Siemens SIMATIC NET PROFIBUS manual Connecting the electric RS 485 bus lines

4 Setting Up 4.4 Installation

4.4.3 Connecting the electric RS 485 bus lines

9	/ n.c.	Ground / 5
		n.c. / 4
8	/ RxD/TxD –N
		RxD/TxD –P / 3
7	/ n.c.
		Ground / 2
6	/ + 5 V Output
		PE / 1

Shield

Fig. 9: Electrical port – assignment of Sub-D sockets

The modules are fitted with an RS 485 electrical port. This is a 9-pin Sub-D socket with a screw lock (inside thread UNC 4-40).

The pin assignment complies with the PROFIBUS stan- dard. At Pin 6 there is a short circuit-proof 5V output for supplying external pull-up/pull-down resistors.

As opposed to the 24V power supply and the casing (ground potential), the RS 485 bus lines RxD/TxD–N and RxD/TxD–P are indirect-coupled (functional separation) within SELV restrictions.

Only use shielded and twisted-pair wiring as a RS 485 bus line as described in the manual ”SIMATIC NET PROFIBUS networks“. Do not exceed the segment lengths given there.

Use a PROFIBUS bus connector plug to connect the RS 485 bus segment. If the module is at the beginning or end of a bus segment, this connector must have an activated bus terminal resistor combination.

All PROFIBUS bus connector plugs in a network must be securely screwed onto the RS 485 interfaces.

Attaching or removing the bus connector plugs, inadequately attached bus connector plugs or loose bus wires within the plug can lead to malfunctions in the optical and electrical networks.

Attach or remove the RS 485 bus connector plug quickly and without twisting them.

Remove the RS 485 bus line from the OLM if a device is not connected to the other end, or there is an OLM which has been disconnected from the power supply. The open line otherwise acts as an antenna and can cause inter- ference.

When connecting a RS 485 bus line to the PROFIBUS OLM in an active network, keep to the following sequence in order to avoid interference:

1.Place the RS 485 bus connector plug onto the device which is to be connected (e.g. to a programming device) and screw it on tightly.

2.Attach the RS 485 bus connector plug to the PROFIBUS OLM quickly and without twisting the connector, and screw it on tightly.

Proceed in the reverse order when removing a device from the network

Ensure that the bus segment connected to the RS 485 interface is terminated at both ends. Only use a connecting cable which is terminated at both ends to connect a single device.

Observe the following safety notice:

Do not connect any bus lines which have been partially or totally laid outside of buildings. Otherwise lightning strikes in the vicinity of the cable could destroy the module. Use optical fiber lines for bus connections which lead out of a building!

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