Modbus Network Allen Bradley

I have a project, that I need to communicate 28 devices (Woodward UMT Power Meters) using MODBUS communication with a Micrologix PLC (Probably MX1400 or MX1500) and a Panel View plus.

We will be reading not more than 10 registers per device, I need your advice because this modbus network is critical for our project, because we are doing Load Shed and Load Add using the Total System Load Demand, Summing the KW registers from each device (1 Register per device) on the PLC.

So the MODBUS communications has to be faster, so I need your recommendations in order to get this network optimized.

fran007 said:

So the MODBUS communications has to be faster,.....

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Why?

You aren't going to be dropping loads on a milisecond basis - more likely in a time frame of minutes per action. You can do a lot of Modbus RTU communications transactions in that kind of time frame. We've used Modbus RTU for similar data exchanges, and a few seconds latency is the worst we've encountered.

It is relatively straightforward to write MSG instructions in the MicroLogix controllers. It takes some good programming discipline to manage MSG instructions, especially when errors occur, but there's no magic to it.

Sequential polls from a single master is the slowest way to implement this system, but the least expensive.

My suggestions for faster communications include:

1. One or more MVI69-MCM communication modules from Prosoft attached to a 1764-LRP MicroLogix 1500 controller. These modules act as Modbus masters independent of the controller and you can have two Masters per module, so you can split up the system into several groups and get faster polling.

2. A 1769-SDN DeviceNet scanner in the MicroLogix 1500 polling multiple DNet/Modbus gateways like the 1782-JDM (Western Reserve Controls), the CDN-367 (MKS Instruments) or the AnyBus Communicator (HMS Fieldbus). Each DNet gateway can be polled in milliseconds and can perform its own master polling on Modbus to an individual power meter.