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Configuring the TSXCUSBMBP Driver
24
35011984 02/2006
Configuring the Slave Response Timeout
Introduction The Slave Response Timeout is used by the TSXCUSBMBP each time it sends a
request from an application out the Modbus Plus network to a slave device.
Slave Response
Timeout
This timeout is variable due to the possibility of a radio link or serial link somewhere
in between the TSXCUSBMBP device and the slave device it is communicating with:
z The interface between the application and the TSXCUSBMBP driver has the
same characteristics of an actual serial port.
z One request must be completed before another request can be sent out.
z If the slave device does not respond to the TSXCUSBMBP, it must time out from
waiting on the response before it can accept another request from the application.
Timeout Value The value for the Slave Response Timeout needs to be determined by adding
together the following times:
1. The maximum amount of time it can take for a request to be sent to any slave
device that the TSXCUSBMBP will be communicating with. This time must
include any delays induced by modems, RF links, network bridges, etc.
2.
The maximum amount of time it can take for any slave device to process a request.
Note:
Start operations on certain PLCs can take as long as 4 seconds or more to
process. If the Slave Response Timeout value is lower than this, timeouts may occur.
3. The maximum amount of time it can take for a response to be sent from any slave
device back to the TSXCUSBMBP. This time must include any delays induced by
modems, RF links, network bridges, etc.
4. Some additional time to provide a ‘safety zone’ to allow for the ‘unexpected’. A
value of 500 ms is adequate if all slave devices are connected on the local
Modbus Plus network or through a high-speed bridge. If any slower network
interfaces are in the communication path between the TSXCUSBMBP and any
slave device, an adequate value is usually between 1000 and 2000 ms.
Note: When performing a Modbus network scan using ProWorx or Concept,
changing the Slave Response Timeout value from its default of 5000 ms to
600 ms before the scan will greatly improve performance. However, the Slave
Response Timeout must be set back to the proper value before attaching to a
PLC in programming mode and performing Start operations.