Testing a 1488 line driver...

3 of the 4 sections of the 1488 are NAND gates with 2 inputs. Those 2 inputs must both be drivein somehow (they should npt be left floating). Basically there are 3 things you can do : 1) Link then together, and to the logic signal (for example TxD from the Z80-DART), thus turning that section of the 1488 into a NOT gate with RS232 level outputs. This is the most common thing to do. 2) Connect the logic signal to one of the inputs, connect the other input to +5V, maybe through a resistor. 3) Drive the 2 inputs from separate logic signals. Maybe the TxD line and a separate 'force break' line. Or the main and back channnel signals in a modem. My point was that you'd checked that the TxD line from the Z80-DART had got to the 1488, but you didn't say what happened to the other input of that bit of the 1488, if indeed there is a second input (the TxD could be using the single-input part of the 1488, of course). If case (3) applies, you could have had a problem in the logic that drives the other input, for example. Right. That, of course, is somethign you need to correct. Maybe the converter has failded. But 0.6V sounds like a diode-drop to me, so it might be that a chip has failed as is now acting like a forward-biased diode from the -12V line to ground, pulling it down. -tony