User`s guide

Page 94

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

6.2.3. Time Allocated Process Control (Allocated Time Slices)

The “

Time Allocation Process Control” operation acts as a process scheduler. There are two primary

processes (scanning the network and servicing the PLC) and each one gets a dedicated “

time slice”.

The 1746-C uses this to ensure the primary processes are serviced in a deterministic and controllable

manner.

The user adjustable configuration parameter in table [0] – word [7] determines the number of

”

Additional Valves to Scan in the Network Scan Time-Slice” before starting the PLC process time slice

(before interfacing with the PLC … servicing the PLC interface process).

Values for the parameter …

• The default value is 2.

This means 2 additional valves (3 total) will be scanned in the scan time slice before the

PLC process is serviced in its time slice (when a M0/M1 transfer sequence from the PLC

can be handled).

• The minimum number is 0 (maximize the amount of PLC interfacing time).

• The maximum number is the lesser of

o 9

o 1 less than the max number of valves on the network

(

can’t scan more valves than are on the network)

The larger the number, the faster the scan period but the fewer interfaces with the PLC.

The smaller the number, the scan period is slower but there are more frequent interfaces with the PLC.

If the number is too large, it will take too long to service the PLC and control capabilities by the HMI

may fall off (ex: they could generate a “control/placement” error in which the process control can’t

move a modulating valve to a specified position within a specified time frame).

In other words, if too much time is allocated for scanning valves before servicing the PLC, then the PLC

might not be serviced often enough for adequate control. However, servicing the PLC too often is a

waste of time if it can’t adequately process any differences between the services. That waste of time

could be better spent polling more valves for data.

This parameter is end-user adjustable in Table [0] and is 100% dependent upon the application.

Therefore, the operator must adjust (tune) this parameter to the specific application to get the best fit of

• best scan rate

• best service rate for the PLC block transfers

• control via the PLC

• the PLC getting table information (not skipping tables)

NOTE: To help prevent accidental delays to servicing the PLC due to taking too long to scan the

desired number of valves in the valve-scan time slice, a 650ms timer is in place. This timer is

reset every time the 1746-C finishes servicing the PLC process. In between polling individual

actuators during the valve scan time slice, if the 650ms timer ever expires then the PLC

process will get serviced. In other words, there is a time-based limit to how long the network

can be scanned before servicing the PLC.