Serial DTR/RTS confusion

Well, I measured again. I see +0.03VDC and -0.03VDC on RTS Hi/Lo (pin4) and -0.001VDC and -0.02VDC RTS Hi/Lo (pin7).

So maybe that explains the loopback test passing, but that's no where near 3 volts and the literature says those readings are considered to be "transitory" voltage.

On Wed, 31 May 2006 17:32:00 GMT, Vey put finger to keyboard and composed:

IME these voltages toggle between approximately +10V and -10V. If the COM port is idle, then the RTS and DTR outputs should be sitting at one or the other of these levels. Just to be sure, are you using pin 5 as the signal ground?

I've written the following program to test COM ports:

It runs in DOS only. Each output pin is toggled at 1Hz, 0.5Hz, and

This old post of mine describes how to use the DOS Debug command to directly control the DTR and RTS pins:

- Franc Zabkar

Are you measuring the output pins at the serial connector without a load such as a modem on them. Remember a wrap block places the load of a receiver onto the transmitter. Take a bare jumper and wire DTR to DSR and look at the voltage level. You can do the same with RTS to CTS. +3VDC is too low. I have seen good ones run anywhere from + or -

Van Gardner

Pin 5? As a ground? I had two different circuit diagrams I was working from. Neither one mentioned connecting Pin 5 to the circuit's ground. They both showed just the ground symbol.

Then when I was measuring, I was using the clam for a ground. Once I used pin 5, everything measured right and then I connected Pin 5 to the circuit and it started working and then just to check, I ohmed out the clam to the ground on the PC and got nada.

Looking at a third similar circuit, it does mention ground going to pin 5.

Thanks Franc!