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The RS485 Network Keeps Having Communication Errors

I have a question about something


Mr. Krantz

Thanks a lot for your help.

In our factory we have a system to make glue for corrugated cartons.

We have a weighting scale machine connected to a CPU using an RS485 Network. After time, when we make a patch of glue, one or two of these scales show many communication errors. We have changed one of these scales, and the system returns to normal, but after a while the same error happens on another weighting scale.

We contacted the system supplier find out what is happening, and they sent us a cable with termination resistors connected to the end and start of the network, but the system still shows the same error. But when we cut one of these resistors the system worked fine till now.

So is there any reasonable cause for this problem?

Signed M M

RS485 Communication Network

Connecting different machines together, an RS485 communication network is a pair of wires. When no signal is being sent by any machine, there is no voltage or signal on the wires. However, when a machine sends data down the line, it sends its signal in the form of electrical impulses (data). On the RS485 network, all of the other machines receive the electrical impulses.

Like beads on a necklace, the RS485 network is a series string connecting the machines together. At each end of the network is a resistor. As electrical impulses come down the network, the purpose of this resistor is to absorb the impulses. Without this resistor, the electrical impulses would bounce off the end of the network and interfere with newer electrical impulses, preventing proper communications.

These resistors are called terminating resistors because they "terminate" the RS485 network at each end. If there are extra resistors in the network, the extra resistors will cause interference.

Problem

I don't know what is inside the machines you have, but sometimes these machines have internal terminating resistors. If so, there is usually a switch by the network connections or a switch on the circuit board inside the equipment so the terminating resistor can be switched in or out of the circuit.

Check your equipment instruction manuals for this.

Look at the whole RS485 network at once. Remember, the RS485 network is a series string; all equipment is located along this series string like beads on a single strand necklace.

The whole RS485 network should only have two terminating resistors: one at each end of the series string. There shouldn't be a terminating resistor between the ends of the RS485 network, either at the end of one of the interconnecting wires or inside any piece of equipment.

Possible Solution

What you have to do about the terminating resistors is to make sure there is one terminating resistor at each end of the RS485 network, and at the same time no piece of equipment should a terminating resistor switched on. Not having enough resistors or having too many resistors on the network will cause interference with the electrical impulses, giving you an error message.

Douglas Krantz
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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