User`s guide

Page 91

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

6.1.5. Using the “Scan Period” Value

The “Scan Period

” value is placed in table [0] for the end user (HMI) to dynamically adjust the network

behavior to the needs of the application. This value indicates the size of the “Scan Period” … the

number of network scans before the 1746-C performs a diagnostic scan to try and “bring back on-line"

any units that had stopped communicating.

During normal operations, the 1746-C will skip any actuator that it has already determined is "off line"

(this is to save time when trying to gather data from the other actuators that are running). The only

time it will bring any "off line" systems back on-line is

• During the diagnostic scan

• After reset.

If the end-user (application, HMI, ...) believes that allowing off-line units to come back on-line it is less

important than normal data gathering (or just not something to be concerned with), then it should

increase the scan period size (200 is the maximum ... indicating that a diagnostic scan will happen after

every 200 network scans for data).

If however, the speed of units coming back on line is more critical than the increased interruption due to

increasing the rate of diagnostic scans, then the HMI should lower the scan period number (5 is the

minimum ... indicating that a diagnostic scan will happen after every 5 network scans for data).

In other words, recovery time to get an actuator back on line increases as the scan period value

increases. Therefore, to speed up the recovery time, make the scan period value low. To lengthen

the recovery time, make the scan period value high.

RECOMMENDATIONS:

Typically, EIM will not recommend any particular value for the scan period value … EIM cannot make

that type of decision for any end user. However, EIM will work with a user to explain what happens if a

value is chosen. The user must then make the choice of value that best suits their application.

Basically, they need to evaluate the priority levels (and effects of) the time period of having an actuator

off-line vs. the interruptions in the normal data gathering to perform system diagnostics (allowing off-line

systems the chance of coming back on-line).