3-30
3
3.8.9 Floating Point Variables
Some new Modbus devices like Watlow's EZ PM series controllers use two
consecutive register to control a value or to read back a process variable. The
two registers hold an IEEE-754 32-bit floating point word. The registers are
read and written to in the low word-upper word order.
3.8.10 Floating Point Write
The WF command writes the num value in floating point format to two con-
secutive registers starting with the low word register.
WF 2160, 75
'writes to registers 2160 and 2161
3.8.11 Floating Point Read
The RF? query reads a 32-bit floating point value from two sequential register
in low word-upper word order. The RF? does not require the number of register
to read since it is fixed at two registers.
RF? 360
'reads registers 360 and 361
3.8.12 Setting Modbus Device Timeouts
The D command sets the time that the 8099 waits to receive a response from the
Modbus device. If the 8099 does not receive a response within the time period,
it assumes that the Modbus device is not responding and sets the timeout error.
Timeout errors can be determined by reading the 8099's Modbus Error Register
with the E? query. If the error code is 101 (Modbus timeout) then the timeout
period should be lengthened. The command to change the timeout period is:
D 500
‘
sets timeout period to 500 ms
The default time period of 300 milliseconds has proved to be satisfactory for
most Watlow controllers but should be verified carefully for your specific
Modbus device. Some devices fail to respond within the default time period
because they perform periodic calibrations. The recommendation is that your
program should have a built-in recovery routine to handle modbus communi-
cation errors.
