Looking for MAINDEC-08-D3RA-D TC01/TC08 DECTREX1 Random Exerciser Documentation

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Bernhard Baehr

Aug 28, 2020, 11:17:52 AM8/28/20
I'm implementing a TC08 DECtape device for the PDP-8/E Simulator (https://www.bernhard-baehr.de/pdp8e/pdp8e.html).
Normal operation already works, now it must pass the DECtape MAINDECS.

Does anybody have the MAINDEC-08-D3RA-D documentation and can provide a scan of it? I haven't found it anywhere.
The binary is available at various places and currently produces an error that I want to diagnose.

Here is a preview screenshot of the TC08 stopped with that error: https://www.bernhard-baehr.de/pdp8e/resources/pdp8e_simulator_tc08_preview.jpg

Thanks in advance!


Aug 28, 2020, 7:29:13 PM8/28/20
Anything helpful in here?


Otherwise, I am still looking. I remember finding a site with a ton of MAINDEC documentation, I am trying to re-track it down.

William Cattey

Aug 29, 2020, 12:27:26 AM8/29/20
Vince Slyngstad has a site where he's inventoried PDP-8 Maindecs:

But the only TC08 Maindec there is maindec-x8-ditca.
The writeup there is a bit difficult to read but may give some insight:

I'm guessing that ditca is a basic exerciser, and that d3ra is either a missed-OCR, or a more advanced random exerciser that came along later.

You can get an image of the bin format paper tape at Vince's site, and also at John Wilson and Lee Nichols's site:

See: ftp://ftp.dbit.com/pub/pdp8/paper/

Hope this helps,


Volker Herrmann

Aug 29, 2020, 2:39:18 PM8/29/20
I found an old print of Maindec-08-D3RA-D from 1967. Are you satisfied with the links, or shell I send you a scan? If so do you also need the listing?

Bernhard Baehr

Aug 30, 2020, 6:07:52 AM8/30/20
Hi all, thanks for the links. But the sites only have binary MAINDEC-08-D3RA-PB, not the documentation MAINDEC-08-D3RA-D of the random exerciser.

The TC08 Maintenance Manual DEC-08-H3DA-D is the main source of information for my TC08 implementation.

The TC08 runs all the TC01 MAINDECs, although the hardware differs in various aspects.

The problem is narrowed down a little bit by fixing a funny bug: two DTXA STOP MOTION FORWARD / READ ALL CONTINUOUS executed immediately after another caused the TC08 to continue processing DECtape lines while the tape motion was already stopped, causing a compare buffer in memory to be overwritten. Now the error printout makes sense: there is a offset between the words read from tape and the expected values, READ ALL starts reading the tape too early.

Volker, so the documentation would still be very valuable, preferably including the listing. And thanks for your hilarious 50th PDP-8/E anniversary video. And greetings from Hannover to Berlin... :-)



Aug 30, 2020, 5:43:31 PM8/30/20

I looked for the source of the maindec documentation I found before, unfortunately I wasn't able to find it. I thought I had saved a reference to it but I can't find it. I will keep trying, tho. It seemed like a great source.

One idea, and this is probably a stretch, but you might get some value out of this:


It is a write up on MAINDEC-15-D3RA-D. Yes, it is for the PDP-15, not the PDP-8. However I noticed that MAINDEC-xx-D3RA seemed to always be consistent in terms of being a DECTape exerciser diagnostic. The source code listing begins on page 18 and it looks remarkably PAL-8-ish. The PDP-15 was an 18 bit machine but apparently the basic instruction format was pretty similar to the -8.

In addition, you can find what reports to be a binary disassembly of maindec-08-d3ra here:


Of course it's not as useful as the original source code but this, in conjunction with the PDP-15 version, might give you enough to figure it all out.

Best of luck,

-- steve

Vincent Slyngstad

Sep 4, 2020, 9:05:19 PM9/4/20
On 8/29/2020 11:39 AM, Volker Herrmann wrote:
> I found an old print of Maindec-08-D3RA-D from 1967. Are you satisfied with the links, or shell I send you a scan? If so do you also need the listing?

I have obtained the document from Volker and placed a version on my
wedsite at:

It should be noted that, though it works fine on the TC08, it was
originally written as a diagnostic for the TC01, so that's how it is
described there. (You'd look for maindec-08-d3ra-d.pdf or "TC01 Random

(The OCR output from "ocrmypdf" is pretty lame.)


Bernhard Baehr

Sep 8, 2020, 12:05:21 PM9/8/20
Volker, thanks again for the scan, and Vincent, thanks for the OCR.

At the moment, the PDP-8/E Simulator runs the DECtape Basic Exerciser, the Extended Memory Exerciser, and the Random Exerciser (MAINDEC-08-D3RA) with the help of documentation. The problem was READ ALL that starts to read tape words at the beginning of the next blocks data area. But when READ ALL starts between two blocks, the TC08 immediately begins to increment the word count and the memory address (memory locations 07754 and 07755). This case is checked by the Random Exerciser.

With the DECtape Formatter, there are currently timing problems because the PDP-8 must very precisely execute DTXA between two DECtape data lines (within approx. 31 microseconds). The PDP-8 and the TC08/TU55 are separate threads in the simulator that must synchronise.

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