I’m not keen on the low input impedance of the probe circuit. That represents a loading of several TTL loads. It might cause a problem with more than 4 or 5 boards plugged into the buss. I have long used a Mullen TB-4 Extender Card / Logic Probe. This board uses an op-amp / comparator to attain high input impedance and also offers the ability to see open / tri-state / “TTL ½” signals. That would undoubtedly take more space, but it would be much more useful and would not add significant load to the buss. Fortunately, I still have my TB-4 manual and schematic, so if you would be interested in seeing how the comparator is used, I could pass that along.
From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On Behalf Of Don Caprio
Sent: Sunday, March 22, 2015 1:08 PM
Subject: [N8VEM-S100:6636] S100 Breakout Board - Version 3
What do you think about this new version of the breakout board?
I doubled the number of test pads and added six built in logic probes.
You can monitor up to six signals with the 7 segment LED showing
H (High) or L (Low). To monitor a signal you place a jumper from the
signal post to a bus pin.
I tried to use a simple logic probe design to conserve on board space.
Design uses 7404. I'm open for suggestions to improve probe circuit.
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