How Load Resistance Affects Level & Pressure Transmitter Communication

DC control and communication circuits can be finicky little beasts. They demand the right cables, they insist on maximum distances for trunk and/or branch lengths, and sometimes, they go so far as to stop working without proper load resistance.

But what in the world is “load resistance”? And what is it for? And why is this control network refusing to work without it?

At the most basic level, load resistance is the cumulative resistance of a circuit, as seen by the voltage, current, or power source driving that circuit. This includes the resistance of the wires and the resistance of any devices connected to those wires. Everything between the “place where the current goes out” and “the place where the current comes in” contributes to load resistance.

Sometimes, this even includes a load resistor. A load resistor is a resistor that has the sole function of increasing the load resistance of the circuit to a specific level.

So that’s the “what.” To understand the “what for,” let’s look at two ways load resistance can be critical to proper operation of a DC control or communication circuit.

1. Line Separation
2. Signal Conversion

Line Separation

Modbus control networks, and others similar to Modbus, use two wires for communication. The voltage relationship between the two lines (A higher than B, or B higher than A) is an integral part of how the communication between devices works. For effective communication between master and slave units, the voltage between the two lines must be consistent across the entire network.

Modbus networks use load resistors at each end of the network to accomplish this voltage stabilization. (Since these resistors are placed at the ends, we call them terminating resistors, instead of load resistors.) However, if the master device of a given network is at one end, rather than at an intermediate point, the internal resistance of that master device behaves as the terminating resistance. So the terminating resistor at the opposite end of the network will need to be matched to the internal resistance of the master device.

“But why?” you ask. This seems a bit arbitrary and far-fetched. Why can’t we use just any old terminating resistor at the far end of the line?

Fair question. Let’s look at it this way: say we have two parallel wires, with a resistor connected between them at each end. If we apply a DC voltage at one end, and the resistors are matched, the voltage at the other end will be (for all intents and purposes) the same. But, if the resistors are not matched, especially if they are significantly different in size, the voltage at the far end will be different, causing a mismatched current to flow in the circuit, disrupting the communication.

Thus, matching the load resistance to the source resistance is extremely important for this type of network.

Signal Conversion

Other communication and control networks, such as HART, use load resistors to convert current signals to voltage signals.

For instance, a HART transmitter that sends a 4-20 mA signal can’t communicate directly with a HART analog input card that detects 0-5 VDC signals. However, running the 4-20 mA signal through a 250 Ω resistor will create a 0-5 VDC signal that the input card can understand. So, in this instance, rather than using the load resistor to maintain voltage, we’re using it to create a voltage.

These are just two ways that control networks are dependent on specific load resistors. If you are having trouble with your control network communications, be sure to check the documentation for your devices to see if load resistance could be part of your problem.

We come across this issue more often than you might think. Because it is a detail, it can be a difficult one to identify – causing downtime and frustration.

Contact us if you have questions regarding the wiring of your APG devices. This is one of several examples of wiring and networking issues that can alter or altogether ruin an otherwise good reading from a healthy sensor.

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top photo credit: Nicolas Buffler via flickr cc - cropped