PDP-11/10 repair started

tony duell ard at p850ug1.demon.co.uk
Tue Oct 6 11:35:17 CDT 2015

>     > Yes, it is a pity that the later board set (a) has the jumper to
>     > disable the built-in console port and (b) has the switchable divider
>     > allowing higher baud rates so you generally don't need to :-)
> Well, except for those of us who don't have any 20mA gear, and want to
> standardize on EIA... :-)

Converting between genuine 20mA loop and RS232 is not that hard. Converting between
a DEC current loop port (where you can make assumptions about the voltage swing and 
which side will be driven, etc), is even easier. Or if you don't mind modifying the hardware
you can pick up TTL level signals at the UART.

I can't rememebr if DEC routed those to the BERG pins on the 11/10. Maybe not. From a 
quick glance at the GT40 prints (older version CPU without the disabling link) it appears 
that the needed signals are at least brought out to backplane pins, but I have not checked
any further

>    > Adjusting the RC clock is not hard given a frequency counter, and it
>     > doesn't have to be that precise.
> The prints actually give the time for the pulse width on the two different
> speed groups, so a well-calibrated 'scope should do it.

Indeed. It is not that difficult. Heck, the first serial interface I ever made (to drive
a 110 baud 'Teletype' [1] used a 555 as a clock. I think I set it by tweaking it one way
until I got bit errors, then tweaking it the other way until I got bit errors, then setting it
midway between, Still works...

[1] A Data Dynamics 390, which uses ASR33 mechanics with Data Dynamics electronics 
(in place of the Teletype Call Control Unit). I remember a single-step facility for the reader
and a few more goodies like that.

>     > The grants are the only (I think) unibus signals to be active high.
> Yup. A source of great confusion to me when I started working with the QBUS,
> where they are asserted _low_!

Now that I had forgotten. How evil....

> Interestingly, I think the M9302 was a _later_ solution to this problem in
> the 11/34. In the early ones, there's a 'SACK Timeout Module' (M8264) which I
> think performs the same function, but at the _start_ of the bus. (I say
> 'think' because this module is poorly documented - e.g. I don't know of
> anything which definitely states which slot to plug it into.)

I have one of those somewhere.... I remember a 4 bit counter and LEDs, possibly to
count errant grants, but it's been a long time since I looked at those prints too. I 
always assumed it went in the last SPC slot, but I am not sure.


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