RS232 signal 1

[atlas466]

Hi,

Sorry if this is obvious but I'm new to this.

I have a Moxa’s NPort W2004 wireless device server connecting serial ports to our network.

The software monitoring the port can recognize inputs on the DSR pin (we are not currently using this pin)

The server describes the interface as: "RS-232/422/485, with 15 KV ESD for all signals"

The machine with the serial port has a 24 volt DC light that I would like to monitor.

Can I connect this 24 volt line directly to the DSR pin on the MOXA device server, or will I have to isolate, and or modify this signal?

Thanks for any advice,
Dan

 

[Skogsgurra]

Most RS 232 receiver chips have nominal input voltage swing +/-15 V. So, if there is any punch behind your 24 V, chances are that you will destroy the receiver chip.

Also, a simple series resistor probably won't work because it is not guaranteed to take the input down to zero if your 24 V opens.

Put a voltage divider there. Use 1 kohm from DSR pin to ground and connect, say, 3.3 kohm to +24 V. That will give you plenty of signal (you need at least +3 V) in active state and a good connection to GND when +24 V disappears.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[atlas466]

Someone just recommended using a voltage regulator.

Is this better or worse?

 

[Skogsgurra]

Voltage regulators are 1: overkill 2: not fast 3: don't produce a connection to gnd when +24 V drops out. I think it is a bad idea produced by someone that knows enough to be dangerous but not enough to be useful.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[djs]

A true RS-232-C signal must go negative to change the state. Going from a positive voltage to zero should not change the state of a RS-232-C signal. So even if you reduce the volatge, you still should not see the signal change.

 

[Skogsgurra]

Yes, if you head the spec, which says that limits are +3V and -3V. But, for practical reasons, most 232 devices have the lower limit just above 0 V, so you do not need to go below 0. And that is especially true for the control signals, where BTW logic is positive in contrast to Tx and Rx, where it is negative.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...