Communication between AB PLC and Honeywell DCS – seeking experts

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I would like to thank everyone in advance{:3_57:} for helping to find a solution – currently, the people are still by the desert and unable to go back for the New Year. There are two sets of PLCs, type AB, at the site; the communication modules used are Heyouxun NT-50-RS-EN, which enable MODBUS RTU 485 communication with the Honeywell DCS. The DCS serves as the master station, while the PLC acts as the slave station; the master station can only read from the slave station and cannot write to it. The problem now is that at the DCS communication cabinet, using modscan32 to simulate the master station allows it to receive data from the slave PLCs, and the data is correct; however, when connecting to the DCS itself, it fails to receive that data. The details are as follows: 1. Previously, a colleague came to the site for individual debugging and told me that the PLC and DCS could communicate with each other. 2 I went to the site for joint debugging; the AB CPU has two network port connectors, one for communication and one for the HMI. Since the communication was working well before, I removed the network cable connecting the AB PLC to the Heyouxun communication module and connected it to my computer to carry out startup testing. After the testing was completed, I reconnected the cable to the communication module, only to find that communication was no longer possible. 3 Later, in the input/output configuration – Ethernet of the AB programming software RSLogix 5000, it was found that NT-50-RS-EN had a yellow question mark in that directory. I checked, and it turns out that RSLogix 5000 does not recognize the communication module tags from Heyouxun (all previous communications were carried out using a colleague’s computer; I don’t have any Heyouxun software installed on my computer). So I went to the HeyouXun official website to download the relevant tags and software. Then the RSLogix 5000 can be recognized. 4 But communication was still not possible, so I wondered if the communication module from Heyouxun needed to be reconfigured. Several attempts failed, but after working with Heyouxun’s manufacturer on the configuration, it now works in the Honeywell communication cabinet, and the correct data from the PLC can be read using the modscan32 software. 5 The Honeywell engineers began to suspect that there was a problem with the voltage on my communication lines; upon checking the PLC cabinet, it was found that indeed one of the wires was in contact with the instrument’s casing. After making the necessary adjustments, the voltages for one set were 0.2V and 3.2V with respect to ground, while for the other set they were 1.2V and 4.2V with respect to ground. 6 Currently, the Honeywell engineers plan to use modsim32 to simulate the addition of data from the station to the communication cabinet, in order to see whether the DCS can receive it properly. Question: One of Honeywell’s engineers said that communication was possible before, but it’s not working now; he hasn’t made any changes, and he suspects it’s a problem with the PLC. 2 But modscan32 can read it, and the voltage is normal; I think there’s nothing wrong with the PLC. 3 The strangest thing is that it really hasn’t changed at all; I can still read it even after reconfiguring it, so why can’t there be a communication link? (Baud rate 9600, data bits 8, stop bits 1, no parity bit; we have checked these basic parameters countless times together.)