modbus tcp connection refused on Raspberry pi 3 about iot-edge-modbus HOT 3 CLOSED

This module is a Modbus 'slave' so there must a Modbus 'master' somewhere.
You refer to the localhost IP address. Is there a Modbus application running on the RPi?
Otherwise, find the right IP address of your Modbus master.

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It's been quite a while since the last post, but in case others may have seen a similar issue, I figured it would help to chime in:

Assuming that the IP address is set to the SlaveConnection desired property field, if the intention behind setting 127.0.0.1 is to poll the host (IoT Edge device) as the Modbus device/slave/server, using 127.0.0.1 may not be the right option. This is because the Modbus module, as an IoT Edge module, is (by default) placed under the Docker daemon's azure-iot-edge network.

You can determine the desired IP address by examining the details of the azure-iot-edge network bridge and compare that to the network interfaces of your host device. Here is an example in an Ubuntu device I've used for testing.

// "172.18.0.1" would be the IP address of the Modbus device
$ sudo docker network inspect azure-iot-edge
[
    {
        "Name": "azure-iot-edge",
        "Id": "f04958f1d3bf175e64d2d4ec696c79afac9572c76012122722ed392c287ca2fc",
        "Created": "2021-07-16T02:42:45.303029015Z",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": [
                {
                    "Subnet": "172.18.0.0/16",
                    "Gateway": "172.18.0.1"
                }
            ]
        },
 <snip>

$ ifconfig
br-f04958f1d3bf: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 172.18.0.1  netmask 255.255.0.0  broadcast 172.18.255.255
<snip>

Hope this helps.

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