# Is it possible to dynamically set length for 'buffer maker' and ' buffer parser'

**URL:** <https://discourse.nodered.org/t/is-it-possible-to-dynamically-set-length-for-buffer-maker-and-buffer-parser/74134>\
**Category:** General\
**Created:** [24 January 2023 18:43 UTC](https://discourse.nodered.org/t/is-it-possible-to-dynamically-set-length-for-buffer-maker-and-buffer-parser/74134 "2023-01-24T18:43:26Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Dyslexicbloke](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@Dyslexicbloke](https://discourse.nodered.org/u/Dyslexicbloke)\
**Post date:** [24 January 2023 18:43 UTC](https://discourse.nodered.org/t/is-it-possible-to-dynamically-set-length-for-buffer-maker-and-buffer-parser/74134/1 "2023-01-24T18:43:26Z")

</div>

Hi folks,  
I am putting together a flow to read and write 32bit floats from/to a PLC using Modbus TCP.

To be honest it took mw way longer than it should have to get my head around the buffer manipulation nodes but I have them working now and can read and right floats... Which is nice!

However if I want to change the quantity of data read I have to alter the trigger for the flex\_modbus node and also adjust the lengths for both the buffer nodes.

For context my read ...  
my read gets 'some', lets say 6, words from the PLC, these are 'raw' values which I treat as uint16.  
This data is used to create a buffer, 12 bytes long.  
The buffer is then interpreted as a float32, which is how the PLC is interpreting the same data.

Writing out is a similar process that prepares an array of two uint16 for the flex\_write node.

As I said both processes work but I would ideally like to create sub flow that wrap the process in much the same way as the flex-modbus nodes work.

I have tried copying the 'length' path and setting that with a change node but couldn't get it to work.

 ![flow](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/5/8/58d9b79b31d2f0e084c58ca977ec09067bf703d0.png)

```auto
[
    {
        "id": "8f6909ec361ec4c9",
        "type": "tab",
        "label": "W/R int32 using Modbus",
        "disabled": false,
        "info": "",
        "env": []
    },
    {
        "id": "ac265d7f498243d9",
        "type": "debug",
        "z": "8f6909ec361ec4c9",
        "name": "debug 20",
        "active": false,
        "tosidebar": true,
        "console": false,
        "tostatus": false,
        "complete": "true",
        "targetType": "full",
        "statusVal": "",
        "statusType": "auto",
        "x": 1140,
        "y": 120,
        "wires": []
    },
    {
        "id": "08ac5471b622f9c9",
        "type": "modbus-flex-getter",
        "z": "8f6909ec361ec4c9",
        "name": "",
        "showStatusActivities": false,
        "showErrors": false,
        "showWarnings": true,
        "logIOActivities": false,
        "server": "0ca896640dce56e2",
        "useIOFile": false,
        "ioFile": "",
        "useIOForPayload": false,
        "emptyMsgOnFail": false,
        "keepMsgProperties": false,
        "delayOnStart": true,
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        "y": 360,
        "wires": [
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                "cbe085c3e3b99e77"
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            []
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    },
    {
        "id": "e6a52ab120b8f69b",
        "type": "modbus-flex-write",
        "z": "8f6909ec361ec4c9",
        "name": "",
        "showStatusActivities": false,
        "showErrors": false,
        "showWarnings": true,
        "server": "0ca896640dce56e2",
        "emptyMsgOnFail": false,
        "keepMsgProperties": false,
        "delayOnStart": false,
        "startDelayTime": "",
        "x": 970,
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        "wires": [
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                "t": "delete",
                "p": "payload",
                "pt": "msg"
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            {
                "t": "set",
                "p": "payload.unitid",
                "pt": "msg",
                "to": "255",
                "tot": "num"
            },
            {
                "t": "set",
                "p": "payload.fc",
                "pt": "msg",
                "to": "modbus.wrd.fc_wn",
                "tot": "global"
            },
            {
                "t": "set",
                "p": "payload.address",
                "pt": "msg",
                "to": "0",
                "tot": "num"
            },
            {
                "t": "set",
                "p": "payload.quantity",
                "pt": "msg",
                "to": "2",
                "tot": "num"
            },
            {
                "t": "set",
                "p": "payload.value",
                "pt": "msg",
                "to": "value",
                "tot": "msg"
            }
        ],
        "action": "",
        "property": "",
        "from": "",
        "to": "",
        "reg": false,
        "x": 780,
        "y": 120,
        "wires": [
            [
                "e6a52ab120b8f69b"
            ]
        ],
        "info": "{ value: msg.payload, \r\n'fc': 5, \r\n'unitid': 1, \r\n'address': 0 , \r\n'quantity': 1 }"
    },
    {
        "id": "1280270bd9f81893",
        "type": "inject",
        "z": "8f6909ec361ec4c9",
        "name": "",
        "props": [
            {
                "p": "payload"
            }
        ],
        "repeat": "",
        "crontab": "",
        "once": false,
        "onceDelay": 0.1,
        "topic": "",
        "payload": "70000",
        "payloadType": "num",
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    },
    {
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        "type": "inject",
        "z": "8f6909ec361ec4c9",
        "name": "",
        "props": [
            {
                "p": "payload.connectorType",
                "v": "TCP",
                "vt": "str"
            }
        ],
        "repeat": "",
        "crontab": "",
        "once": false,
        "onceDelay": 0.1,
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        "wires": [
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        "type": "change",
        "z": "8f6909ec361ec4c9",
        "name": "",
        "rules": [
            {
                "t": "set",
                "p": "payload.unitid",
                "pt": "msg",
                "to": "255",
                "tot": "num"
            },
            {
                "t": "set",
                "p": "payload.fc",
                "pt": "msg",
                "to": "modbus.wrd.fc_rn",
                "tot": "global"
            },
            {
                "t": "set",
                "p": "payload.address",
                "pt": "msg",
                "to": "0",
                "tot": "num"
            },
            {
                "t": "set",
                "p": "payload.quantity",
                "pt": "msg",
                "to": "6",
                "tot": "num"
            }
        ],
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        "reg": false,
        "x": 380,
        "y": 360,
        "wires": [
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        ],
        "info": "{ value: msg.payload, \r\n'fc': 5, \r\n'unitid': 1, \r\n'address': 0 , \r\n'quantity': 1 }"
    },
    {
        "id": "5c2edcb1a6eb0eba",
        "type": "buffer-parser",
        "z": "8f6909ec361ec4c9",
        "name": "",
        "data": "payload",
        "dataType": "msg",
        "specification": "spec",
        "specificationType": "ui",
        "items": [
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                "name": "item2",
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                "offsetbit": 0,
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            }
        ],
        "swap1": "",
        "swap2": "",
        "swap3": "",
        "swap1Type": "swap",
        "swap2Type": "swap",
        "swap3Type": "swap",
        "msgProperty": "value",
        "msgPropertyType": "str",
        "resultType": "value",
        "resultTypeType": "return",
        "multipleResult": false,
        "fanOutMultipleResult": false,
        "setTopic": true,
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    {
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        "specificationType": "ui",
        "items": [
            {
                "name": "item1",
                "type": "int32be",
                "length": 1,
                "dataType": "msg",
                "data": "payload"
            }
        ],
        "swap1": "",
        "swap2": "",
        "swap3": "",
        "swap1Type": "swap",
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        "swap3Type": "swap",
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        "x": 370,
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    },
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        "active": false,
        "tosidebar": true,
        "console": false,
        "tostatus": false,
        "complete": "true",
        "targetType": "full",
        "statusVal": "",
        "statusType": "auto",
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        "y": 160,
        "wires": []
    },
    {
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    {
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    {
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    },
    {
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        "showErrors": false,
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        "server": "0ca896640dce56e2",
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    {
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        "type": "change",
        "z": "8f6909ec361ec4c9",
        "name": "",
        "rules": [
            {
                "t": "delete",
                "p": "payload",
                "pt": "msg"
            },
            {
                "t": "set",
                "p": "payload.unitid",
                "pt": "msg",
                "to": "255",
                "tot": "num"
            },
            {
                "t": "set",
                "p": "payload.fc",
                "pt": "msg",
                "to": "modbus.wrd.fc_wn",
                "tot": "global"
            },
            {
                "t": "set",
                "p": "payload.address",
                "pt": "msg",
                "to": "0",
                "tot": "num"
            },
            {
                "t": "set",
                "p": "payload.quantity",
                "pt": "msg",
                "to": "2",
                "tot": "num"
            },
            {
                "t": "set",
                "p": "payload.value",
                "pt": "msg",
                "to": "value",
                "tot": "msg"
            }
        ],
        "action": "",
        "property": "",
        "from": "",
        "to": "",
        "reg": false,
        "x": 780,
        "y": 220,
        "wires": [
            [
                "5ece56966b7c67b7"
            ]
        ],
        "info": "{ value: msg.payload, \r\n'fc': 5, \r\n'unitid': 1, \r\n'address': 0 , \r\n'quantity': 1 }"
    },
    {
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        "props": [
            {
                "p": "payload"
            }
        ],
        "repeat": "",
        "crontab": "",
        "once": false,
        "onceDelay": 0.1,
        "topic": "",
        "payload": "-3.142",
        "payloadType": "num",
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        "wires": [
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    },
    {
        "id": "964ddf22a39acc72",
        "type": "buffer-parser",
        "z": "8f6909ec361ec4c9",
        "name": "",
        "data": "payload",
        "dataType": "msg",
        "specification": "spec",
        "specificationType": "ui",
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                "type": "uint16be",
                "name": "item2",
                "offset": 2,
                "length": 1,
                "offsetbit": 0,
                "scale": "1",
                "mask": ""
            },
            {
                "type": "uint16be",
                "name": "item1",
                "offset": 0,
                "length": 1,
                "offsetbit": 0,
                "scale": "1",
                "mask": ""
            }
        ],
        "swap1": "",
        "swap2": "",
        "swap3": "",
        "swap1Type": "swap",
        "swap2Type": "swap",
        "swap3Type": "swap",
        "msgProperty": "value",
        "msgPropertyType": "str",
        "resultType": "value",
        "resultTypeType": "return",
        "multipleResult": false,
        "fanOutMultipleResult": false,
        "setTopic": true,
        "outputs": 1,
        "x": 550,
        "y": 220,
        "wires": [
            [
                "8a56fe0f46ed61d6",
                "d6fa49d4481da465"
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    },
    {
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        "type": "buffer-maker",
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        "name": "",
        "specification": "spec",
        "specificationType": "ui",
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            {
                "name": "item1",
                "type": "floatbe",
                "length": 1,
                "dataType": "msg",
                "data": "payload"
            }
        ],
        "swap1": "",
        "swap2": "",
        "swap3": "",
        "swap1Type": "swap",
        "swap2Type": "swap",
        "swap3Type": "swap",
        "msgProperty": "payload",
        "msgPropertyType": "str",
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        "wires": [
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                "d966885f3eab5331"
            ]
        ]
    },
    {
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        "tostatus": false,
        "complete": "true",
        "targetType": "full",
        "statusVal": "",
        "statusType": "auto",
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        "wires": []
    },
    {
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    {
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            }
        ],
        "swap1": "",
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        "swap3": "",
        "swap1Type": "swap",
        "swap2Type": "swap",
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        "msgPropertyType": "str",
        "resultType": "keyvalue",
        "resultTypeType": "return",
        "multipleResult": false,
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        "wires": [
            [
                "255f8b7a82b99a42"
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        ]
    },
    {
        "id": "07802434ebcdba9f",
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        "z": "8f6909ec361ec4c9",
        "name": "",
        "specification": "spec",
        "specificationType": "ui",
        "items": [
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                "name": "item1",
                "type": "uint16be",
                "length": 6,
                "dataType": "msg",
                "data": "payload"
            }
        ],
        "swap1": "swap32",
        "swap2": "swap16",
        "swap3": "",
        "swap1Type": "swap",
        "swap2Type": "swap",
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    {
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    {
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        "name": "debug 30",
        "active": true,
        "tosidebar": true,
        "console": false,
        "tostatus": false,
        "complete": "payload",
        "targetType": "msg",
        "statusVal": "",
        "statusType": "auto",
        "x": 1140,
        "y": 360,
        "wires": []
    },
    {
        "id": "0ca896640dce56e2",
        "type": "modbus-client",
        "name": "bycplc",
        "clienttype": "tcp",
        "bufferCommands": true,
        "stateLogEnabled": false,
        "queueLogEnabled": false,
        "failureLogEnabled": true,
        "tcpHost": "192.168.10.19",
        "tcpPort": "502",
        "tcpType": "DEFAULT",
        "serialPort": "/dev/ttyUSB",
        "serialType": "RTU-BUFFERD",
        "serialBaudrate": "9600",
        "serialDatabits": "8",
        "serialStopbits": "1",
        "serialParity": "none",
        "serialConnectionDelay": "100",
        "serialAsciiResponseStartDelimiter": "0x3A",
        "unit_id": "255",
        "commandDelay": "5",
        "clientTimeout": "500",
        "reconnectOnTimeout": true,
        "reconnectTimeout": "5000",
        "parallelUnitIdsAllowed": true,
        "showWarnings": true,
        "showLogs": true
    }
]

```

Thanks for looking,  
Al

---

<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:** [24 January 2023 19:39 UTC](https://discourse.nodered.org/t/is-it-possible-to-dynamically-set-length-for-buffer-maker-and-buffer-parser/74134/2 "2023-01-24T19:39:34Z")

</div>

There are two possible solutions

1. If all of the values to be parsed are of the same type then set length to -1 and it will detect the length of data and return as many of the specified type values as possible.
2. The parser node itself can be 100% automated via dynamic specification. Ie you can send the parser specification in the message along with the data.

See the built-in help and check out the built-in examples (`ctrl-i` -\> examples)

---

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**Author:** ![Dyslexicbloke](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@Dyslexicbloke](https://discourse.nodered.org/u/Dyslexicbloke)\
**Post date:** [24 January 2023 20:03 UTC](https://discourse.nodered.org/t/is-it-possible-to-dynamically-set-length-for-buffer-maker-and-buffer-parser/74134/3 "2023-01-24T20:03:52Z")

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Cool... Feeling a little silly though as I looked at the help and didn't see the bit that explained using a dynamic spec.  
I had realised it was the spec I needed to update, that was how I found it didn't work!

As all the values are the same length/spec, always, I will give the -1, something else I missed apparently, a go....

You star... that works great, thanks a bunch 🤩

I need to get better at reading the help... Sorry

Cheers,  
Al

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**Author:** ![system](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/1X/d073cd938eafa2e558d7c2cd59003b3ef4963033.png) [@system](https://discourse.nodered.org/u/system)\
**Post date:** [7 February 2023 20:04 UTC](https://discourse.nodered.org/t/is-it-possible-to-dynamically-set-length-for-buffer-maker-and-buffer-parser/74134/4 "2023-02-07T20:04:21Z")

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