23 Aug
2014
23 Aug
'14
7:55 p.m.
On 2014-Aug-23, at 7:40 PM, drlegendre . wrote:
Without any termination resistors, an undriven bus line should nonetheless be interpreted
(by the 7404) as high, so the D2 LED should be on.
It would appear something else is pulling it low (or something is wrong with the
7404/LED).
The absence of
termination resistors on D2 just sounds like a mistake.
The open-circuit state for a TTL input is high and the S100 data lines are
active-high, so with no boards on the bus one could expect all the Data
LEDs to be on, with or without termination resistors. With the D2 LED off
it suggests something is pulling the line low. (Are all the data/address
front panel switches in the same position?)
You might try pulling the appropriate D2 line (DI2) high, through a 220 to
330 ohm resistor for a little safety, to see if the D2 LED will come on.
Yeah, there aren't any resistors connected to it, which is why it's sitting
low. I checked that out. So what's the verdict on this, guys? Is this board
- which was modded with
all the termination stuff for a Z-80 based system - going to be OK when I
try to get the old 8080A hardware working in it? Or is it going to screw
things up, like having the Data LEDs on at all (or at least spurious) times?
This is all new to me.