Is The M9312 Boot Module Essential?

[Fritz Mueller]

>> From: Fritz Mueller
>> I think you are thinking of the M9302, Noel: a far-side terminator card
>> with integrated SACK turnaround?

> On Feb 21, 2022, at 7:19 AM, Noel Chiappa via cctalk <cctalk at classiccmp.org> wrote:
> No; the M8264 Sack Timeout module. What's an M8264, you say? ...

Ah, quite interesting, didn’t know about that one!  The KD11-E is pretty bare boned...  Parity handling was also a quad “add on”.

> So if i) a device requests a grant, and then drops the request at _just_ the
> right time (so a grant gets sent out when there's no device waiting to grab
> it), and ii) there's a break in that grant line (maybe a missing grant
> continuity card) before it gets to the M9302, which can turn it around as a
> SACK , then ... the KD11-E CPU will hang!

One small elaboration here: grants on the UNIBUS, unlike everything else, are active high.  So I believe a broken grant chain with an M9302 in place on the far side results in the grant being pulled up at the M9302, and then continuous assertion of SACK, hanging the processor straight out the gate.

Also, in taking a look this morning at the M9302 schematic around page 70 in the commonly available 11/34 engineering drawings set (rev B, Sep 76), there appears to be a mistake!  The output of the SACK turnaround circuit is annotated here BUS SACK H, but I’m almost certain this should be BUS SACK L…  May be worth a look for other versions of the M9302 drawings to verify this.

  cheers,
     —FritzM.