# Bit calculation for CAN Bus Control in Node-Red

**URL:** <https://discourse.nodered.org/t/bit-calculation-for-can-bus-control-in-node-red/95463>\
**Category:** General\
**Created:** [17 February 2025 11:41 UTC](https://discourse.nodered.org/t/bit-calculation-for-can-bus-control-in-node-red/95463 "2025-02-17T11:41:54Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![StefanF1](https://avatars.discourse-cdn.com/v4/letter/s/e9a140/32.png) [@StefanF1](https://discourse.nodered.org/u/StefanF1)\
**Post date:** [17 February 2025 11:41 UTC](https://discourse.nodered.org/t/bit-calculation-for-can-bus-control-in-node-red/95463/1 "2025-02-17T11:41:55Z")

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# **Issues with Bit Manipulation for CAN Bus Control in Node-Red**

**Hi everyone,**

I'm struggling with **bit manipulation for CAN bus commands in Node-Red** and hope someone has a smart solution. I am using red-contrib-socketcan on a Raspberry Pi with a CAN Adapter.

## ✅ **What’s Already Working?**

**Reading and wiriting / sending CAN commands** is working perfectly via Node-RED.

## 🎯 **What Am I Trying to Achieve?**

I want to **send data to the CAN bus** to control, for example, the **interior light** in my Van (VW Grand California) which is equipped with several CAN busses. The only bus I want to use is for comfort functions.  
This already works with the following command:

```auto
cansend can0 12345678#00.02.00.00.00.00.00.00

```

👉 Here, **bit 1 in byte 1** is set, resulting in `0x02` at the second position of the string.

To **turn the light off again** , I reset bit 1 to `0`:

```auto
cansend can0 12345678#00.00.00.00.00.00.00.00

```

## ⚠ **The Actual Problem**

The **same identifier (`12345678`)** is also used to **control other devices** , such as the **satellite receiver**.  
This device is switched on by **setting bit 4 in byte 1 (`0x10`)**:

```auto
cansend can0 12345678#00.10.00.00.00.00.00.00

```

⚠ **Problem:** Sending this command **accidentally turns off the interior light!**

## 🛠 **Solution Approach**

Before modifying a bit, I need to:  
1️⃣ **Read** the current value of **byte 1** from `12345678`.  
2️⃣ **Set or clear** the required bit.  
3️⃣ **Recalculate** the new byte value.  
4️⃣ **Send the updated CAN command**.

### **Example:**

- The **interior light is already on** (`0x02` in byte 1).
- Now, I also want to **turn on the satellite receiver** (set bit 4).
- The new byte value should be **`0x12`** (`0x02 + 0x10 = 0x12`).

**Correct command:**

```auto
cansend can0 18EF70C9#00.12.00.00.00.00.00.00

```

👉 **Now the satellite receiver is turned on, and the interior light stays on!**

To **turn off the receiver** , I need to remove bit 4 what leads to:

```auto
cansend can0 18EF70C9#00.02.00.00.00.00.00.00

```

## 🤔 **My Challenge in Node-Red**

Right now, I'm struggling to properly implement this **bit manipulation in Node-Red**. 😕

Does anyone have an idea **how to read the current value, modify the required bit, and generate the correct CAN command?**

Any help would be greatly appreciated! 🙏

**Best regards,**  
Stefan

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<div class="post-metadata">

**Author:** ![StefanF1](https://avatars.discourse-cdn.com/v4/letter/s/e9a140/32.png) [@StefanF1](https://discourse.nodered.org/u/StefanF1)\
**Post date:** [20 February 2025 13:53 UTC](https://discourse.nodered.org/t/bit-calculation-for-can-bus-control-in-node-red/95463/2 "2025-02-20T13:53:53Z")

</div>

SOLVED - can be closed

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<div class="post-metadata">

**Author:** ![E1cid](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/e1cid/32/77971_2.png) [@E1cid](https://discourse.nodered.org/u/E1cid)\
**Post date:** [20 February 2025 14:10 UTC](https://discourse.nodered.org/t/bit-calculation-for-can-bus-control-in-node-red/95463/3 "2025-02-20T14:10:12Z")

</div>

You could add how you resolved this so future reads can find a solution to a similar issue. The ethos of the forum is to help and share with others.

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<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:** [6 March 2025 14:10 UTC](https://discourse.nodered.org/t/bit-calculation-for-can-bus-control-in-node-red/95463/4 "2025-03-06T14:10:50Z")

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