This document addresses a rare communication issue encountered in older PLCs and instruments when using RS485 UART for ModbusRTU communication. It focuses on a specific problem with a Modicon 584 PLC approximately 30 years old, describing the investigation and resolution of data dropouts occurring sporadically every 30-45 minutes in a daisy-chained RS485 network involving DA10 and other devices
Applicable Products and Prerequisites
The problem and solution are relevant for several hardware models including DA10, DA30, DA50, DA70, all HMIs, all RTU models, and any hardware communicating via ModbusRTU over RS485. Users should be familiar with Crimson software versions 2.0 through 3.2 to follow the troubleshooting steps
Original Problem Description
A customer added a DA10 device to communicate with the Modicon 584 PLC acting as the Modbus master over RS485. The network included multiple slave devices. The issue was intermittent data loss, showing as “N/A” in the plant SCADA system roughly every 30 to 45 minutes. The data flow would drop out sporadically despite otherwise normal operation
Investigation Process
Item 1: Monitoring the DA10
At this point we had verified all the wiring, devices, and the DA10 would talk successfully to the Modicon 584 by themselves
Item 2: Polling Mechanism Analysis
Now we turned to the Modicon 584 Polling mechanism. Found out that the polling was done by Function Blocks inside of a ladder logic program. What cause the problem was the polling rate for the DA10 was at 100ms and the rest of the devices was at 1000ms. We slowed the polling of the DA10 down to match the other devices, and the problem has gone away.
Need Further assistance? Submit a support ticket here.
https://support.hms-networks.com/
© HMS Networks AB 2026
