Siemens SIMATIC NET PROFIBUS Termination, Restriction when using the 12M bus terminal at 500 Kbps

Passive Components for RS-485 Networks

Termination

The termination must be activated on the first and last node on the bus segment. If termination is activated (termination on), the connection between the incoming (A1, B1) and outgoing (A2, B20 segment is interrupted. The advantage of this is that if a bus terminating resistor is activated incorrectly, the stations after the bus terminal can no longer be accessed. When a segment is started up, you can then make sure that no bus terminating resistors are activated that are not located at the beginning or end of the network.

Note

Restriction when using the 12M bus terminal at 500 Kbps

This restriction only affects segments longer than 80 m.

If the 12M bus terminal is operated at a transmission rate of 500 Kbps along with the RS-485 bus terminal with a 3.0 m tap line (6GK1 500-0AB00), a minimum clearance of 5 m (= 5 m PROFIBUS cable) must be maintained between the RS-485 bus terminal with the 3.0 m tap line. The 12M bus terminals can be arranged at any point in the segment; in other words, no minimum clearance needs to be maintained. The 12M bus terminal can also be included between two RS-485 bus terminals with a 3.0 m tap line. The only important point in this respect is that the PROFIBUS cable between the two RS-485 bus terminals with 3.0 m tap lines must be a total of 5 m long.

4-54	PROFIBUS Networks SIMATIC NET
	6GK1970-5CA20-0AA1 Release 2 05/2000