In just about 4 weeks time we come to the 50th anniversary of the PDP-8 Minicomputer. (23rd March 1965).
As a tribute to this game-changing machine, in the last week I have been thinking about the user interfaces used on minicomputers of the 1960s and how the early computer users interacted with their machines.
It was common in this era for the machine to have a large front panel consisting of lights and switches, allowing the contents of registers to be examined and simple programs to be toggled into memory using the key switches. The machine that most typifies this period of computing was the PDP-8 - first introduced in 1965, and by many, said to be the first minicomputer.
Here's how it started 50 years ago:
It got me wondering if a similar front panel consisting of LEDs switches could be created cheaply - as a nostalgic retro computing accessory shield for the Arduino.
LEDs and switches can be easily serviced using shift registers - an ideal application for them, which saves on I/O lines - as per my posting "A Little Bit Shifty" .
A good example of this is in the recreation of the KenBak 1 computer - using an Arduino as the KenBak-uino- here.
Why should I not like a minimalist computer system that is virtually named after me :)
These are becoming very popular as a means of creating custom hardware and emulation of old microcomputer/microprocessor hardware.
I had thought about implementing a PDP-8 cpu on an FPGA, and then thought whether I really wanted to get involved with PDP-8 assembly language - probably not. However it would be neat to have a front panel reminiscent of those from the mid-1960s, where you could see the value of the program counter and the various registers, lit up in lights, and blinking sequences as the program is run.
Having tinkered around with the SIMPL language on Arduino, I have added a few LEDs, and it is perfectly possible for an Arduino to mimic the LED register displays typical of the PDP-8. A few simple instructions can be used to echo the instructions stored in RAM up to the LED display.
The standard Arduino is limited to 18 I/O lines, if you assume that digital 0 and digital 1 are committed to the serial port. This allows a single 12 bit LED array, plus 6 inputs for either analogue or digital signals.
I wrote a little bit of SIMPL code to send a 12-bit number directly to 12 LEDs conected to Digital2 - Digital 13. This works well if you just want a quick binary number LED display. For larger displays and keyswitch inputs, it's generally best to resort to using shift registers. My blogpost of May 2014 describes how to write out patterns to both 8 bit and 16 bit shift registers and how to control stepper motors from SIMPL using commands to drive a shift register.
More LEDs and Switches
On the Arduino MEGA, there are 52 usable digital I/O lines plus 16 analogue inputs - that would be enough to recreate lots of blinkenlights and switches, without having to add extra ICs as LED drivers.
The first Arduino MEGA 2560 I bought cost about £45. They are now less than £12 from ebay/China, which makes the MEGA a very attractive, low cost starting point for a variety of simple computing projects.
However, it is unlikely that an AVR could satisfactorily emulate a PDP-8. The PDP-8 was a 12 bit machine, originally with 4K of RAM. The AVR cannot directly execute a program out of RAM, unless it is an interpreted language, such as SIMPL, Bitlash or Tiny BASIC.
It might be possible to program an Arduino with a PDP-8 machine code interpreter, written in C, but I suspect that the speed would be at best 1/10th that of the original PDP.
So it might be best if any efforts along these lines was done as a tribute to the computers of the 1960s, rather than a true emulation.
A penny a thought, - a pound for 32 bit thoughts.
The humble Arduino based on the 8-bit AVR ATmega328 is now showing it's age (21+). A new generation of small, very low cost 32 bit microcontrollers have arrived and are benefiting from a smaller architectural geometry and low pin count packages. If the Arduino was being designed in 2015, instead of 2005, it would use a 32 bit mcu. They are better than half the cost of the 8 bit AVR parts, and include more peripherals and better ADC resolution.