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Troubleshooting

9-21

SIMATIC 545/555/575 System Manual

9.8 Checking RS-485 (Twisted Pair) Cable Installation

To locate problems with RS-485 communication links, use a digital or analog

meter capable of measuring resistance between 0 and 150 ohms.

Measure the line-to-line and line-to-shield resistance as follows:

1. Remove power from the local base and all remote bases.

2. Disconnect the CPU and all RBCs from the cable by removing the 9-pin

D-connector.

3. Measure the resistance from either isolated wire to the other isolated

wire (line-to-line) and record this value.

4. Measure the resistance from one line to the shield wire and record this

value.

5. Measure the resistance from the other line to the shield wire and

record this value.

6. Compare the values you recorded with those in Table 9-6.

Table 9-6 RS-485 Cable Resistance

9860 or 9271 9182

Trunk Cable Type

Min. Max Min. Max

Line-to-line 52 ohms 70 ohms * 63 ohms 87 ohms *

Line-to-shield 100 Kohms – 100 Kohms –

* This table assumes two terminating resistors.

Line-to-line resistance that is below the minimum specified in Table 9-6

may be caused by the following:

• Incorrect termination resistors are installed.

• More than two termination resistors are installed.

• A short exists in the cable.

Line-to-shield resistance that is below the minimum specified in Table 9-6

may be caused by the following:

• One of the 9-pin connectors is connected to an RBC or CPU.

• Cable line wires are shorted to the shield.

Using Digital or

Analog Meter

Resistance below

Minimum

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