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 includes some new features:

1. 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.
2. 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.
3. A jumper option lets you choose whether the CP-A powers up running or stopped. The default is stopped.
4. Use of inverting buffers to drive the address bus, status byte, and control signal LEDs to reduce bus loading.

Other important changes include:

1. Moving the voltage regulators and heat sinks to the top of the board helps to keep heat away from the components and vent into the cabinet.
2. Moving the flat cable which connects the MPU 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.
3. 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 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.

The CP-A Rev-5 is currently in development. As soon as a release date is determined, it will be posted here. We were not satisfied with the first engineering samples and decided to delay the product release and make more radical changes. These changes will result in a front panel that will work in more systems without the usual headaches. Rev-5 will be more than “just another revision.”

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

Description	Part Number	Availability	Unit Price (USD)	Quantity	Ordering
CP-A Rev-5 data bus cable, 6-in (150-mm)	CABLE W	In stock	$8.00	1
CP-A Rev-4 data bus cable, 18-in (450-mm)	CABLE K	In stock	$10.00	1
Switch bracket and screws for any IMSAI CP-A	CP-A BKT	In stock	$57.00	1
Mounting hardware for CP-A PNL or CP-A-R	CP-A HW	In stock	$20.00	1
LED and spacer kit for any IMSAI CP-A	CP-A LED	10 day lead time	$24.00	1
Switch set for any IMSAI CP-A	CP-A SW	In stock	$129.00	1

More Information

Some of the more technical Rev-4 bug fixes and changes are:

1. Because the address bus, status byte, and control signal LEDs are now driven by the CP-A’s power supply instead of the bus, a second 7805 voltage regulator was added.
2. The STOP operation is guaranteed to stop on an M1 machine cycle regardless of the processor board.
3. The PROGRAMMED OUTPUT port address is now decoded from the low 8 address lines (A7 through A0).
4. Redesigned the processor interface logic which disable the data in drivers. This can mitigate the problem where bus pin 53 is driven to 0V in an IEEE 696 system. This is one of several changes made to make the CP-A more robust in IEEE 696 systems.
5. Redesigned the PROGRAMMED OUTPUT circuit, so it no longer requires an 8212 IC.
6. The PROGRAMMED OUTPUT latch is connected to the data out bus, reducing loading of the processor bus.
7. Implemented the CP-A modification suggested by Josh Bensadon to fix a glitch in the EXAMINE state machine which was previously mitigated by a capacitor filter.
8. Fixed a bug in the Rev-4 where EXAMINE after a SINGLE STEP can produce unpredictable results.

While some of the circuitry is unchanged from the Rev-4 design, the Rev-5 design is significantly different in many respects. This has resulted in a complete rewrite of the theory of operations section in the user manual.