MicroVAX 3300/3400

Glen Slick glen.slick at gmail.com
Tue May 25 17:50:32 CDT 2021

On Tue, May 25, 2021 at 1:32 PM Zane Healy <healyzh at avanthar.com> wrote:
> It turns out that the oddness was the CPU Board, and a 16MB Clearpoint RAM board.   What I thought was the CPU, is really the header panel over the RAM and TK70 controller.  There is also a M9715 between the CPU and the powersupply that I’d missed seeing.  The cables to get to the DSSI connector by the Powersupply are hanging out the front.  Basically it’s a cable going from the KA640 to a ribbon cable to a cab kit, then DSSI cable from that to the connector next to the power supply.

Sounds like somewhat of a mongrel system. An M7624 KA640 CPU would
typically be found in a 12-slot BA213 as a MicroVAX 3400, or in a
smaller 6-slot BA215 as a MicroVAX 3300.

As far as I can tell a 12-slot B400X is essentially exactly the same
thing as a BA430, where it is a BA430 when used as a VAX 4000-200 with
an M7626 KA660 CPU, and a B400X when used as a Q-bus and storage

An interesting thing about the BA430 backplane is that in addition to
the M7626 KA660 CPU DSSI bus being present on the 50-pin connector, it
is also present on the C/D connectors, and the BA430 backplane routes
the DSSI bus on slot 1 C/D connectors to the DSSI storage device bays
without the need for a DSSI cable.

If I remember correctly the DSSI bus is only present on the 50-pin
connector of the M7624 KA640 CPU, not on the C/D connectors, so a DSSI
cable is needed with that CPU.

The hidden M9715 board in slot 0 between the CPU slot 1 and the power
supply provides termination power to the internal DSSI and SCSI buses
of the BA430 / B400X backplane. It also has a connector which routes
to the DSSI bus on the C/D connectors. In the BA430 nothing connects
to the M9715 connector as the DSSI bus should be coming from the M7626
KA660 CPU C/D connectors. In the B400X there should be an external
bulkhead DSSI connector with a ribbon cable to the M9715 connector. In
both the BA430 / B400X there should be another external DSSI connector
to the left of slot 12, which is the other end of the DSSI bus.

So it sounds like you have a B400X with the M9715 internally cabled to
the external bulkhead DSSI connector as usual, and the M7624 KA640 CPU
internally cabled to an external bulkhead DSSI connector, and then an
external DSSI cable between those two bulkhead connectors. If that is
basically how the DSSI things were cabled up, did you also have a DSSI
terminator on the bulkhead connector to the left of slot 12? Without
checking this myself with a M7624 KA640 CPU you probably get some sort
of errors somewhere if the end of the DSSI bus is not terminated.

If you were to find an M7626 KA660 CPU you could do away with all of
the DSSI cabling between the CPU and the M9715 (in addition to having
a CPU that is around twice as fast).

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