21 Dec
2016
21 Dec
'16
7:44 p.m.
On Wed, Dec 21, 2016 at 7:37 PM, Brent Hilpert <hilpert at cs.ubc.ca> wrote:
The counters may be the beginning of a video timing
divider chain or a clock divider for the LUCY chip.
(e.g. a diagram in the document you mentioned shows a div-6 between a 6MHz crystal osc
and the LUCY chip.
The division factor may vary of course in the executel depending on the crystal.)
Also to keep in mind as you trace it out, 749x counters were often used with gates
between the
outputs and reset inputs to change the count modulus / division factor.
I seem to remember Adrian said that the reset pins on these counters (assuming
they are 7492s, and those pins are reset) come from an inverted version of
the 8085 reset. I think it's unlikely that the 8085 will be
continually reset, so
I would think the counters were not reset by a combnation of their outputs.
[Unknown resistor]
You can try measuring resistors in circuits like this
with a DMM.
Switch the DMM leads to get both polarities through the resistor.
It's not guaranteed as it depends on the connected circuitry and the R value, but
one of the readings will often be your R value, or may confirm or give direction as to
the value.
It's worth remembering that if you put the DMM leads across the resistor,
then the reading can only be affected by other paths in parallel, and these
will reduce the displayed resistance reading, never increase it.
So if you have a resistor that might be either 2k2 or 33k (think about it)
and you measure it in-circuit as 25.4k then, assuming it is not faulty,
it must be 33k.
-tony