Quote:
Originally Posted by ianbartie
TB2 to TB3 measures about 5.45V instead of about -9V & TB2 to TB measures the same instead of 0V.
The bit that has me stumped is both RTD Explorere & Hyperterminal work fine. I can see the data & I can change settings.
Any suggestions?
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Ian,
Did you mean -5.54V? Default "mark state" level should lower than -3 although 0 has been know to work. But if it was positive then that makes it look like a "space state". Might want to disconnect the 3 wires and measure the controller TXD and LCD TXD seperately. Also do not measure with DMM if controller is transmitting as the TXD line (receive for LCD) will not measure correctly.
Levels aside, I feel the mosfet event probably compromised the RS232 transceiver on the controller. Output levels may have fallen gradually over time and finally reached the "threshold" where the RS232 chip on the LCD no longer recognized it as valid. RTD and hyperterminal both use the R232 tranceiver on the laptop which may have a lower "threshold" allowing it to continue working (for now). I have a 120 ohm resistor in the comm ground path on the LCD (R6). Just provides a little protection in the event of ground loop between LCD and controller. It is probably dropping a bit of voltage too which would account for the higher threshold required for the RS232 chip.
As a test you could short across it and see if the serial comm works. But if the RS232 chip on controller is dying a slow death, this will only postpone the inevitable.