This modular computer can run RomWBW, which includes a very good implementation of CP/M. It can also run the Small Computer Monitor.
Both firmware options can be installed at the same time as the memory module has two sockets for FLASH memory chips. A switch selects which firmware is used.
SC691 meets the specification for an RCBus “Alpha” computer.
- SC691 – Assembly guide
- SC691 – Compatibility
- SC691 – Firmware, RomWBW RCZ80_std (RomWBW v3.5)
- SC691 – Firmware, RomWBW RCZ80_sc (RomWBW v3.4)
- SC691 – Firmware, SCM S7
- SC691 – Parts list
- SC691 – User guide
- SC600 series information
- SC600 series support
Downloads
See individual product pages for downloads, such as kit contents sheets, schematics, PCB design files and PCB Gerber files.
Errata
In May 2025 the SC691 kit was upgraded to include a two channel serial module with baud rate options (SC603), replacing the original single channel module (SC610).
Suppliers
| Kits | Website | From | Currency |
| Small Computers Direct | SCDirect | UK | GBP |
| Stephen C Cousins | Tindie | UK | USD |
| Small Computer Central | Lectronz | UK | Euro/USD |
| PCBs | Website | From | Currency |
| Small Computers Direct | SCDirect | UK | GBP |
| Stephen C Cousins | Tindie | UK | USD |
| Small Computer Central | Lectornz | UK | Euro/USD |
| Assembled and Tested | Website | From | Currency |
| Not available | |||
| Components | |||
| See parts list |
Tindie does not collect VAT for EU countries
Lectronz does collect EU VAT for orders up to 150 EUR
Parts List
The base system consisting of:
- SC601 – Z80 CPU plus clock and 11 status LEDs
- SC602 – Memory module, 1 x 512KB RAM, 2 x 512KB ROM
- SC603 – Serial module, Z80 SIO/2, two serial port
- SC611 – Micro SD storage module
- SC673 – Backplane, 10 vertical sockets, 1 horizontal socket, 5 volt power input
Once built this system contains:
- Z80 processor
- 512k bytes RAM
- 2 x 512k bytes FLASH
- 7.3728 MHz clock
- 11 status LEDs
- Voltage supervisor and reset
- Two asynchronous serial ports
- Baud rate options from 2400 to 115200
- Micro SD card slot for file storage
- Backplane with power input
Recommended accessories:
- SC346 – Serial to USB C adapter
- 5 volt, 1 amp, power supply
- Micro SD card (2GB to 16GB recommended)
Optional modules and backplanes from SC600 series:
- SC604 – Compact Flash storage module
- SC605 – Digital I/O module
- SC606 – Real time clock module
- SC607 – Prototyping module
- SC608 – I2C bus master module
- SC609 – Servo control module
- SC610 – Serial module, 68B50 ACIA, one serial port
- SC671 – Backplane, 6+1 slots with 5 volt input
- SC672 – Backplane extension
Many of the modules designed for the standard 40-pin bus and the 60-pin enhanced bus can also be used with this system. See Compatibility below.
User Guide
This section refers to the system as a whole. Details of each of the above modules can be found on the individual product pages.
There are a number of options to power the system. These are described here. This is very important, so please read!
The memory module (SC602) has two ROM sockets, selectable with a switch. Typically, one ROM contains RomWBW and the other contains the Small Computer Monitor.
Both firmware options assume a terminal (or computer running terminal software) is connected to serial port A. Typically, this is achieved with a USB to serial adapter (the small PCB shown below) connected to the computer with a USB C cable.
The default serial port settings are: 115200 baud, 8 data, 1 stop, no parity, RTS/CTS hardware flow control.
A brief guide to using RomWBW can be found here.
Full documentation and source code for RomWBW can be found here.
Small Computer Monitor details are here.
Assembly Guide
Build the backplane and modules as described on each product page.
- SC673 – Backplane 10+1, 5 volt input
- SC601 – Z80 CPU with clock and status LEDs
- SC602 – Memory module, 512kB RAM, 2 x 512kB ROM
- SC603 – Serial module, Z80 SIO/2, two serial port
- SC611 – Micro SD storage module
Fit jumper shunts to all header pins as illustrated below
There are a number of options to power the system. These are described here. This is very important, so please read!
If you are intending powering the system via the jack (or “barrel”) socket on the backplane, first check the supply is providing between 4.75 and 5.25 volts at the jack plug. Check the centre pin is positive.
If you intend powering from the screw terminals on the backplane, first check the supply is providing between 4.75 and 5.25 volts.
Do not fit any modules to the backplane. Connect the power supply to the backplane and turn on with the toggle switch. The power LED should light. Turn the power off.
Fit the Z80 CPU module SC601 to any socket on the backplane.
Fit the memory module SC602 to any socket on the backplane.
Turn the power on. The set of 8 LEDs on the CPU module should show a test sequence. The switch on the memory module select which ROM is used. When SCM is selected the sequence should be each LED lighting up for a fraction of a second in sequence. When RomWBW is selected the LEDs should light in sequence and stay on until all are on. However, without the serial module connected the sequence will not complete.
Turn off. Fit the serial module SC603 and then turn on again. Check the LED sequence again.
Turn off. Fit the USB to serial adapter to port A and connect to a computer running terminal emulation software. Set the terminal software to 115200 baud, 8 data bits, 1 stop bit, no parity. For the first test select no flow control. Switch on. The startup text should appear in the terminal window. The system should respond to key presses. Select RTS/CTS hardware flow control and repeat the test.
Connect the MicroSD card module SC611 but without a MicroSD card fitted. Switch on. The startup text should appear in the terminal window.
Fit a MicroSD and switch on. The startup text should appear in the terminal window. The presence of the MicroSD card should be indicated in the startup text.
The system is now be complete.
This section refers to the system as a whole. Details of each of the above modules can be found on the individual product pages.
When using RomWBW each ‘drive’ on the MicroSD card needs to be initialised with the CLRDIR command from CP/M. With recent releases of RomWBW you may need to use FDISK80 first.
SCM does not support MicroSD cards.
Compatibility
This system conforms to the RCBus specification v1.0 for RCBus-2014 (40-pin).
The following table indicates electrical compatibility with SC691 with default options. Check firmware and software details for any required software support.
| Product (80-pin) | ? | Compatibility notes |
| SC126 Z180 motherboard |  | Function conflict |
| SC701 Backplane 6+1 | | Function conflict |
| SC702 Backplane 6+1 |  | Adds 6 module sockets RCBus 80-pin |
| SC703 Power from 12v | | Power supply and reset module (8 to 15v input) |
| SC704 I2C bus master | | Adds I2C bus master and I2C EEPROM |
| SC705 Serial ACIA | | Adds one serial port Typically set to 0x40 |
| SC706 Z80 CPU | | Function conflict SC730 included |
| SC707 Memory 128k | | Function conflict SC721 included |
| SC708 Z80 SBC | | Function conflict |
| SC709 Backplane 12+1 | | Function conflict |
| SC710 Backplane 12+1 | | Adds 12 module sockets RCBus 80-pin |
| SC711 Prototyping | | Prototyping module |
| SC712 Power from 5v | | Function conflict |
| SC713 RCBus tool | n/a | Simple reference tool No electrical function |
| SC714 Memory 512k | | Function conflict SC721 included |
| SC715 Compact flash | | Function conflict SC729 included |
| SC716 Z80 SIO/2 | | Adds two serial ports Typically set to 0x84 |
| SC717 Z80 PIO | | Adds parallel I/O using Z80 PIO |
| SC718 Z80 CTC | | Adds counter/timer using Z80 CTC |
| SC719 Digital I/O | | Adds digital I/O Eight in, eight out |
| SC720 Z80 motherboard | | Function conflict |
| SC721 Memory 512k | | Function conflict |
| SC722 Z180 CPU | | Function conflict |
| SC723 Backplane 3 | | No male header pins for expansion |
| SC724 Breakout | | Prototyping breakout module |
| SC725 SIO+CTC | | Adds two serial ports and 4 counter/timers |
| SC726 Clock generator | | Generates CLK and CLK2 bus signals |
| SC727 Real time clock | | Adds real time clock for time and date |
| SC728 Module riser | | Simple module riser for RCBus 80-pin |
| SC791 Z80 system | | Function conflict |
| SC792 Z180 system | | Function conflict |
| SC794 Z80 system | | Function conflict |
| Product (40 pin) | ? | Compatibility notes |
| SC114 Z80 motherboard | | Function conflict |
| SC130 Z180 motherboard | | Function conflict |
| SC133 Backplane 11+1 | | Function conflict |
| SC134 LED output port | | Adds eight LED outputs |
| SC135 Digital output port | | Adds eight digital outputs |
| SC136 Digital input port | | Adds eight digital inputs |
| SC137 I2C bus master | | Adds I2C bus master |
| SC139 Serial ACIA | | Adds one serial port Typically set to 0x40 |
| SC141 Backplane 12+1 | | Adds 12 module sockets RCBus 40-pin |
| SC142 Power from 12v | | Function conflict |
| SC143 Flash ROM | | Function conflict |
| SC145 Compact flash | | Function conflict |
| SC147 Backplane 6+1 | | Adds 6 module sockets RCBus 40-pin |
| SC148 Power from 5v | | Function conflict |
| SC149 Z80 CPU | | Function conflict |
| SC150 Paged RAM 128k | | Function conflict |
| SC151 Paged ROM 128k | | Function conflict |
| SC601 Z80 CPU | | Function conflict |
| SC602 Memory module | | Function conflict |
| SC603 Serial module | | Adds two serial ports with baud rate options |
| SC604 Compact flash | | Adds Compact Flash card for file storage |
| SC605 Digital I/O | | Adds digital I/O Eight in, eight out |
| SC606 Real time clock | | Adds real time clock and calendar |
| SC607 Prototyping module | | Prototyping module |
| SC608 I2C bus master | | Adds I2C bus master |
| SC609 Servo controller | | Adds two servo control outputs |
| SC610 Serial port | | Adds one serial port at 115200 baud |
| SC611 MicroSD card | | Adds MicroSD card file storage |
| SC612 Serial port | | Adds one serial port with baud rate options |
| SC613 Compact flash | | Adds Compact Flash card for file storage |
| Other suppliers | ? | Compatibility notes |
| RC2014 Mini system | | Function conflict |
| RC2014 Pro system | | Function conflict |
| RC2014 Zed system | | Function conflict |
| RC2014 Micro module | | Function conflict |
| RC2014 Backplane 5 | | If header pins fitted it adds 4 x 40-pin sockets |
| RC2014 Backplane 8 | | No male header pins for expansion |
| RC2014 Backplane Pro | | No male header pins for expansion |
| RC2014 Z80 CPU | | Function conflict |
| RC2014 SIO/2 dual serial | | Function conflict Address conflict |
| RC2014 Compact flash | | Function conflict |
| RC2014 Dual clock | | Generates CLK and CLK2 bus signals |
| RC2014 Pageable ROM | | Function conflict |
| RC2014 64k RAM | | Function conflict |
| RC2014 Memory 512k | | Function conflict |
| RC2014 Digital I/O | | Adds eight LED outputs and eight push buttons |
| RC2014 Real time clock | | Adds real time clock for time and date |
| RC2014 RP2040 VGA terminal | | Connect VGA monitor and USB keyboard |
| RC2014 Pi Pico terminal | | Connect VGA monitor and USB keyboard |
| RC2014 YM2149 sound card | | Adds authentic retro sound |
| TMSEMU3 – Graphics and terminal module | | Adds serial terminal and graphics |
| Weird electronics Flock v2 | | Adds floppy disk and real time clock |
Notes
- This product is designed for hobby use and is not suitable for industrial, commercial, or safety-critical applications.
- The product contains small parts and is not suitable for young children.
- RomWBW is copyright Wayne Warthen and has been provided free of charge with his permission.
- The Compact Flash interface circuit has been designed by Tadeusz Pycio and reproduced with his permission.
- RC2014 is a trademark of RFC2795 Ltd.
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