PDP-11/23 and H9276-A

[gslick]

It took some looking around to find the DSSI bus pinout documented anywhere. I finally found one place that it is documented in the KA640 CPU Module Technical Manual. Is there a DSSI specification available anywhere?

EK-KA640-TM-001_Tech_Sep88.pdf
Table 2-3 DSSI Connector (J2-Upper) Pinouts
(The table starts at Pin 51 instead of Pin 1 because it is a dual stack of 50-pin connectors)

On the first 11 positions GND is on the even pins. On the middle 3 positions Termination Power is on both the even and odd pins. On the last 11 positions GND flips to the odd pins.

With a 12-29258-01 DSSI terminator attached to DSSI bulkhead connector I measured around 95 Ohms from each of the signal pins (even those listed as Unused in the table) to GND, and around 87 Ohms from each signal pin to Termination Power. Those values don't seem to agree with the data I found for the SIP terminator arrays on one of my M7769 KFQSA boards. I should pull one of them off and measure it, and cross check against one of my other M7769 KFQSA boards.

 

[PG31]

I've been following this thread, very interesting. If I read correctly because KFQSA is MSCP I can use it in a PDP-11 for RT11, RSX11 etc?

 

[gslick]

I looked at this further and realized that the 12-29258-01 DSSI terminator resistance values that I measured between the signals pins, ground, and the termination supply are consistent with the use of three Bourns 4610M-104-121/281 120/280 ohm dual terminator arrays on board the M7769 KFQSA board. (10-pin SIP package, 8 dual terminators in parallel between ground and the termination supply in each package).

I was only considering the 120 or 280 ohm resistance between a signal pin and the termination supply or ground and neglected to consider the effect of that resistance being in parallel with ((23 parallel instances of (120 + 280)) in series with 280 or 120). Taking that into consideration the calculated resistance values measured at the terminator between a signal pin and the termination supply or ground should be 85.5 ohms and 92.2 ohms, and that is consistent with the values that I actually measured.

While the theory of building your own DSSI cable harness and termination network may be simple, in practice it might be more of a mechanical construction challenge.