I have a conveyor scale transferring its data (weight) through a serial cable connected to the communication adapter. From the adapter the data gets sent to a control logix controller. I can see weights of the boxes as they come in instantly in the logic of the PLC and as a new box comes onto the scale a new weight is being seen but quite frequently what I am seeing is as a the new box comes onto the scale the prior weight is being reported onto the new box. Our support vendor is telling us the scale might not be sending the message fast enough to the PLC. Could it be possible the message is being held in the communication adapter and this why I am receiving duplicate weights. I have another Comms adapter on hand to replace but before I go this far I wanted an expert opinion.
Hello Michael,
Modbus is a polled protocol. Generally the modbus messages on a EN2SE is setup Cyclicly, with the default rate of 1 second (100x10ms ). You can try adjusting this in your configuration to increase the poll rate. You may also be limited by the speed that the modbus node can respond. In this case you may see LED 5 flashing red due to timeouts. Keep in mind if you increase the poll rate you could cause this issue.
Also if you have other parameters being polled they can add a delay.
Ok. Thanks for the response back. Our current setup would fall under a Modbus RTU since serial and we are communicating through a TCP/IP device. So would I need to setup the I/O device also to respond(poll) at the same speed to ensure it can handle passing the message to the PLC? It also looks like I could possibly increase the speed and increase the retries to prevent a timeout.
I’m fairly new to some of this stuff and learning as I go. Appreciate the help once again.
From the PLC side to the Linking device the data is exchanged cyclically based on the RPI. This is in ms and should be faster then the serial network. The serial side of the linking device works independently polling the nodes for data. To be updated when a response is received you can enable the trigger byte in the response.
There are many factors that can come into play for how often a message is sent. I would recommend to take a log of the serial traffic to see how much time there is between each query and see if this is slower then you are weighing devices.
You may have to also take into account how much the node, which i assume is a scale, can both weigh items and how fast it can respond to requests.