# Problem switching between printers

**URL:** <https://klipper.discourse.group/t/problem-switching-between-printers/21932>\
**Category:** General Discussion\
**Created:** [February 10, 2025, 11:32pm UTC](https://klipper.discourse.group/t/problem-switching-between-printers/21932 "2025-02-10T23:32:49Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![nilchet](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@nilchet](https://klipper.discourse.group/u/nilchet)\
**Post date:** [February 10, 2025, 11:32pm UTC](https://klipper.discourse.group/t/problem-switching-between-printers/21932/1 "2025-02-10T23:32:49Z")

</div>

### Basic Information:

Printer Model: Ender 3 s1: Ender 3 max neo  
 MCU / Printerboard: original : v4.2.7  
 Host / SBC sonic pad running jpe230 debian  
 klippy.log  
[printer 2 log.log](https://klipper.discourse.group/uploads/short-url/y61kJBiHhusj8ZOIWYz7b2B7M7R.log) (580.1 KB)  
[printer 1 log.log](https://klipper.discourse.group/uploads/short-url/7AABN2fPJWEokggFEE70jX6OWwZ.log) (465.3 KB)

### Describe your issue:

i recently added a second printer and both printers run fine,  
but when i switch between the printers the one that was connected loses connection as soon as i connect to the other.

Lost communication with MCU ‘rpi’  
Once the underlying issue is corrected, use the  
“FIRMWARE\_RESTART” command to reset the firmware, reload the  
config, and restart the host software.  
Printer is shutdown

This is the error it gives and i cant figure it out.

---

<div class="post-metadata">

**Author:** ![nilchet](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@nilchet](https://klipper.discourse.group/u/nilchet)\
**Post date:** [February 11, 2025, 5:51am UTC](https://klipper.discourse.group/t/problem-switching-between-printers/21932/2 "2025-02-11T05:51:03Z")

</div>

i managed to solve it by commenting these lines  
#[mcu rpi]

#serial: /tmp/klipper\_host\_mcu

#[adxl345]

#cs\_pin: rpi:None

#spi\_speed: 2000000

#spi\_bus: spidev2.0

#[resonance\_tester]

#accel\_chip: adxl345

#probe\_points:

# 122, 115, 10

Will having Commented these have impact on my imput shaping or will it work?  
[printer (1).cfg](https://klipper.discourse.group/uploads/short-url/5cKXf2oKQyI66NRXmAKexe9PhPt.cfg) (6.3 KB)

---

<div class="post-metadata">

**Author:** ![Sineos](https://yyz2.discourse-cdn.com/free1/user_avatar/klipper.discourse.group/sineos/32/18_2.png) [@Sineos](https://klipper.discourse.group/u/Sineos)\
**Post date:** [February 11, 2025, 6:43am UTC](https://klipper.discourse.group/t/problem-switching-between-printers/21932/3 "2025-02-11T06:43:23Z")

</div>

Reason for your error:

1. You are running TWO independent instances of Klipper on your SBC.
2. By following [RPi microcontroller - Klipper documentation](https://www.klipper3d.org/RPi_microcontroller.html), you have created ONE instance of the “virtual” MCU with its endpoint on `/tmp/klipper_host_mcu`.
3. When using both Klipper instances, they will “battle” for exclusive access to `/tmp/klipper_host_mcu`, and the one that loses will react with the above error message.

Try the following approach (untested):

```auto
sudo cp /etc/systemd/system/klipper-mcu.service /etc/systemd/system/klipper-mcu2.service
sudo nano /etc/systemd/system/klipper-mcu2.service

```

Modify `/etc/systemd/system/klipper-mcu2.service` as follows:

```auto
# Systemd klipper linux mcu Service

[Unit]
Description=Starts the MCU Linux firmware for klipper on startup
Documentation=https://www.klipper3d.org/RPi_microcontroller.html
Before=klipper.service
ConditionFileIsExecutable=/usr/local/bin/klipper_mcu

[Install]
WantedBy=multi-user.target

[Service]
Type=simple
# Create a new endpoint for the second Klipper instance
Environment=KLIPPER_HOST_MCU_SERIAL=/tmp/klipper_host_mcu2
RemainAfterExit=yes
ExecStart=/usr/local/bin/klipper_mcu -r -I ${KLIPPER_HOST_MCU_SERIAL}
ExecStop=sh -c 'echo "FORCE_SHUTDOWN" > ${KLIPPER_HOST_MCU_SERIAL}'
ExecStop=sleep 1
TimeoutStopSec=2
Restart=always
RestartSec=5

```

Enable and start the second instance of the SBC MCU:

```auto
sudo systemctl enable klipper-mcu2.service
sudo systemctl start klipper-mcu2.service

```

Modify the `printer.cfg` of your second printer to:

```auto
[mcu rpi]
serial = /tmp/klipper_host_mcu2

```

---

<div class="post-metadata">

**Author:** ![EddyMI3D](https://yyz2.discourse-cdn.com/free1/user_avatar/klipper.discourse.group/eddymi3d/32/1981_2.png) [@EddyMI3D](https://klipper.discourse.group/u/EddyMI3D)\
**Post date:** [July 7, 2025, 7:06pm UTC](https://klipper.discourse.group/t/problem-switching-between-printers/21932/4 "2025-07-07T19:06:18Z")

</div>

Closed on July 7, 2025, 7:06pm UTC