28 Jun
2001
28 Jun
'01
6:12 p.m.
--- Michael Davidson <michael_davidson(a)pacbell.net> wrote:
I am just about to embark upon designing a small qbus
interface
card, and am trying to work out what to use for bus drivers and
receivers.
The 8641 quad bus transceiver is still available, but it appears
that the 8640, 8881, 8131 and 8837 are all obsolete and out of
production.
Right. I think I need a pair of 8837s, in fact, to repair my H-11
floppy controller.
Also any suggestions for substitutes for bus drivers
and receivers?
I seem to remember the 7438 being capable of sinking enough current
to drive a 120 ohm bus line effectively, but I'm not sure.
I cannot cite chapter and verse, but in one of the designer handbooks, I
recall seeing the 7438 listed as a functional equivalent, but not a pin-
compatible equivalent, for one of DECs approved bus driver/receiver chips.
If you are trying to design a device without the proper handbook and/or
chip-designer's guide, you are probably going to have lots of hidden
gotchas appear unless you happen to clone the bus interface of an existing
design.
For COMBOARDs, we used 8641s and DC013s for the UNIBUS products, and the
real-deal, expensive DC010, DC004, DC005 chips for the QBus product. I
think I remember seeing third-party boards using 74LS240s for bus buffers,
but I do not know how well they'd work in a loaded box (or any VAX newer
than the uVAX-II).
-ethan
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