I am laying out wiring for a data collection module. The digital input module has a 3000 ohm input impedance, a 4 volt turn on threshold and a 1 volt turn off threshold. I am sending a 24 vdc signal through a set of contacts on an energy meter back to the input module to be counted. I am using a 22 AWG shielded twisted pair cable. Anyone have a recommended maximum cable length for this cable. I am concerned with the noise that could be on the cable.
Recommended Cable Length
- Thread starter Stoney57
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gar
Senior Member
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- Ann Arbor, Michigan
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- EE
100902-1238 EST
Stoney57:
You have described a very poor design.
With 24 V available the midpoint of the threshold should be about 12 V. Then maybe 12 V hysteresis on top of the mean. This means an on point of 18 V and an off point of 6 V.
You have not mentioned data rate. Is it once per second or 100 times per second?
I assume your energy meter switch contacts are near zero resistance when closed and near infinite when open, and are isolated.
Also assume your cable can be considered as a lumped capacitance rather than a transmission line.
When the transmitter switch closes, because of its low impedance, it very quickly brings the voltage at the receiving end to 0 V. When the switch opens the voltage changes toward 24 V more slowly. This is determined by the RC time constant of the cable capacitance and the 3000 ohm input resistance.
Assume 70 pfd per foot and 1000 ft, then the time constant is 0.07*3000 = 210 microseconds.
Your noise immunity is very good when the transmitter is closed (shorted) relative to capactive coupling, and poor when it is open. But with a large hysteresis in the receiver detector it will take a large coupled voltage to cause an error. Your twisted pair reduces magnetically coupled noise, and the electrostatic shield reduces capacitive coupled noise.
.
Stoney57:
You have described a very poor design.
With 24 V available the midpoint of the threshold should be about 12 V. Then maybe 12 V hysteresis on top of the mean. This means an on point of 18 V and an off point of 6 V.
You have not mentioned data rate. Is it once per second or 100 times per second?
I assume your energy meter switch contacts are near zero resistance when closed and near infinite when open, and are isolated.
Also assume your cable can be considered as a lumped capacitance rather than a transmission line.
When the transmitter switch closes, because of its low impedance, it very quickly brings the voltage at the receiving end to 0 V. When the switch opens the voltage changes toward 24 V more slowly. This is determined by the RC time constant of the cable capacitance and the 3000 ohm input resistance.
Assume 70 pfd per foot and 1000 ft, then the time constant is 0.07*3000 = 210 microseconds.
Your noise immunity is very good when the transmitter is closed (shorted) relative to capactive coupling, and poor when it is open. But with a large hysteresis in the receiver detector it will take a large coupled voltage to cause an error. Your twisted pair reduces magnetically coupled noise, and the electrostatic shield reduces capacitive coupled noise.
.
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