10 Nov
2003
10 Nov
'03
2:22 p.m.
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