Testing a 1488 line driver...

I am hampered by the facts that (a) this machine is not at all common in the UK, so I've never seen one and (b) I don't have schematics. But I don;t believe that for an isntant. I do not believe that the pin that's convnetioanlly DTR is driven by a combination of the RTS and TxD signals. RTS on its own would be reasonable. Let me re-state what I am asking. The 1488 has 4 sections. These sections are independant of each other, sharing only pwoer connections. 3 of the secions are NAND gates with 2 inputs each, one is an inverter (NOT gate) with only 1 input How are thse sectiions used? In particualr, the section that drives the TxD ouput (pin 2 on the DB25 connector), does it have 1 or 2 inputs? If the latter, where do _both_ of the inputs go? Are they linked together, do they come from separate signals? Again, I don';t know wnough about the machine. Is the -12V line used by anything else apart from the RS232 drivers? How many 1488s are there on the board? You want to remove either the DC DC converter or all the loads on it. If the altter are just 1 or 2 1488s, it might be easier to remove those. If the -12V line is still missing with those off the board (and again assuming tyhat -12V line doesn't power anytthing else), you then want to remvoe any capacitors between that -12V line and ground (they can short too). If it's still missing, then the converter is bad, but I'd still test it after getting it off the board. If you remvoe the converter first, and it tests fine out-of-circuit, you then need to find which of the on-board compoentns that connect to the -12V line (1488s, decoupling capacitors between the -12V line and ground, anything else?) is shorted. aybe by re-fitting the covnerter and removing the components until the -12V line comes back. -tony