area is only 480x200.

1.10 PRINTER INTERFACE
This is a single RS423 serial port which is compatible with RS232 for printer duty. The port consists of a 9 way female D connector on the OPD control unit (only 3 pins of the connector are used).

The ICL package printer was an OKI thermal, colour printer with a draft printing speed of 80 cps and 40 cps in NLQ.

Epson RX80 compatible printers are catered for in the software and printer configuration programs. The normal BT Tonto printer is the Merlin M1880, described as a standard graphics printer The M1880 appears to be MOA printer MP1711 (WM80) and to be a Shinwa SP80 badged for BT. The MP1713 (WM100) DMP was also issued with some Tontos.

The daisywheel printer recommended for the Tonto was the Dyneer DW16.

1.11 BATTERIES
The OPD has 2 batteries:

BAT1 is a 9 volt, PP3, manganese battery which backs the telephone and the Real Time Clock. It is fitted under the microdrive cover and is user replaceable. The Operating System checks BAT1 voltage during power-up and under the HOUSEKEEPING function.

BAT2 is a 3 volt lithium unit (with a life of at least 5 years) which backs the PERMANENT STORE CMOS RAM. It is soldered to the motherboard; is not considered user replaceable and is not checked directly by the Operating System.

1.12 KEYBOARD
The 73 key keyboard uses the standard QWERTY layout with 10 of the keys colour coded to act as function keys and double as a 'telephone style' numeric pad. The keyboard was manufactured by ALPS using individual switches of rubber membrane construction.

Keyboards with black QWERTY keys, and a red ENTER key, are export units intended for Australian.

Keyboards in standard OPD/Tonto Colours but with inverted ' ' and '#' symbols, and 3 letter 'telephone' groups on the front faces of the number pad keys, are thought to be export units for North America. Later versions of the keyboard have blocking diodes on 5 keys.

1.13 ROMPACK
The ROMPACK included, as an option, XCHANGE in ROM. The original 2 slot ROMPACK has 5 x 32K ROMs. Later, 4 slot versions, and TELESTORE, use a high capacity ROM to make room for the additional slots. ROM capsules mount in the slots and can be 8, 16 or 32K.

+5 volt supplies, address and data buses, plus READ/WRITE and EXTINT lines are available on the 30 pin 'slot' connectors to allow their use as a simple expansion ports.

1.14 ROM CAPSULES
The ROM capsules are normally constructed on a single sided PCB, with the ROM IC, a resistor and a supply decoupling capacitor surface mounted, to obtain minimal capsule thickness. The ROM IC must be de-soldered to remove it.

A capsule's presence is signalled by a link on its 30 way connector which enables that 'slot's' section of the PLA de-coding (in the ROMPACK). Capsule ROMs have a specific firmware header which the OS checks at Power-up.

The System verifies that the capsule ROM is a valid OPD device and, if valid, 'logs' it onto the OS. (Detail of capsule construction is given in Appendix 6).

1.15 PHYSICAL PRESENTATION
The OPD is made up of 4 units:

Control Unit
Modem
ROMPACK
VDU and Power Supply

The control unit includes the main PCB, 'piggy-back' CPU board, keyboard, telephone hookswitch, loudspeaker, 2 batteries and 2 microdrive units.

The telephone handset is wired directly to the modem. The modem and the ROMPACK plug into the rear of the Control unit.

The monitor includes the power supplies for itself and the other units.

Size
Control unit 440mm x 250mm x 95 mm
Mono Monitor 250mm x 280mm x 280mm

Weight
Control unit 3 kg
Mono Monitor 4.15 kg.

1.16 SPECIAL COMPONENTS
The OPD main board uses the 2 Sinclair QL ULAs (ZX8301 and ZX8302 or equivalents). Each microdrive uses a QL Ferranti/Sinclair 2GO075ES device. Two ICL 'ULAs' are employed. One is ULA3 on the main board which controls the non-QL functions i.e telephone, speech synthesizer, battery backed static RAM etc. The second ICL 'ULA' is the PLA device for address de-coding and interfacing in the ROMPACK.

5 types of ICL PLA have been identified:

KVAB02 - Standard 2-slot ROMPACK
KVADO1 - Standard 4-slot ROMPACK
KVAE01 - MEU
KVAG01 - MEU
KVAL01 - TELESTORE 4-slot
The 4 and 6 slot TELESTORE use a PCML CMOS PLA.

The Texas vocabulary ROM is an OPD special. The modem uses six thick film, hybrid circuits. Excluding Capsules there are ten ROM ICs in an original OPD with XCHANGE. Identified ICL ROMs are listed in Appendix 4.

2. FIRMWARE
The design concept was that, for ease of operation, fast loading and optimisation of the relatively small RAM, the major application programs would be provided in ROM. This was in keeping with the targeted, non-technical executive market. The intent being that the OPD would not rival a desk-top PC for general computing. The following ROM firmware was produced for the OPD:

2.1 OPERATING SYSTEM (OS)
The OS is an ICL multi-tasking 'special' allowing up to 5 tasks to be run concurrently. It is not QL QDOS nor is it an industry standard which can take advantage of existing software. The OS is contained in 4 X 32K ROMs on the CPU board. A further 14K is carried in the ROMPACK. ICL called the OS 'BFS' (Basic Functional Software)

The following make up the BFS:

KERNEL - manages the hardware, the memory map, input output device control, and also handles interrupts.
DIRECTOR - is higher level firmware handling applications and the telephone. It controls START, RESUME and REVIEW key functions and allocates resources.
TELEPHONE HANDLER - manages the 'nuts and bolts' of telephone usage.
TELEPHONE DIRECTORIES - 2 directories are managed, one for telephone voice calls and one for computer services. CALCULATOR - A simple 16 digit calculator with memory. (CALCULATOR firmware is in the ROMPACK).
SCREEN IMAGE PRINTER - A screen dump to printer on a single keystroke.
FIELD EDITOR - provides cursor and text editor control.

3. PROBLEMS IDENTIFIED WITH THE ONE PER DESK
These relate to the OPD as released in 1984.

- Serious memory shortage and fragmentation of the available memory as the work session proceeds.

- PSION XCHANCE is not integrated with the Operating System. QUILL cannot access the modem or the real time clock etc.

- BASIC is not in ROM and has to be loaded from microdrive, absorbing scarce RAM.

- Multi-tasking is limited by memory shortage and the Operating System e.g. in-coming telephone calls can abort Operations in progress such as microdrive formatting.

- The Operating System is an ICL 'special', So preventing the use of standard software.

- ICLBasic is a curtailed subset of Sinclair SuperBASIC but is not compatible with it e.g. ICLBasic does not include SuperBASIC graphics such as LINE, CIRCLE and ARC.

- The standard mono monitor is not of high quality. It has no brightness control, in consequence, the display can be difficult to optimise. (The optional colour monitors provide an improved display and are preferred by most business users).

- PSION XCHANGE, in ROM, is not fully developed and is inflexible regarding update and improvement by third party software e.g. QUILL cannot be modified for spelling checkers and cursor acceleration programs such as SPELLBOUND, FILEBOUND and TURBOQUILL+ while ARCHIVE cannot save the faster '_pro' programs.

- The numerical routines for access by menu, together with the hierarchical menu structures, are cumbersome and lack mnemonics or similar logical features to help the operator remember them.

- The keyboard has a number of multi-function/shifted keys; some keytops having 3 legends.

- Device names, such as the microdrives, change between normal OPD operation and operation in BASIC.

- An OPD (with 4 ROM capsules) can have over 400K of firmware in ROM (14 ROM ICs). This yields benefits in speed and simplicity of operating but up-dating to later, improved versions of the firmware can be expensive when PROMs rather than re-programmable EPROMs are used i.e. new ROM devices have to be obtained and the old devices discarded.  In general early OPDs used EPROMs but the bulk of the main OPD production used PROMs.

- Many of the OPD functions have a raw, unfinished feel as if they needed refining to reach release standard.

- The OPD was not provided with a dedicated expansion port. The ROMPACK connector, or a ROMPACK capsule slot, is used to link expansion hardware to the internal circuitry. The connector on the main board, for the ROMPACK, (J5) is provided with a variety of interface connections which are not used by the ROMPACK (or fed through it) but can be used for comprehensive expansion if the ROMPACK is not in place.

APPENDICES:

Fig 1: Memory Map

Appendix 1: Increasing the number of Applications

There is a limit to the number of cartridge and capsule applications that the OPD can handle. If this is exceeded the error message TOO MANY PROGRAMS appears on the initialisation display. The INTERFILE capsule includes a 'patch' to correct this. The special code runs automatically at power-up and normally causes a second initialisation.

The INTERFILE code does not cater for multiple-applications on a cartridge. The error code for this is TOO MANY and appears when a Cartridge menu is requested. To correct carry out the following:-

Load BASIC
Enter the following command

set_pse 28,n

n should be a number from 2 to 5.
If n = 2, approximately 20 cartridge applications can be handled, if n = 3 'approximately 30 etc..

The above alters the programme handling instructions in the PERMANENT STORE to provide multiple menu screens but this data is only recognised during power-up INITIALISATION of the OPD. To implement the changes to the PERMANENT STORE the computer must be switched off and then re-energised so that the changes can be recognised by the Operating System.

Appendix 2: OPD ROM format

1. Block Header

An OPD ROM is divided into blocks. Each block starts (at its lowest address) with a 4 byte header. The header allows the Operating System (OS) to recognise an OPD ROM as valid firmware. The header must start on an even address byte.

Bytes 0, 1 and 3 of the block header are always the same, being:

hex A5, 4F and 00 respectively. Byte 2 is the block size in multiples of 8K

e.g. A 32K block OPD ROM starts with the header:

A5 4F 04 00

2. Block trailer
The block ends with a Trailer of 16 bytes:
Bytes 0 to 11 are spaces
Bytes 19 to 15 are zeros

3. Programs

One ROM block can contain several programs between its block header and the block trailer. Each program must be a multiple of 2 bytes in length and must start on an even byte number. There must be no spaces between the block header and the first program, between programs or between the last program and the block trailer.

4. Program Header

Each program has a header consisting of 26 or more bytes. The total header format is complex but for the manipulation of existing programs knowledge of two aspects of the program header are generally sufficient i.e.

a) Bytes 0 to 11 give the program name

b) Bytes 12 to 15 give the total program length in bytes including the program header

The format of a machine code program on cartridge is similar to a ROM program. The Cartridge program does not have a ROM block header or trailer. It is possible to convert some machine code cartridge programs to ROM programs by adding a ROM block header or trailer. This can be done with the ICL Basic cartridge to provide improvements in memory workspace & speed when working in BASIC.

The length of the ICLbasic program varies depending on the version. Version 5.02 and 7.00 are approximately 38k & 36k respectively. All versions exceed the capacity of an 27256 EPROM (32k). ICLbasic, therefore requires to be contained in two 27256's. These must be mounted, in correct order, in two consecutive slots in the memory map. The obvious location is the 64k reserved for the expansion of the OS.

Additional firmware such as VT-LINK or the MEU programs can be used to fill the remaining space in the EPROMS carrying Basic.

Appendix 3: Setting up for INTERFILE

The INTERFILE capsule contains an enhanced version of the T-LINK protocol and this requires the Data-auto answer application to be reset to LF and the auto-answer Protocol to LI so that INTERFILE can function correctly. The factory default values of these are CTXFR and L respectively. To set for INTERFILE run the CONFIG program on the BASIC cartridge, change to the above values on the MAIN SYSTEM CONFIGURATION SCREEN and reset the PERMANENT STORE to retain the new data. If the INTERFILE capsule is removed the above values should be returned to the factory default settings via the CONFIG program, as above.

Appendix 4: ROM list

Notes:

1) The final digit in the ROM identification is the issue number.

2) There are 2 series of identifications, an initial 'Q' series for EPROMs which carried on to the first PROMs, and a later 'X' series for PROMs only.

3) Location abbreviations are:

mb = main board
rpk = ROMPACK
pb = 'piggy-back' CPU board
MEU = Memory expansion unit / ICL
ACU = Asynchronous Comms. unit / ICL

Appendix 5: Main board

1. DEVICE LISTING
 

1.1. ULA1 and 9 (IC 25 and 29) are Sinclair custom ICs for the Sinclair QL Home Computer.

1.2. ULA1 (CLA 230/ZX 8301). Performs the following functions:

• VDU picture generation
• Master clock
• DPAM control
• DATA bus Transceiver control

• Real time clock
• Interrupt control
• Microdrive control
• RS432 serial data for printer

1 4. ULA3 is an ICL custom device for control of peripheral functions not present in the QL. These are:

• Voice Synthesiser
• 2K static PAM
• Telephone/modem

a). VR1 - Speech Synthesiser Oscillator Tuning.
b). VC1 Real Time Clock fast/slow trim.

J1       Keyboard
J2       Modem
J3       VDU/PSU
J4       Printer
J5       ROMPACK
J6       Microdrives
J7       9v telephone battery
J8       Loudspeaker connector - only on early boards
J9       Hookswitch/loudspeaker

3.1. J3 - POWER AND VIDEO

15 way miniature 'D' Plug on Motherboard
 
 

Notes:
Hold Pin 14 (Powerdown) at +5 volts for operation.

3.2 J4 - PRINTER (9 way miniature D socket on motherboard)

Connections for Epson or compatible serial printer
 
 

3.3 J6 -  MICRODRIVES

14 way IDC connector, header plug on motherboard
 

Pin	Function
1	SELCLK
2	MDSELD
3	RAW1
4	Earth/0 volts
5	RAW2
6	Earth/0 volts
7	MDRW
8	Earth/0 volts
9	Earth/0 volts
10	Earth/0 volts
11	+5 volts
12	Protect
13	+12 volts
14	Erase

3.4 J9 -  LOUDSPEAKER AND HOOKSWITCH

0.1 inch inline header on motherboard
 

Pin	Function
1	Hookswitch in
2	+5 volts
3	LED1
4	LED2
5	+5 volts
6	Loudspeaker
7	Hookswitch out

Notes:
a) Hookswitch closes when handset is lifted
b)    Pins 2, 3 & 4 allow LED to be mounted on case top but to do this LED1 & LED2 must be removed from the motherboard
c)    Loudspeaker is connected between pins 5 & 6
d)    Pin 1 is to the front of the motherboard (nearest to the operator)

4. TEST POINTS

TP1 RESET : Short to zero volts for RESET
TP2 Voice synthesiser sampling oscillator : Set VR1 to give 100Hz at TP2
TP3 CSYNC (Composite sync) : Not used on standard OPD

5. ICL MODIFICATIONS

The following modifications have been seen on the mainboard.

5.1 C10 changed from a 220n, 40v tantalum to a 220n film capacitor in series with a 1ohm resistor
5.2 R47 changed from a 1Kohm resistor to a 2n2 ceramic capacitor
5.3 Field effect transistors FT3 & FT6 are removed
5.4 R11 changed from 33ohms to 200ohms
5.5 A 1Mohm resistor fitted in parallel with the 15MHz crystal

6. HOME BREW MODIFICATION OPTIONS

The following changes can be made:

6.1 Change IC34 from a 74LS240 to a 74LS244 to invert the video & sync. outputs from the main board to allow the use of a standard monitor
6.2 Connect TP3 to pin 15 of J3 (solder pad above Q7) to provide a composite sync. signal for a monitor

Appendix 6: Capsules

OPD Capsules provide optional ROM firmware and connect to the computer via 30 pin 'slots' in the ROMPACK. The original ROMPACK has provision for connecting 2 capsules. The later design of ROMPACK accepts 4 and the latest TELESTORE memory expansion accepts 6. The 30 pin 'slots' also serve as expansion ports for the OPD e.g. for connecting disk drives etc.

Each capsule contains a ROM IC which can be 8, 16 or 32K. The IC is normally an EPROM, surface mounted to obtain minimal capsule thickness. A typical capsule schematic is shown in Fig. 1. The Capsule includes a supply decoupling capacitor C1 and a 3K3 bias resistor R1. There is a link in the capsule between its pin B15 (ground, 0 volts) and pin A14 (HSD*:L). When a capsule is inserted into the ROMPACK this link enables the appropriate section of the ROMPACK programmable logic array (PLA) IC5. The PLA is a peripheral management IC. When the 'link' is in place the PLA generates an enable signal for the capsule ROM (RC*:EN:L) at the appropriate memory address. The link also enables related CPU interface signals, DTACK:L and VAPA:L, which advise the CPU that data is available at the capsule address and that the address is a valid peripheral address.

The PLA pin connections associated with capsule control on a 2-slot ROMPACK are:

PLA Pin	Function
15	RC1: EN:L
16	RC2: EN:L
8	HSD1:L
7	HSD2:L

Appendix 7: Using XCHANGE in a 2-slot ROMPACK

XCHANGE 2.5 was released for use in the 4 slot ROMPACK and the MEU/SEU memory expansion. It was also employed in all versions of TELESTORE. For the 4-slot ROMPACk, and the MEU, XCHANGE 2.5 was in the form of a 1M PROM {XFAA01) and a 256k PROM (XBAJ02). There was no general release of version 2.5 for the original 2 slot ROMPACK. However, some scrap, non-working MEUs and 4-slot ROMPACK have appeared on the surplus market. These generally have faulty PLA IC.'s for which replacements are difficult to obtain. However, the ROM/EPROMs can be in good condition and their XCHANGE programs reusable if a  means can be found to use them.

XCHANGE 2.5 is suitable for 'breaking-down' into 5 27256 EPROMs and mounting in a 2--slot ROMPACK. 'XFAA01/1' is the lowest 32K address block of the 1M XFAA01 PROM and 'XFAA01/4' is the highest 32K address block.

The OPD/TONTO memory map allocates 160K for XCHANGE plus that part of the Operating System which mounts in the ROMPACK (CALCULATOR etc.). 'XFAA01/1' should start at the lowest address in the 160K block and 'XBAJ02' finish at the highest. address in the 160K block. To achieve this the 5 EPROMs carrying XCHANGE 2.5 should be mounted in the ROMPACK as shown below