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A running PDP-8 Straight 8 in Australia

1944GPW

1944GPW

Veteran Member
Joined
Apr 26, 2011
Messages
810
Location
Brisbane, Australia
A few months ago I managed to get to see what is claimed as Australia's oldest running computer, a PDP-8 Straight 8. The machine was used by the Australian Department of Defence in Adelaide from 1967. After a long and detailed recent restoration it is now complete and runs FOCAL and chess, shown here. Needless to say it is a real beauty! The current loop interface had to be adjusted down I believe from 60mA to 20mA to work with the VT220. Apart from the VT220 it has a PC05 portable reader and a ASR33 teletype attached. The owner would like some ideas on an easy way to transfer code from a laptop to this machine - presumably a USB to 20mA interface, if such a thing exists?

PDP8Straight8_1.JPGPDP8Straight8_2.JPGPDP8Straight8_3.JPGPDP8Straight8_4.jpg

Steve.
 
Very pretty! Is the restoration documented anywhere?

As you're probably aware, there are any number of current loop-to-serial adapters and likewise many serial-to-usb adapters (FTDI among the best). I'm not aware of a direct current loop-to-usb adapter but there probably is one somewhere. Malcolm Macleod - also Australian and the owner of a straight-8 - will probably comment as well. He's done a loop-to-serial adapter; maybe he'll do a usb version as well.

Jack
 
G'day Jack,
Thanks for your comments. I know Malcolm through this board and spoke to him on the phone some time back about 11/05's. He mentioned an 8 to be restored, but I had no idea it was a Straight 8. I had asked the owner of this machine whether he and Malcolm knew eachother but I don't think that was the case(?). Malcolm was working on a RS232 to 20mA interface - I saw the circuit and was interested for my 05s, not sure what the state of progress is. I'm sure he'll chime in :)
There is a write-up on this machine's restoration as a Word document, but I'm reticent to release it without permission, and the owner I think may not frequent this Board. I believe a lot of diodes were bad and had to be replaced (hence the label tags seen attached to the boards), are they germanium in the Straight 8? Also the front panel lights I recall, and the power supply caps reformed.

Steve.
 
Maybe a paper tape punch that can print from a modern system would be more in order? I have been working on using paper tape to transition form modern systems to my PDP-11 systems, although my work has been progressing at a snail’s pace. Can at least write and load simple stuff with paper, and once you get something on paper tape makes it easy to load again in the future.
 
60mA current loop - now that takes me back...

As you have already converted the 60mA loop to 20mA - then you are now in the correct range for off-the-shelf solutions (unless you specifically want to build something yourself to keep the costs down).

You have the option of purchasing a 20mA to RS232 converter box and then using a USB/RS232 serial port interface cable. Alternatively, there are straight USB to 20mA current loop converters available (see http://www.wut.de/e-8wwww-10-inus-000.php).

If you want to construct something yourself - obtain a copy of the schematic diagrams for the Cromemco TUART serial card (available from http://www.s100computers.com/Hardware Folder/Cromemco/TU-ART/TU-ART.htm) and look at the serial I/O circuitry for the RS232 and TTY (20 mA) inputs and outputs.

Don't forget that 20mA comes in active and passive modes (i.e. which end of the link supplies the current). If you interconnect two passive links together it tends not to work very well due to the lack of current!

Another (simpler) option may be to make a light-sensitive interface for the 20mA serial link on the straight-8 (a photodiode/photocell) and place that in front of the PC's screen. You can then write software on the PC to turn an area of the screen alternatively white and black simulating the 1's and 0's of the serial link (including the start bit, data bits, parity bit and stop bit(s) as appropriate. The software for the straight-8 can then be assembled on a PC and transferred to the real straight-8 when required.

I am looking into this method myself for booting old and ancient computers of different types.

Dave
 
Dave and Ray, thanks for your comments, I'll pass them on to the owner. Ray, are you saying you read and punch tapes from a PC? Sounds interesting, I know for one I'd like to know more about how you're achieving this.
Dave - thank you very much for that link, indeed there are USB-20mA interfaces, I think that may be just the thing. And the photocell idea is very neat, sort of like tapping in at the optoisolator between the current loop and TTL sides of the circuit.
Thanks again,

Steve.
 
What I have is a keyboard/printer and paper tape reader and punch 'all-in-one' with an RS232 serial interface that I have connected to my laptop via a USB/RS232 interface. I now have a wide range of options open to me:

1) I can download programs from the internet as files on the PC and punch them onto tape as required.
2) I can read pre-existing tapes from the reader and store them as files on the PC.
3) I can develop software on the PC (using emulators or PC cross assembler tools) and transfer the resulting image files to tape.

My punch is a bit more 'hardy' than normal - so I use mylar tape rather than paper tape - as it will last longer and is less susceptible to damage than paper tape.

I am not sure from the photograph exactly what peripherals are on the ASR33 - but (if it has a reader and punch) the owner should be able to do the same thing as me and interconnect his ASR33 to his PC (obviously going via a current loop converter box in his case).

Dave
 
Paper tape is slow but with a serial port on modern systems and the DEC system it’s a great way to move small programs. Cant do a operating system or something where the system need to recall and read from the drive, at least with what I am playing around with but you can store, edit on the modern systems by just keeping everything as a .txt file and push to the DEC system. Thought I saw a paper tape reader with the eight? Lot like what you would have done back in the day with your ASR-33
Biggest hassle today appears to be finding blank paper tape. Beyond that there are still plenty of old paper tape punches and readers with serial ports from the old CNC days so your modern system can be connected to that and you transfer the tape from that to the DEC.
 
In the UK try:

http://www.gnt.co.uk/papertape.htm.

If you are editing files on a PC - beware that the PC will use ASCII whereas the machine that is going to use the data file may not. I have come a cropper once or twice where a system looks to be using ASCII but there is the odd difference (just to keep everyone on their toes)! I think the straight-8 many have been one of these machines - although the details allude me at the moment.

Dave
 
The parity bit is always set on paper tapes for all PDP-8 machines (not just straight-8), so you need to add 200 octal to the "expected" value. "A" (decimal 65) is 101 in octal, 301 in "DEC ASCII".
 
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