CP-A Rev-4
The IMSAI CP-A Rev-4 is a faithful reproduction of the 1975 CP-A front panel (A.K.A. the “operator interface”) by IMS Associates, Inc.
The CP-A is designed to work with the 8080A microprocessor and compatibles (e.g. 8085, Z80).
The board’s mounting, LED, and switch mounting holes were meticulously measured from a 1975 IMSAI CP-A bare board.
There is a newer version of the CP-A available. Check here for more information.
The vast majority of the parts, including the button head screws and LEDs, are identical to the 1975 originals.
Mechanical parts like the switch bracket and display panel components have been carefully reverse engineered and sourced
to be indistinguishable from the 1975 originals.
The graphics on the front panel are from a precision 2400 DPI scan of an original CP-A photo mask from 1976,
and are rendered onto a modern photographic film.
A notable exception to the compatibility with the original are the switches.
Although they are an excellent visual match to the C&K paddle switches originally used, the switch paddles are not interchangeable with C&K nor NKK.
These switches are interchangeable with Alcoswitch and E-Switch.
We suggest you do not mix and match different manufacturers in the same CP-A. There are always color differences.
If different switch models are next to each other, these differences will be obvious. This is why we offer switchless kits.
If you have switches you like better – maybe they’re all black – maybe you found a stash of new old stock C&Ks – get a switchless kit.
The CP-A Rev-4 is licensed for sale by IMSAI from RetroTechReboot.
It is entirely produced and shipped by Parastream Technologies.
Note that it has been updated to include all IMSAI ECNs and ECOs, so it is no longer necessary to apply those to the board.
When the User Manual is ready, it will be available for download.
Complete Kits
Order one from the list below. The kits are complete with everything you need except solder and tools unless otherwise stated.
They even come with a professionally printed user manual. Be sure to hover over any
icons for more information.
Most builders won’t need anything else besides one of the above complete kits.
To skip over the options go to More Information to get links to useful resources.
If you're an advanced builder and want to see all the options, read on.
Options
Bare Board
You may order a bare printed circuit board for you to use your own parts.
Be advised that individual parts are not available to bare board customers unless they are explicitly in the parts list below.
IMSAI Parts
Parts
IC Datasheets
IEEE 696 Compatibility
The CP-A is designed to work in an original S-100 bus system. When the IEEE working group were hammering out the S-100 specifications in 1979, they made a decision not to support some of the very 8080 specific features of the bus in favor of more generic protocols. This decision severely impacts the way the CP-A operates, and requires several changes to make it work — or at least not do bad things to the attached processor board.
- Cut the trace going to the bus pin 20. This pin is attached to the CP-A’s internal tie-high line. When pin 20 is connected to ground (per the IEEE 696 specification), it basically disabled the CP-A because all of the tie-high logic points are now ground. The original intent was to have the CP-A hold the UNPROT (pin 20, memory unprotect) line high to keep addressed memory unprotected. In retrospect, this was never a good idea. This is probably a modification that all CP-A users should do.
- Cut the trace going to bus pin 53. The IEEE 696 specification connects pin 53 to ground. This pin is driven low by the CP-A to disconnect the processor board’s data bus from the S-100 DI bus, which allows the CP-A to jam the programmed input switches onto the data bus. If bus pin forced to ground, it will permanently connect the the switches to the data bus, rendering the processor board inoperable. The side-effect of this modification is that the programmed input port (FF hex) will always read as FF hex. No matter what you do, the Z80 cannot use the programmed input or output switches due to the 8080/8085-specific address decoding of the I/O port.
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