# Serial port loopback test RPI

**URL:** <https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284>\
**Category:** General\
**Created:** [15 December 2022 20:13 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284 "2022-12-15T20:13:29Z")\
**Posts on this page:** 20\
**Page:** 1

<div class="post-metadata">

**Author:** ![devifast](https://avatars.discourse-cdn.com/v4/letter/d/43a26b/32.png) [@devifast](https://discourse.nodered.org/u/devifast)\
**Post date:** [15 December 2022 20:13 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/1 "2022-12-15T20:13:29Z")

</div>

How to test serial port ttyAMA0  
on RPI by connecting RX and TX. with wire? Thanks.  
 ![Untitled1](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/c/c/ccbdcd706a72675ccdad3df29aa58cc79cb6d5d3.png)  
There is no communication between the two.

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [15 December 2022 20:55 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/2 "2022-12-15T20:55:44Z")

</div>

Export the four nodes and paste them here so we can see how you have configured them.

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<div class="post-metadata">

**Author:** ![devifast](https://avatars.discourse-cdn.com/v4/letter/d/43a26b/32.png) [@devifast](https://discourse.nodered.org/u/devifast)\
**Post date:** [17 December 2022 12:48 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/3 "2022-12-17T12:48:14Z")

</div>

```auto
[
    {
        "id": "7806614b4a6f9ddd",
        "type": "tab",
        "label": "Flow 1",
        "disabled": false,
        "info": "",
        "env": []
    },
    {
        "id": "0f6be78ea759416a",
        "type": "serial in",
        "z": "7806614b4a6f9ddd",
        "name": "",
        "serial": "11234b1dfd906877",
        "x": 530,
        "y": 100,
        "wires": [
            [
                "7b1ddc679dba86e0"
            ]
        ]
    },
    {
        "id": "74af457044185c4e",
        "type": "serial out",
        "z": "7806614b4a6f9ddd",
        "name": "",
        "serial": "11234b1dfd906877",
        "x": 350,
        "y": 100,
        "wires": []
    },
    {
        "id": "792d3f6a9589ccf4",
        "type": "inject",
        "z": "7806614b4a6f9ddd",
        "name": "",
        "props": [
            {
                "p": "payload"
            },
            {
                "p": "topic",
                "vt": "str"
            }
        ],
        "repeat": "",
        "crontab": "",
        "once": false,
        "onceDelay": 0.1,
        "topic": "",
        "payload": "test_serial",
        "payloadType": "str",
        "x": 160,
        "y": 100,
        "wires": [
            [
                "74af457044185c4e"
            ]
        ]
    },
    {
        "id": "7b1ddc679dba86e0",
        "type": "debug",
        "z": "7806614b4a6f9ddd",
        "name": "debug 1",
        "active": true,
        "tosidebar": true,
        "console": false,
        "tostatus": false,
        "complete": "false",
        "statusVal": "",
        "statusType": "auto",
        "x": 700,
        "y": 100,
        "wires": []
    },
    {
        "id": "11234b1dfd906877",
        "type": "serial-port",
        "serialport": "/dev/ttyAMA0",
        "serialbaud": "57600",
        "databits": "8",
        "parity": "none",
        "stopbits": "1",
        "waitfor": "",
        "dtr": "none",
        "rts": "none",
        "cts": "none",
        "dsr": "none",
        "newline": "\\n",
        "bin": "false",
        "out": "char",
        "addchar": "",
        "responsetimeout": "10000"
    }
]

```

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<div class="post-metadata">

**Author:** ![devifast](https://avatars.discourse-cdn.com/v4/letter/d/43a26b/32.png) [@devifast](https://discourse.nodered.org/u/devifast)\
**Post date:** [17 December 2022 12:52 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/4 "2022-12-17T12:52:52Z")

</div>

![serialnodered](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/5/8/5885b14b7899808dbb2482ba2649f2cf546cc4a7.png)

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [17 December 2022 13:50 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/5 "2022-12-17T13:50:57Z")

</div>

You have told the serial node to wait for `\n` before passing on the data, but you are not sending any `\n` characters.

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<div class="post-metadata">

**Author:** ![devifast](https://avatars.discourse-cdn.com/v4/letter/d/43a26b/32.png) [@devifast](https://discourse.nodered.org/u/devifast)\
**Post date:** [17 December 2022 14:00 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/6 "2022-12-17T14:00:00Z")

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If you can please return me the code with the correct settings. Thanks.

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [17 December 2022 14:01 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/7 "2022-12-17T14:01:37Z")

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In fact, rather than splitting on a character, as a test, change that option to use a timeout and see if you get anything then.

![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/1/0/10f5b811618ea04b4fa04c02ba8c66c1de8c633b.png)

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<div class="post-metadata">

**Author:** ![devifast](https://avatars.discourse-cdn.com/v4/letter/d/43a26b/32.png) [@devifast](https://discourse.nodered.org/u/devifast)\
**Post date:** [17 December 2022 14:06 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/8 "2022-12-17T14:06:33Z")

</div>

![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/c/6/c6a8bea660aaf4d987149e32aa7fa5a1e93af2b7.png)  
No data again.

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [17 December 2022 14:15 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/9 "2022-12-17T14:15:08Z")

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Does it work if you use something like minicom to drive the serial port? See this, for example.

> **[Serial Port Loopback Test](https://www.abelectronics.co.uk/kb/article/1110/serial-port-loopback-test)**
>
> Testing the status of the Serial Port on a Raspberry Pi in the AB Electronics UK Knowledge Base

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<div class="post-metadata">

**Author:** ![devifast](https://avatars.discourse-cdn.com/v4/letter/d/43a26b/32.png) [@devifast](https://discourse.nodered.org/u/devifast)\
**Post date:** [17 December 2022 15:16 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/10 "2022-12-17T15:16:35Z")

</div>

![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/a/1/a1f4f032cf4c72ab35d9eef928cfd274e2a001cb.png)  
This is the result. The link turned out to be very useful. Thanks. But problem with Node red is not solved yet.

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [17 December 2022 15:42 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/11 "2022-12-17T15:42:58Z")

</div>

What command line did you use for that?

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<div class="post-metadata">

**Author:** ![devifast](https://avatars.discourse-cdn.com/v4/letter/d/43a26b/32.png) [@devifast](https://discourse.nodered.org/u/devifast)\
**Post date:** [17 December 2022 16:06 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/12 "2022-12-17T16:06:18Z")

</div>

PuTTY terminal. I'm sure it's working because when I remove the short I get no characters.

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [17 December 2022 16:11 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/13 "2022-12-17T16:11:43Z")

</div>

Try it in node-red using the Request node instead of separate In and Out. If you are sending a message and expecting a reply that is the one to use anyway.

There was a similar post recently where someone could not get it going with separate in and out.

Make sure you stop putty before starting node-red.

How did you install node-red?

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<div class="post-metadata">

**Author:** ![devifast](https://avatars.discourse-cdn.com/v4/letter/d/43a26b/32.png) [@devifast](https://discourse.nodered.org/u/devifast)\
**Post date:** [17 December 2022 16:17 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/14 "2022-12-17T16:17:54Z")

</div>

![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/2/1/2143bf02d6ed44c517853796d7f2f73d80590a5f.png)

Yes, that was what was expected. Now I'm sure everything works. Thanks.

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [17 December 2022 16:28 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/15 "2022-12-17T16:28:56Z")

</div>

@knolleary do you want to comment on this? Apparently serial loopback would not work using separate In and Out nodes, but does using a Request node.

I was sure there was a similar issue recently, but can't seem to find it. I can't test it myself as I haven't got any appropriate hardware. Perhaps someone else can try it.

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<div class="post-metadata">

**Author:** ![knolleary](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/knolleary/32/3_2.png) [@knolleary](https://discourse.nodered.org/u/knolleary)\
**Post date:** [17 December 2022 16:43 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/16 "2022-12-17T16:43:28Z")

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> [@Colin](#):
>
> [knolleary](https://discourse.nodered.org/u/knolleary) do you want to comment on this?

I have not used the serial node for a long time. If there's an issue please raise it in the usual manner. @dceejay is more likely to be in a position to investigate.

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<div class="post-metadata">

**Author:** ![devifast](https://avatars.discourse-cdn.com/v4/letter/d/43a26b/32.png) [@devifast](https://discourse.nodered.org/u/devifast)\
**Post date:** [17 December 2022 17:24 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/17 "2022-12-17T17:24:10Z")

</div>

What exactly do you mean, connection between two RPIs? If so, I'll try and find an answer.

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [17 December 2022 18:25 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/18 "2022-12-17T18:25:32Z")

</div>

> [@devifast](#):
>
> What exactly do you mean, connection between two RPIs

Sorry I don't know where I said that.

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<div class="post-metadata">

**Author:** ![devifast](https://avatars.discourse-cdn.com/v4/letter/d/43a26b/32.png) [@devifast](https://discourse.nodered.org/u/devifast)\
**Post date:** [17 December 2022 18:44 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/19 "2022-12-17T18:44:10Z")

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> [@Colin](#):
>
> Sorry I don't know where I said that.

The idea is to connect two separate devices and find out if the individual nodes work. Sorry for my bad English but I use a translator because I can't translate or write everything myself.

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [17 December 2022 20:31 UTC](https://discourse.nodered.org/t/serial-port-loopback-test-rpi/72284/20 "2022-12-17T20:31:28Z")

</div>

> [@devifast](#):
>
> The idea is to connect two separate devices and find out if the individual nodes work

That would be a good test, if you have two devices to try it on. Start with only an Out node on one and an In node on the other.

> [@devifast](#):
>
> I use a translator because I can't translate or write everything myself.

The latest translators are usually incredibly good, but sometimes the result is not completely clear.

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