Unable to communicate with the device via Modbus protocol?

Question

SCADA

Hello
The situation is as follows. There are two devices with an RS-485 interface that communicate via the Modbus protocol (one master, the second slave). There is an interface converter (adapter) USB-RS485 . Windows 10 is installed on the PC, with the software Serial Port Monitor installed. I connect both devices in parallel through an adapter to a PC. But in the program, I see only the response of the slave, I do not see the requests of the master. If I disconnect the slave from the PC, then I see requests coming from the master with a certain frequency.
I connected both devices in order to sniff the traffic and understand in which registers the necessary information is located, because I do not have complete documentation for the devices.
Question #1:why, with two devices connected, I see only answers, although the bus is shared and as I understand it, I should see all the traffic on the bus.

The next problem is that it feels like my requests do not reach the slave if I send them from a PC. Tried using different programs, as well as using the Modbus CLI console utilities. . Also used Modbus Poll . Also tried to send a packet directly from Serial Port Monitor. In general, it feels like the packets go nowhere.
I know exactly which address of the slave device, because I set it myself in the settings.
Tell me what I could do wrong and in which direction to dig?

Answer 1

I first test the slave using owenOPC. free, logs are all visible, writes errors. I check whether the wires are connected correctly, the port is configured, the addresses are registered, the data in the registers is in place. Then I try to read it as a master.