For those of you using one of our SS-50 motherboards and find yourself needing more I/O ports, this board will add an additional six SS–30 ports via a connector already on your motherboard. This extender simply plugs in and you’ll have the full eight slots available for more I/O boards. As you can deduce from this picture, each slot has a fixed address unlike the two slots present on the motherboard:

Two versions were produced of the SS-30 Prototype board, small and large.

Yet another experiment that we needed to develop our own products. This is meant to allow someone to experiment with building their own SS-30 (I/O) boards. The two connectors on the left have +5 and ground, while all the SS-30 pins have connectors on the bottom to make it easy to build a circuit with. Trying to accommodate both the SS-30 and SS-30C busses, this board has labels for both. Due to a lack of double-checking, the board pictured above has a bizarre error… all the lettering below the CORSHAM logo is on the back side of the board in a mirror image! Details, details, details.
UPDATE The board above works fine for our system since we don’t have a chassis, but didn’t work well in a real SWTPC chassis without removing the back panel. A new version was laid out that fits entirely in a genuine SWTPC chassis but I’ve yet to order any blank PC boards for it.

Large board
2019: Customers often come up with good ideas for new products or improvements to existing ones, and this was certainly one of those cases! Basically, he suggested adding more breadboard space, and adding a spring loaded connector on top for running wires to external circuitry. The resulting product is our new board:

On this page:
– SS-50 6800 CPU Power-On Reset
– SS050 FAQs
– SS-50 and SS-30 standards

SS-50 6800 CPU Power-On Reset

Update (2/7/2021): Rather than using a 220uf electrolytic, use a 47 uf tantalum capacitor instead.

Update (2/5/2021): A previous version of this page said to add the capacitor to U1… that was incorrect! It is definitely on U4, the MC6875!

All existing versions, including up to 3A, of the SS-50 6800 CPU Board have a problem where power-on reset does not work. A part is missing. This is not a severe problem, as the user can press the RESET button on the board or the motherboard to do the reset, but it can be easily fixed.

You’ll need to solder a 47 uf, 16v (or higher) tantalum capacitor to the back of the board. The negative side of the capacitor goes to U4 pin 8 (ground) and the positive side goes to U4 pin 12. This is a picture from when I was using a much larger electrolytic capacitor, but I’ve switched to a 47 uf tantalum cap for all new boards:

All future revisions of the board will include this fix.

SS-50 FAQs

Q: Where can I get a copy of the SS-50 standard?
A: There is no standard, just a loose specification manufacturers followed when designing boards. The first SS-50 machine was from SWTPC and had a 6800 processor, so the buss was closely designed around that processor’s architecture. When SWTPC added a 6809 board they more-or-less followed the previous use of most pins on the buss, but also changed the purpose of others. Other companies began making compatible boards based on what was already on the market. To paraphrase Dr. McCoy of Star Trek fame, “Dammit Jim, I’m a buss, not a standard!”

Q: I keep hearing SS-50 and SS-30. Are they the same thing?
A: No, but closely related. The SS-50 buss is the main processor buss which includes all the address, data, and control signals needed to control the buss or respond to the processor board. It has 50 pins. The SS-30 buss is meant only for I/O. Decoding addresses takes a few chips that chew up real estate on a peripheral board and add cost, so SWTPC does the decoding on the motherboard and runs only essential signals to the peripheral boards. This is the SS-30 buss, and has 30 pins. Only a few address lines, data lines, and very few control signals.

Q: What are those strange connectors?
A: Those are Molex connectors which SWTPC used on other, non-computer, products. Hey, use what you’ve got in stock to save development time and stocking cost! They are still available from many sources, but the cost has gone up tremendously since they aren’t widely used anymore. The male connector with 10 pins, as used on motherboards is AMP/ TE Connectivity part number 1-640384-0, while the female connector used on plug-in boards is Molex/Waldom 09-48-2101. There is also a polarizing plug that goes into the female connectors and is Molex part number XXXXXXX.

Q: Which pin is pin 1 on both the SS-30 and SS-50 busses?
A: Technically, there were no pin numbers. SWTPC simply used the name of the line and never referenced a pin number. Many companies use http://www.cs.unc.edu/~yakowenk/swtpc/ss50.html as a guide, but I am aware of at least one board where the pin numbers were exactly the opposite direction.

Q: Boards don’t seem to plug in without a lot of force. Are you sure they are meant to plug into the motherboard?
A: Yes, the Molex connectors offer a lot of resistance at first but eventually get looser with insertions and removals. Make sure the polarizing plug is aligned properly with the missing pin on the motherboard. Some people insert a small screwdriver or used dental tool into each of the female connectors to loosen them up, but be very careful not to over-loosen them or else some of the pins might not make contact.

Q: Why are there so many pieces to build a complete system?
A: This is a clone, so it has an SS-50 buss for the CPU and memory, and an SS-30 buss for the peripheral devices. The original systems had a motherboard, CPU board, memory board(s), serial console board, terminal, etc, which means our system does too. Back in the day, people were driven by cost (just like now) so systems were modular, allowing someone to buy exactly what they wanted (or could afford) while building their system.

Q: What is the bare minimum to get started?
A: The motherboard contains the framework for the other boards to plug into, and is required. To use a terminal for a console, a serial board is required. We have two CPU boards, one having a 6800 and 16K of RAM, and a 6809 CPU with 128K. Only one CPU board is required.

Q: What can the bare minimum systems do?
A: Actually quite a lot, thanks to all the available software on the net. If you want to re-live the early days of the microcomputer revolution, there is plenty of RAM on either CPU board to load up Tiny BASICs, play games, write assembly language code, etc.

Q: How about mass storage?
A: We use our SD Card System. Our CPU boards include the low-level code providing drivers for the SD system, along with monitor commands to boot an operating system.

Q: What OS do you provide?
A: For the 6800, we use FLEX. For the 6809 we use FLEX/9 but have talked to various groups about multi-tasking operating systems. A different (still not released) I/O board is being developed to add additional capabilities for some OSes.

Q: An OS is great, but is there any software for those old systems?
A: Absolutely! Try looking at the FLEX User Group site: www.flexusergroup.com. They also have a large FTP site with a bunch of disk images. Drop those DSK files onto the SD card, mount the file as a disk drive, and you’ve got the disk contents for your use.

Q: So what do you guys do with YOUR systems?
A: I’ve got both 6800 and 6809 systems on the bench, both of which run FLEX, and are both a lot of fun to just program. I enjoy assembly language programming, so I’ve been working on some better tools for writing code.

Q: This stuff is all old technology, so shouldn’t it be cheap?
A: Sorry, this has a long answer. Some vintage parts are getting harder to find, demanding higher prices, or available in small quantities which have higher shipping costs. Some of these parts haven’t been manufactured since the early 1980s. Another big cost factor is that a lot of parts are needed, so while a more modern part might cost a few dollars more, the “more vintage” design sometimes requires several dozen of the older technology devices. Yet another factor is that the cost of printed circuit boards is based on the size of the board and the number being ordered. Ordering a single board might have a $70 set-up fee, and then $50 for the single board. Bumping up to 30 boards still has the $70 set-up fee, but the per-board cost might drop to $10. Given the size of the boards and the fact that each board has only limited functionality (as per the original design), the fixed prices are fairly high. This is a slightly political issue, but keep in mind the 25% US tax (it’s a tax paid by every American) on goods purchased from China, which is where a large percentage of electronic parts come from. Invoices from major US based suppliers show the dollar amount I had to pay in “tariffs” (it’s a tax) for that order.

Q: What else is needed, really?
A: Just a terminal and an RS-232 cable OR a personal computer with a serial port, cable, and a terminal emulator program. At shows we always use Wyse 30 terminals as they are available nicely refurbished for about $200.

Q: If I buy one, how hard is it to get running?
A: Before we ship a complete system, everything is plugged in, the system is brought up, then it is disassembled, packaged, and shipped. It’s really quite simple to plug in the boards and get running. Our suggestion is to set up just the motherboard, power supply, CPU board and serial board. Once your terminal is talking to the system, then it’s easy to plug in the parallel card, connect the SD card system, and boot into FLEX.

Q: Can I use Corsham Tech boards in a real SWTPC system or use SWTPC boards in a Corsham Tech system?
A: Absolutely! I sometimes bring several real SWTPC boards to demos, but they tend to draw a lot of current. Their 8K RAM board draws more than our complete 6809 system with a meg of RAM.

Q: Is there a case to put a system in?
A: No. It is expensive to have a case built unless we buy in very large numbers which we will probably never sell most of.

SS-50 Specification

This isn’t really any definitive definition of the SS-50 bus, as boards were simply produced that worked with other SS-50 boards. When SWTPC devised the bus, they just made things work and other manufacturers made compatible boards. Years ago I found two sites that gave a clear description of the pins for both the SS-50 and SS-50C versions, and since they haven’t been touched in decades, I decided to copy the information here just in case the original pages ever go away.

The SS-50 bus

The SS-50 was the main backplane in 6800 based SWTPC machines, and connected the CPU board with memory, disk controllers, and so on. As was common in that era, a board usually did just one thing, and did it well, so a system would have a CPU board, one or more memory boards, a serial board connecting to a terminal, and either a cassette I/O or disk controller board for program/data storage. Physically, the SS-50 bus consisted of rows of male Molex connectors, spaced 0.156 inches apart. The boards that plugged into it had the corresponding female connectors along one edge, rather than the more modern (and cheaper) printed-circuit contacts. The SS-50 bus had all signals from the processor. For I/O, see the section below about the SS-30 bus.

All signals starting with “/” are active low.

1. /D0 – (complement) data bus line 0
2. /D1 “
3. /D2 “
4. /D3 “
5. /D4 “
6. /D5 “
7. /D6 “
8. /D7 – (complement) data bus line 7
9. A15 – address bus line 15
10. A14 “
11. A13 “
12. A12 “
13. A11 “
14. A10 “
15. A9 “
16. A8 “
17. A7 “
18. A6 “
19. A5 “
20. A4 “
21. A3 “
22. A2 “
23. A1 “
24. A0 – address bus line 0
25. GND – ground
26. GND – ground
27. GND – ground
28. +8V – power line
29. +8V – power line
30. +8V – power line
31. -12V – power line
32. +12V – power line
33. INDEX – no pin – prevents backwards insertion
34. /M.RST – (complement) manual reset
35. /NMI – (complement) non-maskable interrupt
36. /IRQ – (complement) interrupt request
37. UD – user-defined
38. UD – user-defined
39. /Phase 2 – (complement) processor clock 2
40. /VMA – (complement) Valid Memory Address
41. R/W – Read / (complement) Write
42. /RESET – (complement) Reset or power-up
43. BA – Bus Available for DMA
44. /Phase 1 – (complement) processor clock 1
45. /Halt – (complement) halts the processor
46. 110b – 110 baud clock signal
47. 150b – 150 baud clock signal
48. 300b – 300 baud clock signal
49. 600b – 600 baud clock signal
50. 1200b – 1200 baud clock signal

The SS-30 Bus

I/O was handled on a distinct 30-pin bus (the SS-30), which was generally similar to the SS-50 but had a “board select” signal instead of the address bus. The logic to select individual I/O boards in the SS-30 was hardwired to memory-map them into four-byte slots starting with board 0 at address $8000. Very few address lines from the SS-50 bus were present, usually only A0 and A1 (called RS0 and RS1) but the two UD (user defined) pins could have A2 and A3 connected to them.

1. UD – user-defined
2. UD – user-defined
3. -12V – power line
4. +12V – power line
5. GND – ground
6. GND – ground
7. INDEX – no pin – prevents backwards insertion
8. /NMI – (complement) non-maskable interrupt
9. /IRQ – (complement) interrupt request
10. RS0 – register select – like A0
11. RS1 – register select – like A1
12. D0 – data bus line 0
13. D1 “
14. D2 “
15. D3 “
16. D4 “
17. D5 “
18. D6 “
19. D7 – data bus line 7
20. /Phase 2 – (complement) processor clock 2
21. R/W – Read / (complement) Write
22. +8V – power line
23. +8V – power line
24. 1200 baud clock signal x 16
25. 600 baud clock signal x 16
26. 300 baud clock signal x 16
27. 150 baud clock signal x 16
28. 110 baud clock signal x 16
29. /RESET – (complement) Reset or power-up
30. /Board Select

When we began building the first of our SS-50 series of cards, the most common request was for a mass storage option. Those old disk controllers are hard to find on the surplus market and the old disk drives are getting harder to maintain. While we like vintage equipment, sometimes it’s worth using some modern technology to do updates.

Enter our SD Card System.
This is a two board system that provides a very inexpensive, flexible, and long-lasting storage option for our KIM Clone , SS-50 based designs, or almost any third party system.

The main board is our SD Shield. It plugs into an Arduino Mega and provides for insertion of a micro SD card, and also includes a DS3231 based real time clock (RTC). The Arduino does the hard work of providing an interface between the host processor (6502, 6800, 6809, etc) and the SD card and RTC. It connects to the host via a ribbon cable.

The board also contains three LEDs used for status, a RESET switch, a four position DIP switch, a DS3231 RTC, backup battery, and it maintains the Arduino Mega headers so additional shields can be placed on top. Since the source code is available you can add additional drivers for whatever shields you add. Want to add an LCD shield that displays which files are mounted? Just plug in the shield and modify the user interface class in the source code. Only one of the DIP switches has a defined purpose, so the rest are free for you to add your own logic.

The protocol between the host and the SD Card System is completely defined in The Remote Disk Protocol Guide.

In order to use this card on an SS-50 based system, you will also need a parallel card such as ours.

	SD Card System 2015
	SD Card System 2017
	SD Card System 2021
	The Remote Disk Protocol Guide August 2015 V0.1
	The Remote Disk Protocol Guide April 2017 V1.1

Downloads
Drivers for 6502, 6800 and 6089 Flex, Arduino Mega Firmware, SD Images Flex, older versions also
Firmware 2021
Test program for SD Shield
Clock utilities set time and date from KIM-1
Reproduction and uptodate SD Shield by Eduardo Casino (Github copy)

The schematic above is the only version posted. It is not the latest schematic used by Bob (see below). Eduardo Casino (see the download above) reverse engineered the SD Shield and produced an uptodate version 9.

April 5, 2021 New Version of SD Card Shield

This is another board done entirely with KiCad and rolls in a few changes from the previous boards. It is completely compatible with the rev 8 boards, which were compatible with rev 7, which were compatible with… etc, etc, etc.There is no new revision of Arduino code for this board.

No new features but a few changes:

• The CR1220 battery has been replaced with a much more common CR2032 to keep the RTC (Real Time Clock) chip alive.  I did extension market research (ie, I emailed all my nieces and nephews) and found they all had 2032 batteries at home, while none of them had 1220s.My family is used to getting odd emails from Uncle Bob.  The local hardware store was surprised  when I asked about CR1220s, so going to a more common battery makes sense.
• A different micro SD socket. The old one was push-in, pull-out, but this one is pop-in, pop-out.  It also has alignment pins on the bottom making it much easier to get seated exactly in the right spot before soldering. The solder connections are at the back of the socket, not in the front like the old board.  So far every surface mount solder job with the new socket has been perfect the first time, whereas the old sockets needing to be touched-up about 15% of the time.
• The old 5-pin header for an external RTC board has been removed, allowing the LEDs to be aligned in a row.  With the RTC on-board there was little reason to keep the header.
• Added a clearly labeled 4-pin area for adding external I2C devices.
• For those buying the bare board, the LED colors are labeled on the silk screen. BTW, the “bare board” has the DS3231 RTC and the microSD socket already soldered.

The rev 9k1A will eventually be changed to just rev 9, as there is no need to keep KiCad revisions uniquely identified.

March 21, 2018 What’s On the Bench

Sometimes it’s nice to just enhance an existing product, especially when the supply of blank boards starts running low and more need to be ordered.  I’m running low on the SD shields so it was a good time to make some improvements. This is a prototype:

The changes from the Rev 4 boards:

• A DS3231 RTC and backup battery are now on-board instead of having the plug-in RTC.; Too many of those plug-in boards had batteries that died within a year or two, so I decided to just put the chip directly on the board and have a battery connector.  It takes a very common LR44 battery which can be replaced easily when it fails.
• DIP switches are clearly labeled as to which is which, and which direction is on and off.
• A pull-down resistor is temporarily added here to fix a problem where the green and red LEDs don’t work right until the host computer initializes the interface to the SD shield. A pin was left floating and confused the software into thinking the host was sending a byte when it wasn’t.
• Two I2C pull-up resistors were added but do not seem to be necessary.

There will eventually be a Rev 6 with the new pull-down resistor and maybe the two pull-ups removed, but I’ll use the current Rev 5 boards and manually add the pull-down for now.

The RTC is completely software compatible with the current boards so any version of our Arduino code will work with it.