User`s guide

Page 21

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

4.2. Network Interface (Scan) Time-Slice (1746-C

↔

Network

Communication)

4.2.1. Operation: Network Scanning to Gather Valve Actuator Data

The Network Master polls and controls up to 60 valves on the network by sequentially polling

(scanning) each device in sequence from slave actuator address #1 to the last slave address (#n) on its

network.

This polling gathers information from the actuators (including alarms, discrete information and actuator

position) and places it in tables that can be transmitted to the PLC.

The scan operation cannot stop in the middle of a request for data from a particular actuator.

Therefore, the 1746-C can only interface with the PLC once it has finished all required communications

with the actuator it is currently getting data from. Network scanning resumes with the next actuator

after interfacing with the PLC and processing any requests made by the PLC (writes to tables, writes to

actuators, table reads, …).

For example, if the 1746-C is currently scanning actuator [5] and the PLC wants to write a new valve

position setpoint to 22 actuators then the 1746-C will …

1. Finish getting data from actuator [5]

2. Get the new command table from the PLC.

3. Write the new valve position setpoint to all 22 actuators.

4. Restart network scanning where it left off … the next actuator [6] after the last one polled [5].

Standard data is gathered from the actuators by a single Modbus 03 command.

However, a 2

Modbus 03 command is generated if any of the following are true …

• If requesting totalizer data

• If requesting a specific holding register

• If requesting a block of holding registers from this actuator

• If requesting to monitor the discrete outputs on this actuator

• If requesting TEC2000 input data.

This is one reason why additional data should only be requested when needed … it will slow the entire

scan time of the system (by a minimum of 30-50 ms each time an actuator is polled for this data).

For instance, if all 5 of the above are requested for each actuator, an extra 30-50 ms is required for

EACH … meaning that the scan time could be increased by almost a factor of 5! … dramatically

slowing down the system!

A typical scan time (time to scan all the actuators on the network) to gather “standard” information on

60 actuators is less than 10 seconds … depending upon the 1746-C configuration.

When gathering data, the data is stored in the appropriate tables in the 1746-C. Values in the table

for a particular actuator will not be overwritten if there is an error communicating (on both ports) with the

actuator.