Hello again,

Since I don't know for TTL... :-)

...I tried a different strategy. Unless there's a difference between the Z8530PSH that came on the Lisa and the Z8530PS (no H) that I found for cheap somewhere, the SCC was not the culprit after all.

Back to the original strategy: probing! I'll update if I have any discoveries...

--Tom

On Thu, Oct 26, 2017 at 1:37 AM, James MacPhail <gg_@sigmasevensystems.com> wrote:

When no data is transmitted, U9C pin 25 shows 3.66v

For TTL signals, the logic "high" threshold is 2.0V, and "low" is 0.8V.

So, 3.66V is within the specification for a high level TTL signal.

Anything between 0.8 and 2.0V is not high nor low... which is to be expected for a signal in transition but may indicate a problem for a steady state circuit.

High signals are often much closer to 5V, and low signals much closer to 0V; it depends on the parts involved.

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