3 wire cable representation with components

[garyljudd]

I am monitoring a 5VDC signal accross 2 wires. It is being toggled off and on at 1000 baud. The wires are a single pair twisted cable with ground wire shielding. This is being done with a 500' cable. I would like to replace the cable with components that would simulate the cable. How can I do this?

 

[betwixt]

Your description doesn't make sense!

1. you say you have 5V DC across 2 wires but it is being toggled off and on at 1000 Bauds. 5V DC is just that, a steady voltage, do you mean it is a serial signal of 5V amplitude?

2. You say there is also a ground wire, is it connected to either of the other wires?

3. On the two '5V' wires, are they carrying a differential signal (like RS422/432) or a single wire signal with ground reference (like RS232 but at 5V levels)?

4. What is the characteristic impedance of the cable?

5. What is the resistance of each conductor over the 500' length?

6. What is the capacitance per metre/foot between the twisted wires and the shield?

Brian.

 

[garyljudd]

1. It maybe a serial signal of 5V amplitude (with Rx, Tx and ground). Also it may be used simply to monitor open and closures of a switch.

2. The shield wire may or may not be used as a ground, it may be pulled high in some applications connected to the 5 volt wire inside the equipment terminals to which it will be connected. Also, a cable with three insulated conductors can be used.

3. They are carrying a single wire signal with ground reference (like RS232 but at 5V levels). Two applications will be used. A signal wire, data wire, and ground wire. Or two wires eventually shorted at a switch terminal and the remaining wire to the other switch terminal.

4. What is the characteristic impedance of the cable? I'm looking for a model to plug in these various cable parameters.

5. What is the resistance of each conductor over the 500' length? Same as above.

6. What is the capacitance per metre/foot between the twisted wires and the shield? Same as above.

 

[D.A.(Tony)Stewart]

Show us the signal. The twist pair is typical 120 ohms single ended or 240 differential. It will have a certain distributed capacitance and inductance per foot e.g 10pF/ ft. But more importantly there may be other noise sources of ingress and the line may not be balanced for source and load impedance. Ideally it should be. If you are experiencing transient errors, a DSO might capture the waveform if you can trigger off a parity error or use a Tx pattern for BER testing or look at the eye pattern on a scope. Etc.

Depending on the SNR and clock/ data recovery circuit, one can improve results on BER greatly. In the 70's they used RS232 at 1200/9600 baud for 1000ft cable bundles but sometimes had to eliminate some Pairs for crosstalk.

Need more details on issues and goals.

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Is the switch a dry contact with insufficient wetting current (10%)

Is the shield intended to supply 5V power with ground inside?

Is this going to be full duplex ?

Will the Tx and Rx have unknown Ohm line impedances at both ends?

Will it use CMOS?

If any answers are yes, there will be issues that can be resolved by design.