VAXstation TURBOchannel adapter: one more thing

[Roger Ivie <rivie%ridgenet.net@localhost>]

I probably should have mentioned: the VAXstation 4000/60 has a bug that
can cause the bug to hang under certain conditions that involve
simultaneous DMA by a TURBOchannel device and the Ethernet interface
(an FPGA's write-buffer gets full; the addition of an extra DMA device
on the CVAX pin-bus was not considered during design of the FPGA).

The TURBOchannel adapter can work around this by monitoring the DMA
request from the Ethernet controller; it doesn't request DMA when it
sees that the Ethernet controller is requesting DMA.

However, this required addition of a greenwire to the 4000/60
motherboard.

I don't know the logistics of how DEC dealt with this. I don't know if
they installed the greenwire on all machines under service, only on
those that were being used with a DMA-capable TURBOchannel device, or if
they re-spun the motherboard to put the trace in the wiring.

I also don't recall the specifics of the wire; which pin on the Ethernet
controller goes to which pin on the connector the TURBOchannel adapter
plugs into.

In short, if you want to use a DMA-capable device on a /60, you're on
your own.

(that was interesting week; I found bugs in three different DEC chips
that week. FYI, SGEC checks parity on read data that is not selected
(i.e., for which the byte mask is not on), even though the manual
specifically says that it does not.)
--
roger ivie
rivie%ridgenet.net@localhost