Siemens SIMATIC TIWAY 1 User Manual download pdf (Page 93)

System Performance Examples

D-4

D.2 P/C Interlocking Between P/Cs with Different Scan Times

Peerlink

distributes data from each P/C to every other P/C, assuring that

the V Memory tables for each are updated with the latest data once per

network scan. Since the P/Cs on the network are operating asynchronously

and possibly at different rates, some data transmitted may never be seen

by another P/C. For example, data being sent from a Peerlink on a fast P/C

can be changing so fast that a slower P/C may “miss” some of the values.

In many applications, the data is not changing so quickly and this effect is

unimportant. However

, in some applications, certain precautions must be

taken. Following are two scenarios illustrating two different means of

assuring that each P/C is using current data.

D.2.1

Scenario 1

This first scenario simply keeps the data in V Memory constant for several

scans, allowing the receiving station time to get the data. The exact time

required will depend on the difference between the P/C scans involved.

The ladder logic shown below is for a P/C sending data to a slower P/C. The

data to be transmitted has been calculated and placed into locations

V300-V315 previously

. This data is not moved into the allocated Peerlink V

Memory table until a timer has counted down. This timer

’

s preset must be

at least equal to the worst case total scan time (1 P/C scan of transmitting

P/C + 1 network scan + 2 P/C scans of the receiving P/C). In the example

shown below

, the transmitting P/C will prevent the slower P/C from

missing data by only allowing the data to change once every 200

milliseconds.

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Siemens SIMATIC TIWAY 1 User Manual Page 93