On Jan 14, 9:28am, Mark Buettemeier wrote:

> If ECU were to send the poll command to two scopes at
> the same time and they both were to respond on the same wire at the same
> time the two messages would become merged into one confusing mess of bits.
> "Y" connectors for serial cables are generally a bad idea unless the devices
> involved have a pre-designed expection for such a setup -- in which case
> they usually have some sort of addressing scheme which identifies the
> particular device that is expected to respond while all others stay quiet
> and ignore commands.

It's even worse than that. Electrically, each interface is holding each signal
line in some state. If I remember my RS-232 correctly, the "idle" state is
a negative voltage between -3 and -12, typically -5. So if Scope A is holding
its Transmit Data line to the mark state, and Scope B wants to send a space,
there's no telling what the receiver would get. Bottom line: don't just
electrcally "Y" serial or parallel computer interfaces. You need something
more sophisticated.

Mark Cousins

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