At 04:06 PM 4/7/2006, David Mathog wrote:
On Thu, 06 Apr 2006 15:50:34 -0700
Jim Lux wrote:
>
> Except that the piece of hardware the serial port is talking to uses RTS
> and CTS, not as handshaking lines, but to wake up the box, and to return
> status.
In that case be very careful about bit states on power
transitions affecting the USB hub and the controlling computer.
I had all sorts of fun controlling some TrippLite UPS's that
used a similar "bit level" serial line control methodology.
Some computers would come up with the serial port lines
bouncing around at random, which could send an "inverter kill"
signal to the UPS at just the wrong time.
You may well find that when you reboot the master machiine all
hell breaks loose on the USB->serial controlled slave devices.
In this case, that's not a problem. The box uses RTS going asserted and
then deasserted to bring it up out of sleep mode. If nothing happens after
that, it just goes back to sleep. So wild fluctations won't cause any
problems.
But you're right. I have a USB->parallel interface that has just that
problem, and in that particular case, there are "bad things" that can
happen (mind you the same problem exists with a hardware parallel port,
because the BIOS goes out and does a reset). In that case, the external
box doesn't get power applied until the PC is up and running.
Jim
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