Siemens SIMATIC NET PROFIBUS manual Segments 1 and 2 Connected Through

Active Components for RS-485 Networks

Segments 1 and 2 Connected Through

Figure 5-5 shows the connection between two segments via an RS-485 repeater with each LAN cable connected through:

Segment 1

Segment 1

R

Segment 2

Terminating resistor Bus segment 1 off!

Terminating resistor Bus segment 2 off!

Segment 2

Figure 5-5 Connecting Two Bus Segments to the RS-485 Repeater

Note

If you turn off the power supply of a complete segment, the terminating resistors of the connected nodes are also without power supply. This can lead to disruptions or undefined signal states in this segment that are not recognized by the repeater and can then lead to problems in the other segment.

Whenever possible, we recommend the following procedure:

SAlternative 1:

Disconnect the two segments before turning off the power supply using switch 5 (Table “Description and Functions of the RS-485 Repeater”) on the repeater (set to “OFF”).

SAlternative 2:

Connect the repeater to the power supply of the segment to be turned off so that the repeater is also turned off. In this case, make sure that the repeater is not at the end of the previous segment, since the repeater is then the terminating resistor which has no effect if there is no power supply. If this solution is required, use a PROFIBUS terminator with a permanent power supply after the repeater.

SAlternative 3:

If you want the repeater to retain its power supply, use PROFIBUS terminators to terminate the segment you want to turn off since these also require a permanent power supply. You require one terminator if the bus segment to be turned off ends at the repeater, otherwise you require two terminators.

5-8	PROFIBUS Networks SIMATIC NET
	6GK1970-5CA20-0AA1 Release 2 05/2000