# Modbus negative numbers

**URL:** <https://discourse.nodered.org/t/modbus-negative-numbers/4074>\
**Category:** General\
**Created:** [18 October 2018 07:35 UTC](https://discourse.nodered.org/t/modbus-negative-numbers/4074 "2018-10-18T07:35:48Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![RickW](https://avatars.discourse-cdn.com/v4/letter/r/aca169/32.png) [@RickW](https://discourse.nodered.org/u/RickW)\
**Post date:** [18 October 2018 07:35 UTC](https://discourse.nodered.org/t/modbus-negative-numbers/4074/1 "2018-10-18T07:35:48Z")

</div>

I need some help please to get Modbus numbers reading correctly when they go negative.

When the output from my modbus device goes negative it adds 65535 to the number.

I have tried various settings in a few different Modbus input nodes, but they all give the same output.  
I have also tried a few of the function nodes but none seem to solve the problem.

I thought it may be quite simple to write a "function" node, but I have no idea with java script.

What I want is:

if msg.payload \> 2000;  
then msg.payload = msg.payload - 65535;  
else return msg.payload;  
endif;

Those of you who understand thees things will know I am getting all sorts of error messages on each line. I suspect it just needs some minor grammar and layout adjustments but I have not found any java script grammar tutorials that show what I need to do?

Thank you

---

<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:** [18 October 2018 07:39 UTC](https://discourse.nodered.org/t/modbus-negative-numbers/4074/2 "2018-10-18T07:39:11Z")

</div>

Hi @RickW - you're missing a few brackets and braces:

```auto
if (msg.payload > 2000) {
  msg.payload = msg.payload - 65535;
}
return msg;

```

Here's a good guide on the basics of conditional statements in JavaScript - [https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Building\_blocks/conditionals](https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Building_blocks/conditionals)

The other pages around that one cover other useful topics.

---

<div class="post-metadata">

**Author:** ![RickW](https://avatars.discourse-cdn.com/v4/letter/r/aca169/32.png) [@RickW](https://discourse.nodered.org/u/RickW)\
**Post date:** [18 October 2018 07:41 UTC](https://discourse.nodered.org/t/modbus-negative-numbers/4074/3 "2018-10-18T07:41:22Z")

</div>

Thank you knolleary!

I thought I wasn't too far off.

---

<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:** [18 October 2018 07:59 UTC](https://discourse.nodered.org/t/modbus-negative-numbers/4074/6 "2018-10-18T07:59:11Z")

</div>

My mistake... The last line should be `return msg` ... I have edited my previous reply to fix it.

---

<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:** [18 October 2018 08:14 UTC](https://discourse.nodered.org/t/modbus-negative-numbers/4074/7 "2018-10-18T08:14:41Z")

</div>

> [@knolleary](#):
>
> if (msg.payload \> 2000) {

I think it might be better to use 32767 which is the maximum 16 bit signed integer. Also I think it should be 65536 that you take off, not 65535. The hex value 0xFFFF is 65535 which should convert to -1.

---

<div class="post-metadata">

**Author:** ![RickW](https://avatars.discourse-cdn.com/v4/letter/r/aca169/32.png) [@RickW](https://discourse.nodered.org/u/RickW)\
**Post date:** [18 October 2018 08:39 UTC](https://discourse.nodered.org/t/modbus-negative-numbers/4074/8 "2018-10-18T08:39:14Z")

</div>

Thank you.

I did get an error and was trying a few things.  
Is working now.

Much appreciated!

---

<div class="post-metadata">

**Author:** ![RickW](https://avatars.discourse-cdn.com/v4/letter/r/aca169/32.png) [@RickW](https://discourse.nodered.org/u/RickW)\
**Post date:** [18 October 2018 09:30 UTC](https://discourse.nodered.org/t/modbus-negative-numbers/4074/9 "2018-10-18T09:30:19Z")

</div>

I agree with your updated numbers.

My particular application will never have valid readings above 2000, but using 32767 would be better in case other applications have higher valid numbers.

I bow to your argument for for the subtract number. Either number appears to function in a smooth linear trend. My current application doesn't have a direct reference I can compare it against.

Thanks again.