User`s guide
Copyright © 2004 EIM COMPANY, INC. • 13840 PIKE ROAD • MISSOURI CITY, TX. 77489
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Controlinc 1746-C (Version 5.21) Network Master Users Guide (2004-11-18)
4.3.2. Operation: PLC Commands to Write Data to the Actuators
When valve control commands (change setpoint, open, stop or close) are generated by the PLC, the
Network Master sends the appropriate command(s) over the network to the addressed slave device
(actuator).
However, if the command for a particular valve has not changed (value in the table is the same as
before), then nothing is sent to the particular valve (it is skipped). Basically, when the 1746-C starts
acting on a command to write data to the actuators, it compares the current value in the table with the
new one. If the values are different, then the actuator is flagged as one to write information to.
When writing discrete output control commands (directly controlling coils [3-5] or [100-1011]),
the 1746-C does NOT compare the value to the last one. When commanded to write to the
coils, it is performed ONE TIME and then turned off for that actuator (however, monitoring the
status of those outputs is left on for that actuator).
When the 1746-C starts writing data to the actuators, is starts with the lowest address number and
proceeds through to the highest address number. This write operation is completed after all
“appropriate” actuators have been written to.
In general, there is one Modbus command/response set issued for EACH PLC command to write data
to a particular actuator (ex: writing analog output data).
However, as per the specification, when a position control command is issued, the 1746-C is required
to ensure that a “SSR” contactor configuration bit is set on any 320A system prior to receiving a control
command. Therefore, there are up to 3 Modbus command/response sets issued for EACH PLC
command to write position data in any particular 320A actuator:
If the actuator is configured to have an SSR in it (table [13]):
1. A Modbus command is sent to the actuator requesting status information to verify the SSR
configuration coil is set.
2. If the coil is not set, a Modbus command is sent to set it.
3. The Modbus command is sent to indicate the new setpoint.
Note 1: Discrete OPEN / CLOSE / STOP commands.
The discrete OPEN / CLOSE / STOP commands (in table 3) are handled exactly the same as if
a setpoint was sent.
• OPEN (send a 100% open setpoint)
• CLOSE (send a 0% open setpoint)
• STOP (send a setpoint matching the last position read)
(For more information on how this works, refer to the actuator’s operation manual)