ICS ELECTRONICS 4809A, Instruction Manual

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The time that the 4899A waits to receive a response from the Modbus device
is programmable by the D command. This is an internal 4899A timeout
and not the same as the GPIB bus timeout. If the 4899A does not receive
a response within the timeout period, it assumes that the Modbus device
is not responding and sets the timeout error. Timeout errors can be deter-
mined by reading the 4899A'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 300 ‘ sets timeout period to 300 ms
The default time period of 100 milliseconds has proved to be satisfactory
for some Watlow controllers but should be verified carefully for your spe-
cific 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 the
occasional timeout error.
3.8.12 Locking Setup Parameters
All of the 4899A’s configuration parameters can be locked to prevent ac-
cidental change by the end user. These lockable parameters are noted by a
# symbol in Tables 1-1. Locked parameters cannot be queried or changed
while locked. Any command that addresses a locked parameter is not ex-
ecuted, the Command Error bit in the Event Status Register is asserted and
the ERR LED is lit. The lock function is saved by the *SAV 0 command.
An example is:
CAL:LOCK ON ‘blocks unauthorized changes
*SAV 0 ‘saves lock condition
CAL:LOCK OFF ‘unlocks setup parameters
While lock is enabled, the end-user can change and save any non-locked
parameter.
3.8.13 Generating SRQs from Modbus Errors