Reproduction Corsham 6800 for SS50 Systems


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Improved Corsham 6802 Processor Board

 

For comparison this is an actual Corsham 6800 Processor Board.

Bare board of reproduction Corsham 6802 Processor Board

 

Summary

This is an enhanced reproduction of the Corsham 6800 processor board.  Its design was based on the Corsham 6800 REV 3A board.  Improvements include use of a 6802 processor.  The 6802 does not require the hard to find MC6875 clock generator but is otherwise a replacement for the 6800.  At the current time it is easier to source 6802 chips. The board was upgraded to run at 2MHZ.  It has a slow clock circuit that can be selected by jumper to slow the clock speed to 1MHZ for off board access.  This would allow for slower SS30/SS50 boards to be used.  If building a completely new system there would be no need to slow the off board speed down since boards are available for 2 MHZ operation.  The Corsham SD card system, serial IO boards, parallel IO boards all operate at 2MHZ.  If used in a SWTPC system the Peripheral Technology FD-2A floppy controller and SS30-IDE board  function at 2 MHZ.  Some 6800 programs may not be compatible at 2 MHZ since it was common to use software timing loops to slow things down. For these programs you can set the clock speed of the board at 1MHZ until you can modify the programs to run at 2MHZ.  Other improvements eliminated the MC14411 baud rate generator which is becoming harder to find.  The only downside to the elimination of the MC14411 was the loss of 110 baud.  Baud rates from 300 to 38400 may be selected.  The onboard RAM has been retained but may be totally disabled should the need arise.  The EPROM on the Corsham consisted of 8K at E000-FFFF.  This board retains that but allows for the use of a 2764, 27128 or 27256.   Only 8K of the larger EPROMs is mapped to the 6802 at a time.  By using jumpers different portions of the larger EPROMs may be mapped to the 6802 allowing for different operating systems to be supported by changing a jumper to select different 8K sections of the EPROM.    This board can be an excellent processor board to use in either a Corsham style system or to replace a SWTPC MP-A or MP-A2 processor board.

Changes and Improvements

MC68B02 vs MC6800: The 6802 processor is code compatible with the 6800.  The 6802 has an internal oscillator circuit allowing a crystal to be used.  The 6800 required a hard to obtain and expensive MC6875 clock generator chip.

Runs at 2 MHZ. Can be switched to 1MHZ by a jumper.  When running at 2MHZ, off board access can be at either 1MHZ or 2MHZ.  This allows the onboard access of the EPROM/RAM to occur at 2MHZ while allowing slow IO boards to be used.

Removed the MC14411 baud rate generator and replaced it with easy to find parts.  Baud rates of 150, 300, 600, 1200, 2400, 4800, 9600, 19200, 38300 are generated by the new circuit.  The only common baud rate that is missing is 110.  110 is typically only used when using a Teletype (Typically a Teletype 33) for a terminal.  If you should need 110 baud use a SWTPC MP-A or MP-A2 processor board.

The onboard RAM can be totally disabled.  This could be useful when using a mix of older SS50 boards where some of them access part of the RAM address space. An example would be a graphics/video board.

Uses a ground pour to fill in almost all empty areas of the board with ground.

 

2 MHZ Operation

These notes assume that you are running the internal processor board at 2MHZ and the off board memory/IO at 2MHZ. 

 There were no 2 MHZ 6800 based SS50 systems back in the 70's and early 80's.  In fact memory at the time didn't support a 2 MHZ processor speed. So there are some considerations if you want a 2.0 MHZ system.

1- Memory boards.  Since the C6802 processor board has memory no memory boards are needed.  Should you disable the onboard memory,  any memory board you use will need 200ns or faster RAM to work reliably. 

2 -  If you use a floppy controller you will need an FD2A floppy controller.  The FD2 is marginal. The FD2A uses a 74F245 for the data bus interface while the FD-2 uses a DP8304.  No "F" speed of the DP8304 was ever made.  An FD-2 floppy controller may or may not work. Oscilloscope analysis shows there is no safety margin; and it is likely as the board gets hot, it could have read/write errors. The WD2797 FDC chip on the FD-2/A has a bus cycle of 200ns.  Floppy controller boards using an 1771 are not fast enough.  The 1771's  bus cycle time is 500ns which is double the available time. Even if you use the 1MHZ bus speed, original 1970's drivers used software timing loops and would not work until the driver is patched.

3 - Your serial interface card must use an MC68B50; boards using an MC6850 or MC68A50 may not work.  The slower chips may run for a while but could eventually lock up.

4 - Motherboards.  The Corsham MB is fine. The only SWTPC motherboard that has been tested to run at 2MHZ is the PT-MP-B2R motherboard. I do not have an SWTPC 6800 mother board to test with.  While it is possible the SWTPC 6800 motherboards would work, it is not certain.  The PT motherboard uses circuitry that is different from either of SWTPC's 6800 motherboards.

5- You have to ensure that any other boards you want to run will work at 2 MHZ.

6 - The Corsham SD shield, serial board, parallel board do work at 2MHZ.

 

Key Features

Downloads

Corsham 6802 User Manual.pdf 

SWTBUG-master.zip  SWTBUG boots FLEX from  Corsham SD Shield

SWTBUG-PT  SWTBUG boots FLEX from PT-IDE or FD2A

The .bin image of both SWTBUGs is 8K in size and can be copied to a 2764.

 

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Price List Hardware

C-6802-Bare Corsham 6802 Bare board $20
C-6802-ASM Corsham 6802 Board assembled with RAM and EPROM (2764) $99

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