Advanced settings

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4. Advanced settings

4.1. Calculation of minimum slot time

Setting the appropriate (minimum) Tslot_Init value for a given network may significantly shorten the total DP Slave polling cycle. If one of the DP Slaves is out of order or if its response is lost, the DP Master will only wait for a set minimum time before sending another query. The value should be set to maximum to prevent problems.

The calculator on enables you to calculate the RTT (round trip time).

Set the PLC Master to Ethernet interface in the calculator (Profibus protocol timing is based on the last sent byte; time on Master’s RS485 does not figure in this calculation).

RTT for Bridge mode can be used directly; for Router mode the resulting average RTT needs to be multiplied by constant 1.25 to receive the maximum achieved RTT.

Calculate the recommended Tslot_Init as follows:

Tslot_Init       = RTT * (Port speed in bps) / 1000

4.2. Router mode – timing

Router mode web based settings may cause time problems in more complex networks. CLI lets you adjust radio channel access parameters and set up repetition taking into account the number of re-translations in your radio network.

If you only use the Profibus protocol with RipEX and no other broadcast interferes with your network, you can configure certain parameters to shorten the access time to channel using CLI. If you want to use packet acknowledgement on the radio channel, you can shorten the repetition timeout if ACK is turned off.

Set up using CLI:


-ack nTurns on ACK
-retries 2Number of retries 2
-rto-prog fTurns off progressive retries
-rto-fix 10Shortens the retry timeout to the minimum value of 10 Bytes
-rto-var 10Shortens the variable retry timeout to the minimum value of 10 Bytes
-slots-rx 0Will receive immediately after request – random channel access is not used
-slots-tx 0Will transmit immediately after request – random channel access is not used

Same settings should be used for all devices.

To find out more about CLI, see RipEX manual chapter CLI Configuration.

Set the following in Profibus parameters:

Tslot_Init    16383

Note: This setting is only appropriate for certain types of networks; changes should only be made by experienced users!

Connecting RS 485

Connector layout of RipEX COM 2 for RS 485 and the corresponding PIN’s on Siemens Simatic S7.

RS485 connection

Fig. 4.1: RS485 connection