Stephen C. Cousins designed an extended and re-worked version of the RC2014, available for ordering as kit or as pure PCB at Tindie. Cousins also operates an excellent Website with detailed information, as well as assembly and user guides.
The modules from Stephen already incorporate the extended system bus (2x40 pins instead of 1x39 in the original version), and they are more integrated, e.g. combining CPU, RAM and ROM in one PCB.
RC2014 Stephen C Cousins Version (Small Computer, SC)
- SC116 – Modular Backplane (3 Slots)
- SC108 – Z80-CPU, 128k RAM, 32k ROM
- SC110 – Serial Module, Z80 SIO/2 and CTC
- Testing the whole system
- SC112 – Modular Backplane (6 Slots)
- SC129 – Digital I/O Board
- SC115 – Prototype Breakout Board
The first three PCBs (SC108, SC110 and SC116) create a small and lean, but complete Z80 system, to start with the Small Computer Monitor and Basic. Besides the Z80 CPU, RAM, ROM, there are two serial interfaces (and a CTC) available. The serial interface is used to connect to a standard PC (or a MAC in my case) via a terminal programm.
In a second step, the system will be expanded by a Digital I/O module (SC129) and a Breakout Board (SC115) for prototyping. Therefore we need a slighter bigger backplane with 6 slots (SC112). Maybe later this will be expanded by even more modules - let's see.
SC116 – Modular Backplane (3 Slots)
For a start I have chosen the small backplane with 3 slots. The slots form the exptended system bus with 2x40 Pins are compatible with the initial system bus with 1x39 Pins. Not all pins of the second row are actually used, e.g. pins 1-16 are not connected. The second row allows access to additional control lines, like Clock2, BUSACK, HALT, BUSRQ, WAIT, and NMI, further address lines, and additional data lines for 16 Bit CPUs.
The complete specification of the system can be downloaded as PDF from Stephen C. Cousins' website, the Small Computer Central. All details for the SC116 Backplane can be found here.
Besides the three main slots, the backplane also provides inputs for a 5V power supply (incl. green status LED), a power switch, and a reset button. As power input, a regulated 5V supply should be used, either connected with a 2.1mm barrel plug or a screw terminal.
SC116 Modular Backplane (3 Slots)
Assembly of the backplane was pretty „straight forward“. I did not manage to source the required 2x40p sockets at reasonable prices, so I had to improvise using a few 2x17p and 2x6p sockets. To make them fit, I had to file them down a bit - visible and not really beautiful, but hey - it's a hobby project!
SC108 – Z80-CPU, 128k RAM, 32k ROM
The heart of the system is the CPU module, featuring:
- The Z80 CPU
- 128k RAM (64k usable), and
- 32k ROM (will contain the Small Computer Monitor and Basic)
The Z80 CPU will be clocked by a 7.3728MHz oscillator. Depending on the used version of the CPU, you can also operate with different clocks. But then you need a separate 7.3728MHz oscillator for the serial module. If you stick to 7.3728MHz, the clock of the CPU module can be used for the serial module, too.
The ROM (a 27C256 EPROM) will be programmed with the Small Computer Monitor (configuration R3), which contains a Machine Code Monitor, BASIC and a CP/M loader. To use CPM you need e.g. the CF-Modul with the CP/M software on a CF card.
All details about the SC108 CPU module can be found here.
SC108 Z80 CPU, RAM, ROM
Thanks to the excellent documentation on Stephens website, assembly of the module was easy and successful. Attention: the required oscillator comes in a small 8 pin package (8 PDIP) – I accidentially sourced the "usual" one in a 14 pin version - took me about a week to get a replacement.
SC110 – Serial Module with Z80 SIO/2 and CTC
The serial module SC110 is completing the base system and realises the quite important connection to a terminal program running on a standard PC.
The module is based on the Z80 SIO/2 Chip, providing two serial TTL Ports, usable with standard FTDI Seriall/USB converters, as well as one Z80 CTC, providing four counter/timer channels.
All details about the SC110 Serial Module can be found here.
Assembly was again easy – thanks to the excellent guidelines on Cousins' website.
SC110 Serial & CTC
Testing the sytem
After completing all modules, a test of the complete system was done.
Pay attention to the following:
- Each single module should be tested individually first (according to Cousins' assembly guides)
- For the serial connection, Port A of the SC110 module has to be used! The baud rate of Port A is fixed to 115.200 Baud (8/n/1) – the baud rate of Port B is not fixed, but needs to be set with the SCM later
- The FTDI converter needs to be identified by the PC/MAC (my MAC automatically identified the modul sourced at AZdelivery without any special driver)
Consider the following, when connecting the FTDI converter:
- connect TX/RX the right way (TX to RXD and RX to TXD), and
- do not use the 5V connection from the FTDI together with the main 5V supply. Either you leave it unconnected, or you leave the jumper on the SC110 open. Alternatively you can power the whole system from USB via the FTDI. But then do not use a main power supply and set the jumper to the "short" position.
- GND needs to be connected
- DTR/CTS are not used
|FTDI Signal||FTDI Pin||SC110 Pin||SC110 Signal|
If all is done correctly, you will be greated by the SCM inside the terminal with the unspectacular message
Small Computer Monitor – S3
and a very simpol
leads you to the main menu of the SCM , where you can do some basic configurations, gather information, and start e.g. the Assembler, the Basic or the CP/M loader.