The Input Shaper functionality on the Prusa CORE One, and on the Original Prusa MK4/S, MK3.9/S can be calibrated using an optional accelerometer.
The accelerometer assembly consists of a small PCB enclosed in a printed cover, which connects to the printer. Please note that using the accelerometer requires partial disassembly of the printer.
The input shaper is pre-calibrated from the factory, so this calibration is usually unnecessary on a stock printer. However, if you've made any modifications to the printer hardware, it may be beneficial to recalibrate the settings using the accelerometer.
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Prusa CORE One
Temporarily remove the Wi-Fi board cover, by loosening its M3x12 screw.
Connect the jumper cable between the xBuddy connector and the Wi-Fi module connector.
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Remove the circular cover on the back. Then, route the accelerometer cable in the hole.
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Connect the accelerometer cable to the connector at the back of the printer. 
Original Prusa MK4/S, MK3.9/S
On MK4S and MK3.9S, connect the accelerometer to the dedicated port on the xBuddy board.
Alternatively, you can install a short jumper cable between the xBuddy and the Wi-Fi module on the back of the printer. Then, you can connect the accelerometer to the connector on the back of the printer.
On MK4 and MK3.9, connect the cable to the accelerometer's PCB and attach the printed covers. The accelerometer connects directly to the dedicated port on the xBuddy board.
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Accelerometer connection on MK4S and MK3.9S
Original Prusa MK3.5/S
For MK3.5 or MK3.5S, please visit the dedicated guide.
Calibration
The accelerometer is used to calibrate the input shaper parameters. You can launch the calibration from the LCD Menu -> Settings -> Input Shaper -> Calibration.
X-axis
On MK3.9 or MK4, open the fan-door completely before the procedure. Remove the silicone sock, if installed. Identify the part of the accelerometer that is in the shape of the heater block.
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Fit the accelerometer to the heater block, by pushing it from the bottom of the hotend. If there is any resistance, do not attempt to push the accelerometer harder, as excessive force might bend the heatbreak.
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Y-axis (MK4/S, MK3.9/S, MK3.5/S)
Identify the part of the accelerometer that protrudes from the rest. On the front side of the Y-axis, place the protrusion between the steel sheet and the heatbed.
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Results
The results shown on the printer's display at the end of the calibration indicate the optimal input shaping type and frequency found with the calibration.
30 comments
1. Enable IS tuning in the menu
2. After the printer homes and parks the head in the middle, remove the silicone sock.
3. Attach the accelerometer cable to the board, put the board into the same mount as for the Mk4.
4. Put the whole assembly in place of the silicone sock. Just press it gently onto the heater block.
5. Continue with on-screen prompts, the printer will calibrate IS.
There is no need to move the accelerometer to the bed or anything - it will stay on the printhead during the entire calibration process.
After being satisfied with the results, remove the accelerometer, tug the cable back through the hole, plug the hole and replace the silicone sock.
One could assume it's the same as on the MK4(S) for example. However, the nozzles in the Core One sit at an angle relative to X/Y which they don't with the MK4(S).
Please add this important detail!
Connection and nozzle attachment are now included and pictured in the article. The fact that the nozzle sits at an angle on CORE One is regular, and it is still possible to attach the accelerometer even with the CORE One nozzle and heaterblock angle.