Application Example; Modbus - Adding Devices - Siemens SITRANS RD500 Operating Instructions Manual

Application Example

The PV reading from a thermocouple is 3 degrees lower than the actual temperature
when the process is at +200 degrees. It reads only 1 degree lower than the actual
temperature when the process is at +300 degrees.
Desired PV = (PV x Slope) + Offset
Slope = 300-200
299-197
Offset = 200 - (0.980 x 197) = 6.940
A Slope value of 0.980 and an Offset value of 6.940 corrects the sensor error.

Modbus - Adding Devices

Below, see how to add I/O data that originates from external serial Modbus devices. This
allows the RD500's capabilities to be extended to any device equipped with a serial port
and that supports the Modbus RTU slave protocol.
For more information regarding the serial connectivity of the RD500, refer to the
hardware manual found at www.siemens.com/sitransRD500.
After adding each Modbus device, click Apply, and the next device field will become
available.
Driver Settings
• Protocol Type: Selects Modbus protocl: RTU or ASCII
• RTU Framing: Detect via Timing and Track Frame Contents options
• Slave Response Timeout: The time in milliseconds (mS) that the Modbus master
allows the Modbus slave device to respond. If the Modbus master does not receive
a response from the slave in the specified time, the master will enter a timeout error
in the event log.
Page 26 SITRANS RD500 – OPERATING INSTRUCTIONS
= 0.980
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