CP-A Rev-5
The IMSAI CP-A Rev-5 is the next major release of the classic CP-A Rev-4 front panel (A.K.A. the “operator interface”).
The CP-A is designed to work with the 8080A microprocessor and compatibles (8085, Z80).
The board’s mounting, LED, and switch mounting holes were meticulously measured from a 1975 IMSAI CP-A bare board.
The classic CP-A Rev-4 is also available. Check here for more information.
This major revision to the CP-A included some design changes including:
- Use of inverting buffers to drive the address bus, status byte, and control signal LEDs to reduce bus loading.
- Because the LEDs are now driven by the CP-A’s power supply instead of the bus,
a second 7805 voltage regulator was added.
- Implemented a SLOW STEP variant of SINGLE STEP. Press the SINGLE STEP switch down and it works in the usual way.
Raise and hold the switch and it performs about 2 steps per second.
- The power switch may be configured for mains power or a low-voltage remote on/off control.
For mains power, the A and B pads for the wiring to the power supply are located close to the switch for safety.
For remote on/off, the three switch terminals are available on a header at the top of the board.
A cable is included to connect the switch to your power supply.
- A jumper option lets you choose whether the CP-A powers up running or stopped. The default is stopped.
- A jumper option lets you select the status signal used to stop the CP-A on an instruction fetch.
The default is sM1 which should work on all supported CPUs. You may jumper it to DO5 which only works on the 8080A.
- The PROGRAMMED OUTPUT port address is now decoded from the low 8 address lines (A7 through A0).
- Redesigned the PROGRAMMED OUTPUT circuit, so it no longer requires an 8212 IC.
- Implemented the CP-A modification suggested for the Ithica Audio (later called Intersystems) IA-1010 Z80 CPU board running at 4MHz.
Other changes include:
- Moving the voltage regulators and heat sinks to the top of the board helps to keep heat away from the components and out the top vent into the main chassis.
- Moving the ribbon cable which connects the CPU board to the top of the board and reducing its length from 18 inches (45.7 cm) to 6 inches (15.2 cm).
This improves the signal quality on the 8080 internal bus.
- Simplified the construction of the board using SIP resistor networks.
This change eliminates nearly 50 solder connections and reduces visual clutter around the LEDs.
- The PCB has a black silkscreen on a blue solder mask. The blue and black completely disappear behind the red LED filter.
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.
The photo mask for this CP-A revision has the "SLOW STEP" legend above the "SINGLE STEP" legend.
A notable exception to the compatibility with the original are the switches.
Although they are an excellent visual match to the original C&K paddle switches, these switch paddles are not interchangeable with C&K nor NKK.
They are interchangeable with Alcoswitch and E-Switch.
We do not suggest you mix and match different switch 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.
For those who already have a set of paddle switches they like, we offer kits without switches.
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.
When the User Manual is ready, it will be available for download.
Complete Kit
Order from the list below. The kits are complete with everything you need except solder and tools.
| Description | Part Number | Availability | Unit Price (USD) |
CP-A Rev-5 complete kit with manual | CP-A | 3/1/2025 | $399.00 |
CP-A Rev-5 kit with manual (no switches) | CP-A-NS | 3/1/2025 | $320.00 |
You may also add one or more of these options for your kit.
| Description | Part Number | Availability | Unit Price (USD) |
| Socket set for CP-A Rev-5, standard | SOC-CP-A | 3/1/2025 | $10.00 |
| Socket set for CP-A Rev-5, machined | SOCM-CP-A | 3/1/2025 | $30.00 |
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.
| Description | Part Number | Availability | Unit Price (USD) |
| CP-A Rev-5 bare board | CP-A-BB | 3/1/2025 | $45.00 |
You may also add one or more of these accessories and parts.
|