User`s guide

Page 30

Controlinc 1746-C (Version 5.21) Network Master Users Guide (2004-11-18)

Note 2: If writing to a valve and the write fails, then the internal flag to write the value REMAINS

ACTIVE. This means that even though the communication process will prevent the program

from trying to transmit when there is an error, the program will keep this value identified as one

to send out until it is actually sent out or cleared by the PLC.

This also means that the write to this particular actuator will be attempted the next time the PLC

sends this table in with a “write table” command associated with it.

For example … If this was a value in table [5], the 1746-C will retry writing the value to the

actuator the next time the PLC sends table [5] in as a write command.

The 1746-C cannot “cancel” this command because it is not in its scope of responsibility to

directly change process control commands.

Therefore, it is the PLC’s responsibility to write whatever “safe” value is appropriate for the

particular actuator/application at any time in case the actuator “comes back online”.

Note 3: The PLC should limit the rate in which these commands are sent to the 1746-C … only write

as fast as required to adequately control the valve. The faster the PLC requests changes to

valve setpoint positions in the valves, the longer it takes the 1746-C to complete the write

operation and continue scanning the network for data … decreasing the overall performance

concerning updating status information for all actuators on the network.

For example

: Sending write commands once per second might still afford good control and still

update all status information. However, sending write commands 4 times a second might write

the information out faster but delay the response (status) information for the entire network to a

point where it is too difficult to control.

Warning: If the valve has been moved while the PLC is down or the valve was moved while in local

mode, then as soon as the valve is communicating and accepting remote commands, it will

move according to values in tables [3] and [5]. Please adjust the PLC RLL accordingly.