# Writing to Data Block in S7 1200 PLC via node-red-contrib-s7

**URL:** <https://discourse.nodered.org/t/writing-to-data-block-in-s7-1200-plc-via-node-red-contrib-s7/98259>\
**Category:** Industrial\
**Created:** [23 July 2025 04:27 UTC](https://discourse.nodered.org/t/writing-to-data-block-in-s7-1200-plc-via-node-red-contrib-s7/98259 "2025-07-23T04:27:49Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![pathum](https://avatars.discourse-cdn.com/v4/letter/p/c77e96/32.png) [@pathum](https://discourse.nodered.org/u/pathum)\
**Post date:** [23 July 2025 04:27 UTC](https://discourse.nodered.org/t/writing-to-data-block-in-s7-1200-plc-via-node-red-contrib-s7/98259/1 "2025-07-23T04:27:49Z")

</div>

Hello,  
I am trying to read and write to a s7 1200 PLC Datablock.  
I have sucessfully able to read the content of DB1 which contains a array "Test" which contains 20 String variabvles. See the image.

 ![Screenshot_19](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/c/9/c9cd8ef7cd42afe181d42197a361287198b75ce1.png)  
Folowing is the json for reading

```flows
[
    {
        "id": "defa031d7223bc60",
        "type": "s7 in",
        "z": "4606f8b0d0a8d4e9",
        "endpoint": "2f70a4039f09ee82",
        "mode": "single",
        "variable": "Andon Call Test",
        "diff": false,
        "name": "Andon Call Block Read",
        "x": 280,
        "y": 1020,
        "wires": [
            [
                "a35cfb21a3a49967",
                "3c06c3e94034441b"
            ]
        ]
    },
    {
        "id": "78b3252143da9aad",
        "type": "debug",
        "z": "4606f8b0d0a8d4e9",
        "name": "debug 60",
        "active": true,
        "tosidebar": true,
        "console": false,
        "tostatus": false,
        "complete": "payload",
        "targetType": "msg",
        "statusVal": "",
        "statusType": "auto",
        "x": 740,
        "y": 1020,
        "wires": []
    },
    {
        "id": "a35cfb21a3a49967",
        "type": "function",
        "z": "4606f8b0d0a8d4e9",
        "name": "function 8",
        "func": "const buf = msg.payload;\nconst totalStrings = 21; // Number of strings\nconst stringSize = 256; // Size of each string (padded to 256 bytes)\n\nconst parsedStrings = [];\n\nfor (let i = 0; i < totalStrings; i++) {\n const offset = i * stringSize; // Get the offset for the current string\n const actualLen = buf[offset + 1]; // Byte 1 is the actual length of the string\n\n if (actualLen > 0) {\n // Extract the byte array for the string data\n const stringBytes = buf.slice(offset + 2, offset + 2 + actualLen);\n\n // Convert the byte array into a string using ASCII encoding\n let str = '';\n for (let byte of stringBytes) {\n str += String.fromCharCode(byte);\n }\n\n parsedStrings.push(str); // Push the string into the parsedStrings array\n } else {\n parsedStrings.push(''); // Empty string if actual length is 0\n }\n}\n\nmsg.payload = parsedStrings; // Set the parsed strings as payload\nreturn msg;\n",
        "outputs": 1,
        "timeout": 0,
        "noerr": 0,
        "initialize": "",
        "finalize": "",
        "libs": [],
        "x": 500,
        "y": 1020,
        "wires": [
            [
                "78b3252143da9aad"
            ]
        ]
    },
    {
        "id": "3c06c3e94034441b",
        "type": "debug",
        "z": "4606f8b0d0a8d4e9",
        "name": "debug 65",
        "active": true,
        "tosidebar": true,
        "console": false,
        "tostatus": false,
        "complete": "true",
        "targetType": "full",
        "statusVal": "",
        "statusType": "auto",
        "x": 520,
        "y": 980,
        "wires": []
    },
    {
        "id": "2f70a4039f09ee82",
        "type": "s7 endpoint",
        "transport": "iso-on-tcp",
        "address": "10.243.13.61",
        "port": "102",
        "rack": "0",
        "slot": "1",
        "localtsaphi": "01",
        "localtsaplo": "00",
        "remotetsaphi": "01",
        "remotetsaplo": "00",
        "connmode": "rack-slot",
        "adapter": "",
        "busaddr": 2,
        "cycletime": "1000",
        "timeout": 2000,
        "name": "",
        "vartable": [
            {
                "addr": "DB1,B0.5375",
                "name": "Andon Call Test"
            },
            {
                "addr": "DB1,B5376.100",
                "name": "Andon Call Test 1"
            }
        ]
    }
]

```

Now I am trying to write but Cant figure out why S7 Out node Saying

> msg : error  
> "Error: Expected [5375] values for this item"  
> **Incomming msg for S7 Out looks like this**  
> msg : Object
> 
> object
> 
> payload: buffer[5376]
> 
> [0 … 9]
> 
> 0: 0x0
> 
> 1: 0x13
> 
> 2: 0x53
> 
> 3: 0x65
> 
> 4: 0x6e
> 
> 5: 0x73
> 
> 6: 0x6f
> 
> 7: 0x72
> 
> 8: 0x20
> 
> 9: 0x44
> 
> [10 … 19]
> 
> 10: 0x69
> 
> 11: 0x73
> 
> 12: 0x63
> 
> 13: 0x6f
> 
> 14: 0x6e
> 
> 15: 0x6e
> 
> 16: 0x65
> 
> 17: 0x63
> 
> 18: 0x74
> 
> 19: 0x65

Folowing is the code

```flows
[
    {
        "id": "e5cb46dbef0bcd4e",
        "type": "s7 out",
        "z": "4606f8b0d0a8d4e9",
        "endpoint": "2f70a4039f09ee82",
        "variable": "Andon Call Test",
        "name": "Andon Call Write",
        "x": 810,
        "y": 840,
        "wires": []
    },
    {
        "id": "5d183a28c9ef8549",
        "type": "MSSQL",
        "z": "4606f8b0d0a8d4e9",
        "mssqlCN": "814fd0e4912a79ae",
        "name": "LKEK1DB01",
        "outField": "payload",
        "returnType": "1",
        "throwErrors": "0",
        "query": "INSERT INTO [NodeRed].[dbo].[EventCall]\r\n (MachineID, ErrorID, UserID, RecordTime, Status, Shift, Comments, Severity)\r\nVALUES\r\n (\r\n {{payload.MachineID}},\r\n {{payload.ErrorID}},\r\n {{payload.UserID}},\r\n '{{payload.RecordTime}}',\r\n '{{payload.Status}}',\r\n '{{payload.Shift}}',\r\n '{{payload.Comments}}',\r\n '{{payload.Severity}}'\r\n )\r\n",
        "modeOpt": "",
        "modeOptType": "query",
        "queryOpt": "payload",
        "queryOptType": "msg",
        "paramsOpt": "",
        "paramsOptType": "none",
        "rows": "rows",
        "rowsType": "msg",
        "parseMustache": true,
        "params": [
            {
                "output": false,
                "name": "param1",
                "type": "int",
                "valueType": "num",
                "value": "0",
                "options": {
                    "nullable": true,
                    "primary": false,
                    "identity": false,
                    "readOnly": false
                }
            }
        ],
        "x": 370,
        "y": 840,
        "wires": [
            [
                "51ba80916f44af68",
                "a75ed45029b6f074"
            ]
        ]
    },
    {
        "id": "e43002f3de7f33bf",
        "type": "function",
        "z": "4606f8b0d0a8d4e9",
        "name": "function 9",
        "func": "msg.payload = `SELECT [ErrorName]\n FROM [ETD1 Andon Call System].[dbo].[ErrorType]`\n return msg;",
        "outputs": 1,
        "timeout": 0,
        "noerr": 0,
        "initialize": "",
        "finalize": "",
        "libs": [],
        "x": 220,
        "y": 840,
        "wires": [
            [
                "5d183a28c9ef8549"
            ]
        ]
    },
    {
        "id": "aa034cc6676389d8",
        "type": "inject",
        "z": "4606f8b0d0a8d4e9",
        "name": "",
        "props": [
            {
                "p": "payload"
            },
            {
                "p": "topic",
                "vt": "str"
            }
        ],
        "repeat": "",
        "crontab": "",
        "once": false,
        "onceDelay": 0.1,
        "topic": "",
        "payload": "",
        "payloadType": "date",
        "x": 80,
        "y": 840,
        "wires": [
            [
                "e43002f3de7f33bf"
            ]
        ]
    },
    {
        "id": "51ba80916f44af68",
        "type": "debug",
        "z": "4606f8b0d0a8d4e9",
        "name": "debug 61",
        "active": false,
        "tosidebar": true,
        "console": false,
        "tostatus": false,
        "complete": "true",
        "targetType": "full",
        "statusVal": "",
        "statusType": "auto",
        "x": 540,
        "y": 800,
        "wires": []
    },
    {
        "id": "a75ed45029b6f074",
        "type": "function",
        "z": "4606f8b0d0a8d4e9",
        "name": "function 10",
        "func": "var Output = {};\nconst errorMessages = msg.payload.recordsets[0].map(item => item.ErrorName);\n\n// Config\nconst totalStrings = 21;\nconst stringSize = 256;\nconst totalSize = totalStrings * stringSize; // 5376\nconst buffer = Buffer.alloc(totalSize); // Full buffer\n\nfor (let i = 0; i < totalStrings; i++) {\n const offset = i * stringSize;\n const message = errorMessages[i] || \"\";\n const len = Math.min(message.length, stringSize - 2);\n\n buffer.writeUInt8(0, offset); // Byte 0: unused/reserved\n buffer.writeUInt8(len, offset + 1); // Byte 1: string length\n\n for (let j = 0; j < len; j++) {\n buffer.writeUInt8(message.charCodeAt(j), offset + 2 + j);\n }\n}\n\n// ✅ Send exactly 5376 bytes\nOutput.payload = buffer;\nOutput.variable = \"Andon Call\";\nreturn Output;",
        "outputs": 1,
        "timeout": 0,
        "noerr": 0,
        "initialize": "",
        "finalize": "",
        "libs": [],
        "x": 590,
        "y": 840,
        "wires": [
            [
                "59a8b52b9686d3c9",
                "e5cb46dbef0bcd4e"
            ]
        ]
    },
    {
        "id": "59a8b52b9686d3c9",
        "type": "debug",
        "z": "4606f8b0d0a8d4e9",
        "name": "debug 62",
        "active": true,
        "tosidebar": true,
        "console": false,
        "tostatus": false,
        "complete": "true",
        "targetType": "full",
        "statusVal": "",
        "statusType": "auto",
        "x": 740,
        "y": 800,
        "wires": []
    },
    {
        "id": "2f70a4039f09ee82",
        "type": "s7 endpoint",
        "transport": "iso-on-tcp",
        "address": "10.243.13.61",
        "port": "102",
        "rack": "0",
        "slot": "1",
        "localtsaphi": "01",
        "localtsaplo": "00",
        "remotetsaphi": "01",
        "remotetsaplo": "00",
        "connmode": "rack-slot",
        "adapter": "",
        "busaddr": 2,
        "cycletime": "1000",
        "timeout": 2000,
        "name": "",
        "vartable": [
            {
                "addr": "DB1,B0.5375",
                "name": "Andon Call Test"
            },
            {
                "addr": "DB1,B5376.100",
                "name": "Andon Call Test 1"
            }
        ]
    },
    {
        "id": "814fd0e4912a79ae",
        "type": "MSSQL-CN",
        "tdsVersion": "7_4",
        "name": "SQL LKEK1DB01",
        "server": "LKEK1DB01.CSL.LOCAL\\ETD1_ANDON_CALL",
        "port": "1443",
        "encyption": false,
        "trustServerCertificate": true,
        "database": "ETD1 Andon Call System",
        "useUTC": false,
        "connectTimeout": "1500",
        "requestTimeout": "1500",
        "cancelTimeout": "5000",
        "pool": "5",
        "parseJSON": false,
        "enableArithAbort": true,
        "readOnlyIntent": false
    }
]

```

---

<div class="post-metadata">

**Author:** ![sumitshinde-84](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/sumitshinde-84/32/83795_2.png) [@sumitshinde-84](https://discourse.nodered.org/u/sumitshinde-84)\
**Post date:** [23 July 2025 07:59 UTC](https://discourse.nodered.org/t/writing-to-data-block-in-s7-1200-plc-via-node-red-contrib-s7/98259/2 "2025-07-23T07:59:41Z")

</div>

> [@pathum](#):
>
> Hello,  
> I am trying to read and write to a s7 1200 PLC Datablock.  
> I have sucessfully able to read the content of DB1 which contains a array "Test" which contains 20 String variabvles. See the image.
> 
> ![Screenshot_19](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/c/9/c9cd8ef7cd42afe181d42197a361287198b75ce1.png)
> 
> Folowing is the json for reading
> 
> Flow 1
> 
> Andon Call Block Readdebug 60function 8debug 65
> 
> CopyDownload
> 
> -○+
> 
> Now I am trying to write but Cant figure out why S7 Out node Saying
> 
> > msg : error  
> > "Error: Expected [5375] values for this item"  
> > **Incomming msg for S7 Out looks like this**  
> > msg : Object
> > 
> > object
> > 
> > payload: buffer[5376]
> > 
> > [0 … 9]
> > 
> > 0: 0x0
> > 
> > 1: 0x13
> > 
> > 2: 0x53
> > 
> > 3: 0x65
> > 
> > 4: 0x6e
> > 
> > 5: 0x73
> > 
> > 6: 0x6f
> > 
> > 7: 0x72
> > 
> > 8: 0x20
> > 
> > 9: 0x44
> > 
> > [10 … 19]
> > 
> > 10: 0x69
> > 
> > 11: 0x73
> > 
> > 12: 0x63
> > 
> > 13: 0x6f
> > 
> > 14: 0x6e
> > 
> > 15: 0x6e
> > 
> > 16: 0x65
> > 
> > 17: 0x63
> > 
> > 18: 0x74
> > 
> > 19: 0x65
> 
> Folowing is the code

It looks like the error message is indicating a mismatch between the expected size of the buffer and what is being sent:

> **Error: Expected [5375] values for this item**

This usually happens when the S7 Out node expects an exact number of bytes (5375 in this case) but receives 5376. You're likely sending 1 extra byte in the buffer. Make sure the buffer size matches exactly what the node expects.

Also, for more insights into working with S7 PLCs in Node-RED, I’ve written a blog that might help with related issues: [Getting Started: Integrating Siemens S7 PLCs with Node-RED • FlowFuse](https://flowfuse.com/blog/2025/01/integrating-siemens-s7-plcs-with-node-red-guide/)

Hope this helps!

---

<div class="post-metadata">

**Author:** ![pathum](https://avatars.discourse-cdn.com/v4/letter/p/c77e96/32.png) [@pathum](https://discourse.nodered.org/u/pathum)\
**Post date:** [23 July 2025 08:25 UTC](https://discourse.nodered.org/t/writing-to-data-block-in-s7-1200-plc-via-node-red-contrib-s7/98259/3 "2025-07-23T08:25:36Z")

</div>

Hello Sumitshinde,  
Thanks you for replying.I've read your article several times before. It was very helpful. I did try the suggestions you mentioned earlier, but unfortunately, they didn't work in my case. I experimented with flat arrays, buffers, and regular arrays, but none of them seemed to resolve the issue.

Do you have any other ideas or suggestions I could try?

 ![Screenshot_1](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/b/9/b9a7d8bd2a8947710a94d9c27915c1317f606413.png)

---

<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:** [21 September 2025 08:26 UTC](https://discourse.nodered.org/t/writing-to-data-block-in-s7-1200-plc-via-node-red-contrib-s7/98259/4 "2025-09-21T08:26:23Z")

</div>

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