The way we will focus on in this months newsletter is using MSG (message) instructions. There are a couple different ways that we can configure Allen-Bradley PLCs can talk to each other. There are many different options for communications networks that PLCs may need to talk, but Ethernet is, by far, the most common for new systems. With Ethernet, usually if you have multiple devices on a network, they all connect to a "hub" called an Ethernet switch. Here’s a simple image of how the two PLCs would likely be physically connected with an Ethernet network. If these are Allen-Bradley PLCs (ControlLogix, CompactLogix), the best way to do this is usually to use Ethernet. So let’s say we want to send data from PLC 1 such as the most important boiler temperatures and pressures so that we can display them on the control room HMI screen. Think of PLC 2 as the Plant SCADA PLC that allows an operator in the main control room to get an overall look at the plant from his control room. Let’s say PLC 1 controls one steam boiler and PLC 2 is the main plant PLC that monitors the alarms all the boilers in the plant and other equipment as well. Here’s a simple example of why you’d want to communicate data from one PLC to another. This is actually pretty cool and a fun topic to learn. Specifically Allen-Bradley PLCs, such as ControlLogix and CompactLogix controllers. This month’s blog post topic is all about sending messages between PLCs.
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