Weird data bus write problem

[cjs]

[This was a post on a device that would check for three output levels, rather than just two: "0", "1" and "somewhere in between, so neither." To avoid derailing this discussion, I've moved it to its own thread: A "Poor Man's Logic Analyzer" With TTL Threshold Detection. ]

[GARTHWILSON]

cjs wrote:

To solve this, I was inspired by an idea I saw in the second of Garth's schematics in the Interfacing to SPI and Microwire section of his circuit potpourri page. There he uses op-amps rigged as comparators against a 2.5 V reference to light LEDs when the input voltage is above that level.

Minor correction there: The LM339 shown there is an open-collector comparator (as mentioned in note #10 below the diagram). It's not terribly fast as comparators go, but it's quite a bit faster than an LM324 is.

[Chromatix]

One could even go a step further here, and test against three voltage thresholds: TTL/CMOS low, TTL high, and CMOS high.

These could be processed into a 2-bit code, synchronised with a clock and trigger, and recorded into a sample buffer, which is the essence of a logic analyser, but with some basic analogue-domain information included. If the thresholds can be adjusted by hand, this can also be made useful in an LVCMOS context.

[cjs]

To avoid derailing this thread too much, I've reposted my Octal TTL Logic Probe post as the head of a new thread, and added my further responses there (as has, already, Garth; thanks Garth!).