# Modbus TCP previews correct output, but receiving end gets garbage

**URL:** <https://discourse.nodered.org/t/modbus-tcp-previews-correct-output-but-receiving-end-gets-garbage/78322>\
**Category:** General\
**Tags:** modbus, docker\
**Created:** [10 May 2023 15:22 UTC](https://discourse.nodered.org/t/modbus-tcp-previews-correct-output-but-receiving-end-gets-garbage/78322 "2023-05-10T15:22:11Z")\
**Posts on this page:** 7\
**Page:** 2

<div class="post-metadata">

**Author:** ![Darren](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/darren/32/92202_2.png) [@Darren](https://discourse.nodered.org/u/Darren)\
**Post date:** [19 May 2023 12:22 UTC](https://discourse.nodered.org/t/modbus-tcp-previews-correct-output-but-receiving-end-gets-garbage/78322/22 "2023-05-19T12:22:34Z")

</div>

The more I and my colleague look at it, the more it looks like bytes are being swapped around, similar to the issue observed [here](https://discourse.nodered.org/t/scaled-value-while-reading-modbus/75871/), except my modbus master and slave are entirely in node-red, so I don't know why this is happening.

as a test, I broadcast `12345` on `4008`, and receive as much in my node-red client, but my colleague receives alternating `FF 00` and `00 FF` his end.

He said:

> Almost like it’s only using 8 bits and maxing out at 255, then either shifting or endian swapping it

---

<div class="post-metadata">

**Author:** ![Steve-Mcl](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/steve-mcl/32/4826_2.png) [@Steve-Mcl](https://discourse.nodered.org/u/Steve-Mcl)\
**Post date:** [19 May 2023 12:26 UTC](https://discourse.nodered.org/t/modbus-tcp-previews-correct-output-but-receiving-end-gets-garbage/78322/23 "2023-05-19T12:26:28Z")

</div>

Can you clarify your overall goal?

If you simply want to read/write values from a MitsiPLC, you can use `node-red-contrib-mcprotocol`

If you need to build a modbus server, then you should read the built in docs. For example, would be better off writing to the modbus server using the flex-write.

Built in help...

> You can use the Modbus write nodes (FC) to write data to the server buffers.
> 
> You can use the Modbus read nodes (FC) to read data from the server buffers.

Here is how i would structure it:

 ![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/e/b/eb24598da75bb09d32e136995af6da3e5137e028.png)

---

<div class="post-metadata">

**Author:** ![Darren](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/darren/32/92202_2.png) [@Darren](https://discourse.nodered.org/u/Darren)\
**Post date:** [6 June 2023 09:22 UTC](https://discourse.nodered.org/t/modbus-tcp-previews-correct-output-but-receiving-end-gets-garbage/78322/24 "2023-06-06T09:22:40Z")

</div>

My apologies for the delay in replying; I was called to other tasks by my work.

The overall goal with this system is to re-broadcast signals received from a PLC, but the best my colleague is seeing is the same replies coming back with alternating 8 bit “halves” of the 16 bit data, even when I broadcast `12345`. I think we need to do a byte swap using buffer-parser, but I've never got anything but errors from the buffer-parser node.

Current flow is (possibly broken? been a while):

```auto
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        "g": "49c18e3ad19acbda",
        "name": "Silo Level scale",
        "func": "msg.payload = {\n 'value': msg.payload,\n 'register': 'holding',\n 'address': 2,\n 'disableMsgOutput': 0\n};\n\n// debug\n//msg.payload = {\n// 'value': 1337, 'register': 'holding',\n// 'address': 2, 'disableMsgOutput': 0\n//};\n\nreturn msg;",
        "outputs": 1,
        "noerr": 0,
        "initialize": "",
        "finalize": "",
        "libs": [],
        "x": 1220,
        "y": 320,
        "wires": [
            [
                "8916057c5b4e9270",
                "5d59c82badda5237"
            ]
        ]
    },
    {
        "id": "8512686ef7c21c3e",
        "type": "function",
        "z": "37f1be47da815ae1",
        "g": "49c18e3ad19acbda",
        "name": "Maximum stoker speed scale",
        "func": "msg.payload = {\n 'value': msg.payload,\n 'register': 'holding',\n 'address': 3,\n 'disableMsgOutput': 0\n};\nreturn msg;",
        "outputs": 1,
        "noerr": 0,
        "initialize": "",
        "finalize": "",
        "libs": [],
        "x": 1260,
        "y": 360,
        "wires": [
            [
                "ed36434616147015",
                "5d59c82badda5237"
            ]
        ]
    },
    {
        "id": "62340306b6eaf6f0",
        "type": "function",
        "z": "37f1be47da815ae1",
        "g": "49c18e3ad19acbda",
        "name": "Averaged stoker speed scale",
        "func": "msg.payload = {\n 'value': msg.payload,\n 'register': 'holding',\n 'address': 4,\n 'disableMsgOutput': 0\n};\nreturn msg;",
        "outputs": 1,
        "noerr": 0,
        "initialize": "",
        "finalize": "",
        "libs": [],
        "x": 1260,
        "y": 400,
        "wires": [
            [
                "248c46681d071246",
                "5d59c82badda5237"
            ]
        ]
    },
    {
        "id": "c5a0fdf6b8bb9850",
        "type": "comment",
        "z": "37f1be47da815ae1",
        "g": "49c18e3ad19acbda",
        "name": "[Unavailable] Stoker at minimum speed",
        "info": "",
        "x": 190,
        "y": 380,
        "wires": []
    },
    {
        "id": "8916057c5b4e9270",
        "type": "debug",
        "z": "37f1be47da815ae1",
        "g": "49c18e3ad19acbda",
        "name": "debug 11",
        "active": false,
        "tosidebar": true,
        "console": false,
        "tostatus": false,
        "complete": "payload",
        "targetType": "msg",
        "statusVal": "",
        "statusType": "auto",
        "x": 1520,
        "y": 320,
        "wires": []
    },
    {
        "id": "ed36434616147015",
        "type": "debug",
        "z": "37f1be47da815ae1",
        "g": "49c18e3ad19acbda",
        "name": "debug 12",
        "active": false,
        "tosidebar": true,
        "console": false,
        "tostatus": false,
        "complete": "false",
        "statusVal": "",
        "statusType": "auto",
        "x": 1500,
        "y": 360,
        "wires": []
    },
    {
        "id": "248c46681d071246",
        "type": "debug",
        "z": "37f1be47da815ae1",
        "g": "49c18e3ad19acbda",
        "name": "debug 13",
        "active": false,
        "tosidebar": true,
        "console": false,
        "tostatus": false,
        "complete": "false",
        "statusVal": "",
        "statusType": "auto",
        "x": 1500,
        "y": 400,
        "wires": []
    },
    {
        "id": "3acabffc83517f90",
        "type": "subflow:3decb5f518882686",
        "z": "37f1be47da815ae1",
        "g": "49c18e3ad19acbda",
        "name": "",
        "x": 1830,
        "y": 540,
        "wires": [
            []
        ]
    },
    {
        "id": "c1f1279b6bad34ee",
        "type": "comment",
        "z": "37f1be47da815ae1",
        "name": "Brown & Carroll Modbus re-server",
        "info": "",
        "x": 920,
        "y": 100,
        "wires": []
    },
    {
        "id": "f4d3a0e92cc61648",
        "type": "modbus-write",
        "z": "37f1be47da815ae1",
        "g": "49c18e3ad19acbda",
        "name": "",
        "showStatusActivities": false,
        "showErrors": false,
        "showWarnings": true,
        "unitid": "1",
        "dataType": "HoldingRegister",
        "adr": "60",
        "quantity": "1",
        "server": "06fb380adab7a3ac",
        "emptyMsgOnFail": false,
        "keepMsgProperties": false,
        "delayOnStart": false,
        "startDelayTime": "",
        "x": 1140,
        "y": 440,
        "wires": [
            [],
            []
        ]
    },
    {
        "id": "509b5b5744b44bea",
        "type": "inject",
        "z": "37f1be47da815ae1",
        "g": "49c18e3ad19acbda",
        "name": "",
        "props": [
            {
                "p": "payload"
            },
            {
                "p": "topic",
                "vt": "str"
            }
        ],
        "repeat": "",
        "crontab": "",
        "once": false,
        "onceDelay": 0.1,
        "topic": "",
        "payload": "69",
        "payloadType": "num",
        "x": 970,
        "y": 440,
        "wires": [
            [
                "f4d3a0e92cc61648"
            ]
        ]
    },
    {
        "id": "2b2b6e5bf6a1669d",
        "type": "inject",
        "z": "37f1be47da815ae1",
        "g": "49c18e3ad19acbda",
        "name": "",
        "props": [
            {
                "p": "payload"
            },
            {
                "p": "topic",
                "vt": "str"
            }
        ],
        "repeat": "5",
        "crontab": "",
        "once": true,
        "onceDelay": 0.1,
        "topic": "",
        "payload": "",
        "payloadType": "date",
        "x": 710,
        "y": 560,
        "wires": [
            [
                "67a752c86bc8ca3c"
            ]
        ]
    },
    {
        "id": "67a752c86bc8ca3c",
        "type": "function",
        "z": "37f1be47da815ae1",
        "g": "49c18e3ad19acbda",
        "name": "Debug 12345 to D408",
        "func": "// debug\nmsg.payload = {\n 'value': 12345, 'register': 'holding',\n 'address': 8, 'disableMsgOutput': 0\n};\nreturn msg;",
        "outputs": 1,
        "noerr": 0,
        "initialize": "",
        "finalize": "",
        "libs": [],
        "x": 900,
        "y": 560,
        "wires": [
            [
                "bdc07397e20610c1",
                "5d59c82badda5237"
            ]
        ]
    },
    {
        "id": "bdc07397e20610c1",
        "type": "debug",
        "z": "37f1be47da815ae1",
        "g": "49c18e3ad19acbda",
        "name": "debug 12345",
        "active": true,
        "tosidebar": true,
        "console": false,
        "tostatus": false,
        "complete": "payload",
        "targetType": "msg",
        "statusVal": "",
        "statusType": "auto",
        "x": 1030,
        "y": 520,
        "wires": []
    },
    {
        "id": "d8b07ddb.c44d4",
        "type": "modbus-client",
        "name": "Boiler_PLC",
        "clienttype": "tcp",
        "bufferCommands": true,
        "stateLogEnabled": false,
        "queueLogEnabled": false,
        "failureLogEnabled": false,
        "tcpHost": "192.168.127.2",
        "tcpPort": "502",
        "tcpType": "DEFAULT",
        "serialPort": "/dev/ttyUSB",
        "serialType": "RTU-BUFFERD",
        "serialBaudrate": "9600",
        "serialDatabits": "8",
        "serialStopbits": "1",
        "serialParity": "none",
        "serialConnectionDelay": "100",
        "serialAsciiResponseStartDelimiter": "",
        "unit_id": "1",
        "commandDelay": "1",
        "clientTimeout": "1000",
        "reconnectOnTimeout": true,
        "reconnectTimeout": "2000",
        "parallelUnitIdsAllowed": true
    },
    {
        "id": "06fb380adab7a3ac",
        "type": "modbus-client",
        "name": "nodered_local_server",
        "clienttype": "tcp",
        "bufferCommands": true,
        "stateLogEnabled": false,
        "queueLogEnabled": false,
        "failureLogEnabled": true,
        "tcpHost": "127.0.0.1",
        "tcpPort": "10502",
        "tcpType": "DEFAULT",
        "serialPort": "/dev/ttyUSB",
        "serialType": "RTU-BUFFERD",
        "serialBaudrate": "9600",
        "serialDatabits": "8",
        "serialStopbits": "1",
        "serialParity": "none",
        "serialConnectionDelay": "100",
        "serialAsciiResponseStartDelimiter": "0x3A",
        "unit_id": "1",
        "commandDelay": "1",
        "clientTimeout": "1000",
        "reconnectOnTimeout": true,
        "reconnectTimeout": "2000",
        "parallelUnitIdsAllowed": true,
        "showWarnings": true,
        "showLogs": true
    }
]

```

---

<div class="post-metadata">

**Author:** ![Darren](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/darren/32/92202_2.png) [@Darren](https://discourse.nodered.org/u/Darren)\
**Post date:** [6 June 2023 09:50 UTC](https://discourse.nodered.org/t/modbus-tcp-previews-correct-output-but-receiving-end-gets-garbage/78322/25 "2023-06-06T09:50:49Z")

</div>

I tried doing something like that, but the server doesn't appear in the list of writable objects. I added a Modbus flex getter to the server's input, but that just gave us zeroes, so I had to revert in the end.

---

<div class="post-metadata">

**Author:** ![Darren](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/darren/32/92202_2.png) [@Darren](https://discourse.nodered.org/u/Darren)\
**Post date:** [6 June 2023 14:29 UTC](https://discourse.nodered.org/t/modbus-tcp-previews-correct-output-but-receiving-end-gets-garbage/78322/26 "2023-06-06T14:29:59Z")

</div>

Some the weirdness my end was caused by Modbus broadcasting single-number arrays, rather than single numbers. Fixed with simple functions. [Solution to that issue here](https://discourse.nodered.org/t/modbus-server-with-node-red-docker-port-forwarding-required/78096/12)

Issue still remains with byte-swapping on my colleague's end.

---

<div class="post-metadata">

**Author:** ![Darren](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/darren/32/92202_2.png) [@Darren](https://discourse.nodered.org/u/Darren)\
**Post date:** [26 June 2023 12:21 UTC](https://discourse.nodered.org/t/modbus-tcp-previews-correct-output-but-receiving-end-gets-garbage/78322/27 "2023-06-26T12:21:37Z")

</div>

**Solution:** Turns out there was an error on my colleague's side, where his software was reading from addresses `4001`, where it should have been reading from address `1` as they were already denoted as holding registers to begin with and didn't need the `400` preamble.

---

<div class="post-metadata">

**Author:** ![system](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/1X/d073cd938eafa2e558d7c2cd59003b3ef4963033.png) [@system](https://discourse.nodered.org/u/system)\
**Post date:** [10 July 2023 12:21 UTC](https://discourse.nodered.org/t/modbus-tcp-previews-correct-output-but-receiving-end-gets-garbage/78322/28 "2023-07-10T12:21:50Z")

</div>

Closed on 10 July 2023 12:21 UTC

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