At 05:26 PM 11/7/2004, Pierre Vallee wrote...
>In doing the voltage checks for U1, I find readings of 12.77v on pin 5 (should
>be -8) and 0v on pin 12 (should be 5). I believe the circuit diagram shows
>these pins being part of the transmit circuitry (pin 2). Could U1 be my
>problem? Any suggestions. Thanks
U1 is an EIA-232 transceiver. It converts TTL level to EIA ("RS-232") levels.
The voltages you describe show it is operating correctly - when pin 12 is at 0,
pin 5 should be more than +3 positive ("space" level). When pin 12 is at 5, pin
5 should be more than -3 negative ("mark" level. You'll notice it also inverts
the signal. When idle, the EIA-232 output should be "marking time," or at a
negative voltage.
Your problem seems to be that the signal going to pin 5 is incorrect. It should
sit at a high level, and pulse low when the K2 is transmitting data. Since it's
low all the time, I would suspect a wiring problem (this single goes through
the internal cable to the header connector which plugs into the control board.)
Check for shorts to ground, or a lack of continuity from U1 pin 12 to header
pin 3.
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