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Serial DTR/RTS confusion

When a program asserts these to go high, the voltage is supposed to increase to more than +3VDC, right?

Looking at my multimeter, mine aren't doing anything, yet when I connect DTR to DSR or RTS to CTS, they pass the loopback test.

Yes, my multimeter is set and working correctly. Yes, I am sure I am looking it the right pins. I have tried every pin, just to make sure (it's a 9 pin connection) and I don't see any voltage, plus or minus out of any of the pins. Yet the loopbacks are working just fine.

I'm using Win2k and the B&B Electronics Comtest program.

Reply to
Vey
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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.

Reply to
Vey

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:

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

0.25Hz. A loopback function is also supported.

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

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- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.
Reply to
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 -

6volts to + or - 15 depending on the device.

Van Gardner

Reply to
vangard

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!

Reply to
Vey

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