Interest in TTY interfacing or PDP-8 W076 and/or M452 replacements/enhancement cards?

From: Andreas Holz <asholz_at_topinform.com>
Date: Tue Nov 11 03:55:51 2003

Hi Vince,

a possibilty would be to publish the pcb-layouts and the part-lists.

Andreas

vrs wrote:

>Hi,
>
>I have been fiddling around with the terminal interfaces of my PDP-8/i and
>PDP-8/L (they are essentially the same). For those unfamiliar with those
>machines, they come with 110 baud 20ma current-loop interfaces designed for
>the ASR-33 teletype (with a "reader-run" modification to turn the paper tape
>reader on and off). What I have done is to design and build three small
>cards:
>
>The first of these replaces the W076, which is the 20ma interface card, with
>a version that generates 20ma signals, and also generates RS-232 signals.
>The reader-run feature is mapped into RTS flow control. The connector is a
>40 pin header with the same pinout as the M8650/M8655 cards from the later
>PDP-8/a/e/f/m models. This means you can take a standard BC01 or BC05 cable
>and use it to interface to an RS-232 device or a current loop device,
>whichever your cable was designed for.
>
>The second card replaces the M452, which is the baud rate generator card.
>This new version is loosely based on the baud rate generator of the M8655,
>and generates a multiple of either 110 or 150 baud, up to 19200 baud. The
>selection is made by grounding various MUX inputs with a (socketed) DIP
>switch. You can also replace the DIP switches with a header, or even with a
>single toggle switch to select high/low, where "low" can easily be the
>standard 110 baud.
>
>The third card is designed to go in an adapter box (needs just a 5V supply),
>and converts the TTY's 20ma current loop to an RS-232 DCE interface,
>including the mapping of RTS flow control (from the DTE) into the reader-run
>signal. There is nothing particularly PDP-8 related about this card, and it
>could be used to interface any 20ma TTY to pretty much any computer that
>speaks RS-232. (Just ignore the RTS stuff if your computer doesn't do flow
>control.) The interface is DCE so you can just stick your BC01-style DTE
>cable into it and skip the null modem.
>
>The current status: The first board's RS-232 interface has been checked out.
>I need to build a cable to check the 20ma side. (I ordered one but it never
>came.) The oscillator board has been bench-checked (haven't opened my 8L
>for it yet), and oscillates at what appear (on my scope) to be the right
>frequencies. The stand-alone TTY conversion is still being built (waiting
>on parts).
>
>My vision here is to convert everything to RS-232, then use a pair of RS-232
>switch boxes: one to select the CPU and another to select the TTY. I also
>plan to bring out high/low switches from the 8/i and 8/L to a homebrew
>panel, so I can use a decent baud rate when connected to the PC for
>downloading, but switch over to the TTY when I want a more authentic
>experience.
>
>I should also mention that I have made no effort to be compatible with even
>earlier PDP-8 models, which might have been ECO'd to take a W076 rev. D
>card. I don't have enough info about those machines to know what I'm doing
>there (and would have no way to debug there).
>
>Here are my questions for the group: What level of interest is there in
>these cards? Are there a number of people who need a W076 equivalent? How
>about the M452 oscillator replacement? Is there an audience for the TTY
>conversion?
>
>The reason I am asking is that I would like to make them available, but PCB
>manufacturing doesn't get cheap until you are into dozens of units. So, if
>there are a lot of interested people, it becomes reasonable to have a PCB
>shop crank out 25 or more. If no-one is interested, then 'one-off'ing them
>in my darkroom/shop (time consuming and error-prone as it is) is far less
>expensive.
>
>One thing that will *not* work, is for me to make them in my basement for
>others. It takes all me day to make and stuff one card, and then a good bit
>of the next to debug all the bridges and opens that result from my sloppy
>fab work :-).
>
>So, is there enough interest that I should consider having boards made?
>
>The newsgroup alt.sys.pdp8 appears to be comatose/dead. Should I ask there
>anyway, or is everyone there also here?
>
>Thanks,
>
> Vince
>
>
>
>
Received on Tue Nov 11 2003 - 03:55:51 GMT