23 Aug
2014
23 Aug
'14
1:53 p.m.
On 2014-Aug-23, at 11:56 AM, drlegendre . wrote:
Working away on the Altair 8800 restoration..
So the display/control board ("d/c") is now installed, and I'm running
into
my first roadblock. I ask your understanding in advance, as my familiarity
with digital logic is pretty weak.
This machine has a Processor Tech. MB-1 mainboard in it. Full-width, 16
slots, and some mods I don't understand.
First, there's an LM309 (TO-3 case) mounted to the board, apparently
providing reg'd 5VCD to the normally 8V (un-reg'd) line. Why would that be
there? Every plug-in board I've seen - which isn't all that many,
admittedly - has its own regulator for 5V on-board.
Secondly, on the far left side of the board, there are several columns of
resistors that seem to be doing a couple different things - some are
pulling-up various groups of lines to what seems like 1/2 of VCC, others
are either there for termination or some other unknown function.
All together, close to 100 resistors installed there.. and it's clearly a
mod (at least partly) as a line of 50 new holes was drilled to accommodate
them.
Anyone know what's up with this stuff? Why it's there? At this point, my
sense is that I need to remove all of it, as they don't exist in the
original build - and I may even cut the main board in two - totally lop-off
the leftmost 12 sockets along with the resitors & reg.
I'd suggest double-checking the 309 regulator and the +8 lines on the bus sockets, the
309 might be just providing +5 for the termination network.
It's conceivable the bus and (some) boards were modified for +5 on the bus edge
connectors but it would make for a limited and incompatible system.
The S100 bus is a mixture of OC (open-collector) and tri-state signals:
- OC signals are supposed to have a pull-up resistor somewhere in the system, usually
on the processor board.
- Tri-state signals *may* (optionally) have termination resistor (networks) somewhere
in the system.
- One way to provide such termination is an R per signal to a common supply
regulated to ~ 1/2 Vcc;
- Another way is two Rs per signal: one to ground, one to +5, calculated to
give the target AC impedance for the line assuming negligible
AC impedance between +5 & GND while DC biasing the line to ~ 1/2 Vcc.
See the IEEE-696 spec:
http://www.pestingers.net/PDFs/Other_computers/IEEE%20696%20S-100%20Bus%20S…
page 34, Sec 3.4.1 Driver Types
Sec 3.7.1 Bus Termination
page 15, Table 6
I'd think you should be able to get the sys working before taking the irreversible
step of cutting the Processor Tech bus board. No, the termination isn't strictly
necessary, at least for 'low-speed' (i.e. early) systems (I've seen several
16-slot systems with no termination), but some reading suggests the PT bus may have been a
common upgrade for the Altair bus-boards.