On 18 November 2002, I started to implement the PDQ8, a PDP8 clone. I am certainly not the first clone the processor, and hopefully won't be the last! However, it looks as though I may be the first person for a long time to try and introduce a 12 bit architecture!

The main features of my implementation are:

• Intended to be usable as professional laboratory equipment.
• Built with current components and technology
• A whole system, not just a CPU
• Separate memory and IO busses
• Syncronous RAM bus
• Async IO bus

The main components of phase 1 (PDQ8a) will be

• The CPU - probably very minimal in the first iteration.
• Programmers front Panel (LEDs and Switches) - to ease low level debug of peripherals, bus and processor.
• A PDP8x compatible bus allowing use of Dave Conroy's IOU (Peripheral subsystem)

Dave Conroy's IOU includes an IDE interface, and an RS232 interface used for a TTY. (A PC serial port and Telnet/Hyperterm is more probable than an ASR33)

The BUS is the most important feature of this project in the long term. However, the 8a will have a more limited bus - that defined by Dave Conroy for his PDP8x. The more advanced bus will be introduced later.

I want to support a bus that can be extended to at least the hight of a rack unit. I am not sure how I will do this, because Omnibus was very wide, and ties so closely to the processor that using it slows the processor to bus speed. OTOH, I may need to use Omnibus to implement many of the peripheral in a realistic manner. I would rather use a lighter weight bus, perhaps with Muxed address/data, but how then to do 3-cycle databreak? I want to demonstrate the main processor working before embarking on more compex stuff.

The system will be built with 32k words of static RAM (2 * 8 bit, with 2 bits of each chip unused). This may have battery backup, to simulate core memory. Unlike the PDP8x, I will probably not use a PROM.

Althought there will be 32k words of RAM, in the 8a, only 4k words will be accessible, because of the lack of an MMU.

I am using the Quickstart Technology QST0201 Spartan 2 board. This allows all the IO of the Spartan II to be accessed, and avoids the need for a download cable.

I have started building a "lights and switches" front panel on Veroboard. I hope to make a PCB version if possible. The system will be Eurocard based, and, while it will lie on the desk in bits.

This is a serious engineering project, and I fully expect all parts of the system to function reliably if used as industrial control equipment - well maybe not my 20 year old digidata tape drive :-(

Unlike most of the other FPGA PDP projects, I use schematic capture, and don't bother too much with emulation. I expect to have the processor executing code by 1 Q 2003.

Phase 2 (PDQ8b) will probably include

The first attempt at my bus, and ...

• Memory Management - MC8/i compatible. This will enable 15 bit address range.
• Extended Arithmetic- EAE compatible. This is probably required by most software.
• Printer port - This may not be straitforward, as, at the time of the PDP8, the predominant technology was Data Products (DP), and not Centronics. The DP interface is far more complex than Centronics, and I have not seen a printer with DP interface for over 15 years.
• 1/2" tape interface. (I have two tape drives, and have designed loads of tape interfaces over the years.)
• Flash based RF32 (Drum) Emulation The jury is out as to whether this will be based on DRAM or something less volatile.

I expect the PDQ8b to go in a 3U high standard 19" rack unit.

This system will continue to support the IOU to provide serial and HD interfaces, via an adaptor between the two busses. This machine will be capable of running most PDP8 software, including anything that runs on PDP8x.

There are several other subsystems that I plan to do later:

• Analogue interfaces
• Graphical display emulation onto VGA - this will be compatible with a DEC offering TBD, so the original software can drive it.
• RX02 using 3 1/2" floppy - This should be trivial, but very useful.
• SCSI Interface - again, this has been done before, and I want to promote compatibility.

I am prepared to sell working systems (excluding magtape). Contact me for pricing details!

If you have specific suggestions for features of the PDQ8b, then let me know.

© 2002, Andrew C Grillet, All rights Reserved