Douglas Krantz - Technical Writer - Describing How It Works
Go to the Fire Alarm Home Page of Douglas Krantz -- Describing How It Works
Go to the Fire Alarm Operation Map Page of Douglas Krantz - Technical Writer
Go to the General Electrical Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Description Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Installing Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Maintaining Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Testing Map Page of Douglas Krantz - Technical Writer
Go to the Fire Suppression Map Page of Douglas Krantz - Technical Writer
Go to the Science Article Map Page of Douglas Krantz - Technical Writer
Go to the Writer Home Page of Douglas Krantz -- Describing How It Works

I have a question on how something works.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

I have a Question

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.


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
See how Class A Wiring works

More Articles

What Does E=IR Really Mean? -- If it isn't just a word, and it isn't really something to be memorized in order to pass a test, what do the letters in Ohm's Law really mean?... Read More

Which Way Does Electricity Flow?-- So... In a wire, negatively charged electrons move, and positively charged atoms... Read More

What is an RS485 Communication Network? -- RS485 is a two wire data communication system where both wires are considered the carrier of the signals. As such, there is no ground/return. Two, three, four, up to dozens of devices ...Read More

What is a Fire Alarm System? -- Long ago, as earliest method of spreading the word of fire danger, people shouted "Fire!" Depending on the circumstances, people would run... Read More

PDF Book
PDF of Make It Work - Conventional Fire Alarm Systems
PDF of Make It Work - Conventional Fire Alarm Systems

Douglas Krantz

Describing How It Works

View Douglas Krantz's profile on LinkedIn


Readers Questions

Short Circuit
Free Subscription
I'll Send You the
Fire Alarm


How Does Class A Fire Alarm Wiring Work?-- Fire alarm systems save lives and protect property. Fire alarm systems also break down because... Read More

Just What Is a Signaling Line Circuit (SLC)? -- The SLC (Signaling Line Circuit) is another way of saying Data and Power Circuit. Along with added power to run the sub-computers and their input and output circuits, it's a computer data-buss ... Read More

How is a Buffer Relay Wired Into a Door Holder Circuit? -- Like a door stop, a door holder keeps a fire door open. When smoke is detected, the door holder releases, allowing the door to shut. The door holder looks simple and innocuous enough... Read More

How Does One Find a Soft Ground Fault? -- Normally, we think of resistance like that of a resistor. The amount of resistance is built-in; no matter what voltage is used to drive the electrical... Read More

Can a Magnet Really be Used to Test a Smoke Detector? -- Smoke detectors usually have two ways of being tested. Smoke (smoke particles in the air, or some sort of canned smoke), and magnets (the activation of an internal magnetic... Read More

Electrical Flow

On this website, most references to electrical flow are to the movement of electrons.

Here, electron movement is generally used because it is the electrons that are actually moving. To explain the effects of magnetic forces, the movement of electrons is best.

Conventional current flow, positive charges that appear to be moving in the circuit, will be specified when it is used. The positive electrical forces are not actually moving -- as the electrons are coming and going on an atom, the electrical forces are just loosing or gaining strength. The forces appear to be moving from one atom to the next, but the percieved movement is actually just a result of electron movement. This perceived movement is traveling at a consistent speed, usually around two-thirds the speed of light. To explain the effects of electrostatic forces, the movement of positive charges (conventional current) is best.

See the explanation on which way electricity flows at