How to measure PROFIBUS faults with a multimeter
When you do not have a handheld device or an oscilloscope, you can use a multimeter to perform some principle tests, such as testing of PROFIBUS cables, bus connectors, and loop impedance. These tests cannot provide 100% accurate values, but they can provide a rough indication. At the same time, these measurements must also meet the following prerequisites:
(1) The same components (PROFIBUS cables and plugs) must be used throughout the segment, PROFIBUS components should not be connected, and PROFIBUS cables must be disconnected. You can determine if the cable is broken by measuring the voltage between the shield and the two data lines.
(2) All terminators must be disconnected. If there are permanently connected PROFIBUS components in the system, such as repeaters, these components should be disconnected. Each segment must be measured separately.
Using a multimeter to measure can find and locate the following errors:
●Simple data line "inversion"
●One of the two data lines is interrupted
●Cable shield terminal
●Short circuit between two data lines
●Short circuit between data line and cable shield
1) Measure the loop impedance
Determine the loop impedance by measuring the impedance between the two cores of the PROFIBUS cable. The impedance of the core depends on the cable construction and is also temperature dependent. Specific cable impedance is usually specified in ohms per km (ohms) at a given temperature. It is equivalent to the loop impedance of a 1km long PROFIBUS cable. Typical values for PROFIBUS RS 485 cable type A are 110ohm/km loop impedance at 20ºC. However, for special cables, for example, highly flexible cables, it is possible to deviate from this value. As the temperature rises by 1ºC, the cable impedance increases by 0.4%. Measuring cable loop impedance is relatively simple.
At one end of the PROFIBUS cable, data core A and data core B must be short-circuited (or bridged). Then at the other end of the cable, measure the loop impedance between the two cores. Look in the PROFIBUS cable manufacturer's data sheet for the specific loop resistance (ohm/km) of the cable used. For short cables (less than 50m) the loop impedance may be 0. The length of this cable segment can be evaluated using this specific loop impedance:
Length (km) = measured loop impedance (ohms)/specific loop impedance (ohm/km):
Cable loop impedance can also be evaluated from knowledge of cable length:
Loop impedance, Rloop (ohms) = cable length (km) × specific loop impedance (ohm/km)
2) Test PROFIBUS cable and bus connector
Before starting the test, all stations must be disconnected from the cables and all terminators must be turned off or disconnected. The five steps described below should be completed on each PROFIBUS segment. Step 1 Check that the cable has no voltage coming from the power terminator; Step 2 Check for shorts between the cable wires; For steps 3 and 4, introduce a short between the selected pins in the 1st connector , and make these measurements on each of the remaining connectors. If you find that a short circuit introduced at a connector fails, this is an indication that the cable is bad or incorrectly connected. Steps 1 to 4 must be completed in the correct order in measuring the cable to adequately check that the cable is free of routing errors. Step 5 is used to check the length of this cable by introducing a short circuit between wires A and B on the first connector and measuring the loop impedance on the last connector.
●Step 1
Using the low DC voltage range on your multimeter, check that the voltage between the shield and connector pins A and B is 0. If you see any voltage, either the cable is not disconnected from all equipment or is still plugged into a powered terminator.
●Step 2
Measure the impedance between the connector pins on each connector. If the measured impedance is infinite (failure), it indicates a short circuit or a terminating resistor is present. In order to make a measurement, the cable loop impedance Rloop must be evaluated. This can be accomplished as described previously. Locating a short on a PROFIBUS cable can be difficult because a fault in just one connector will short the entire cable. One solution is to isolate parts of the cable until the short circuit disappears. But it must be noted that the connected terminal resistor will introduce an impedance of 220Ω between line A and line B. Only proceed to step 3 if no short circuits are found and all terminators are disconnected.
●Step 3
The test must be performed by introducing a short circuit between pin 8 (wire A) and the shield on the first connector of the segment being tested. In the first connector, the short circuit is completed by introducing a link from pin 8 to the shield. Then, take these measurements on every other connector.
●Step 4
When measuring, a short circuit must be introduced between pin 3 (wire B) on the first connector of the segment being tested and the shield. In the first connector, the short circuit is completed by introducing a link from pin 3 to the shield. Then, take these measurements on every other connector.
