Host Engineering Forum
General Category => Do-more CPUs and Do-more Designer Software => Topic started by: Garyhlucas on October 21, 2021, 08:41:17 PM
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I am building a machine with 3 BRX 36 I/O. One does all the heavy lifting controlling 3 servos and 9 VFDs while the other two basicly twiddle their thumbs occasionally sensing things and operating solenoid valves during a 1 minute cycle. Would it make sense to move the coms program that is passing simple bit data back and forth over ethernet into one of the lightly loaded PLCs?
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If the "loaded" PLC is doing RX/WX to the other two, if 10 Hz update rate is good enough, PEERLINK can easily share data across all 3, as a type of "global" data.
Basically, each PLC broadcasts its specific PL block of 16 WORDs, up to 16 blocks, so 16 different PLCs could each broadcast a different block, or one PLC could broadcast all 16 blocks to unlimited number of "listen only" "peers". In your case, PLC#1 could take PL block #1, PLC#2 take PL block #2, and PLC #3 take PLC block #3.
Each PLC is broadcasting at the rate of 10 Hz, and then receiving the other 2 packets at the same rate. So, every 100ms, each PLC is sending 1 and receiving 2 packets. Very simple comm load sharing.
What's great is that the ladder logic is extremely simple, a PEERLINK in $Main of the 3 PLCs, along with some logic to possibly process moving their necessary bits into their corresponding block's registers. Casting makes bit picking easy (e.g. PL0:0 would be the first bit of the first block, PL16:0 is the first bit of the 2nd block, PL32:0 is the first bit of the 3rd block).