# Unable to read tmc uart 'stepper' register IFCNT **URL:** \ **Category:** Knowledge Base\ **Created:** [July 10, 2025, 5:24pm UTC](https://klipper.discourse.group/t/unable-to-read-tmc-uart-stepper-register-ifcnt/24192 "2025-07-10T17:24:37Z")\ **Posts on this page:** 1\ **Page:** 1
**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:** [July 10, 2025, 5:24pm UTC](https://klipper.discourse.group/t/unable-to-read-tmc-uart-stepper-register-ifcnt/24192/1 "2025-07-10T17:24:37Z")
# Unable to read tmc uart ‘stepper’ register IFCNT ## Background The error “Unable to read tmc uart register IFCNT” indicates that your printer’s microcontroller unit (MCU) has failed to communicate with a specific TMC stepper motor driver. This communication happens over a serial interface known as “Universal Asynchronous Receiver/Transmitter” (UART). > 💡 **Note:** > > - Klipper can identify which driver it failed to communicate with (e.g., `stepper_x`), but it cannot determine _why_ the communication failed. The root cause is almost always a physical issue with power, wiring, or hardware configuration that must be diagnosed and fixed manually. > - The reference to the TMC register `IFCNT` is a “red herring”. This error is related to a UART communication problem, and not to a specific TMC register. ## Causes and Potential Solutions The `klippy.log` will confirm that this error occurred, but will not indicate the specific cause. A systematic, hands-on approach is required to identify and resolve the issue. Work through the following potential causes, ordered from most common to least common. - **No or Insufficient Motor Voltage (VM) to Drivers** This is the single most common cause, especially on new builds or after wiring changes. The printer-board’s logic may be powered, but if the **supply voltage jumper** is set incorrectly or the motor power is not properly connected, the drivers will not receive power. This error will typically appear for all drivers at once. - **Incorrect Printer-Board Jumper Settings for UART Mode** Most printer-boards use physical jumpers to configure the driver sockets for different modes. For UART communication to work, these jumpers must be set correctly. Carefully check the **communication mode jumpers** against your printer-board’s documentation. - **Faulty Wiring or Loose Connections** A bad crimp, a loose plug, or a damaged wire in the motor cable can interrupt communication, often causing the error for a single axis. Visually inspect all cables and plugs for damage or improper seating (e.g., bent pins). Double-check the orientation and seating of the driver module. If only a single axis is affected, swapping the drivers and connectors (e.g., X with Y) is a good diagnostic step. If the error moves to the new axis, the issue is with that motor or its cable. - **Incorrect `uart_pin` Definition in `printer.cfg`** Even with correct physical wiring, communication will fail if the firmware is set to use the wrong MCU pin. This is a common mistake when using a generic or outdated configuration file that doesn’t match your specific printer-board revision. Match the configuration against the board’s official pinouts, schematics, or default configurations. - **Driver Module Not Enabled for UART** Not all TMC driver modules come ready for UART mode out of the box. Some brands (e.g., Watterott, older MKS or Fysetc versions) require a physical modification on the driver PCB, typically by soldering a bridge across pads to enable the UART pin. Research your exact driver type and consult documentation to determine the required modification. - **Faulty Stepper Driver Module** The driver module itself can be defective out of the box or may fail over time due to static discharge, overheating, or voltage spikes (such as unplugging motors while powered on). If the error is consistently tied to a specific driver regardless of socket, the module may be faulty. Swap the suspect driver module with a known-good one. If the error is resolved, the driver is likely at fault. - **UART Bus Conflicts with Other Devices** This is rare on modern boards with dedicated UART pins for each driver. However, conflicts can occur on older or simpler boards if the same serial pins are used for both a driver and another device, such as an LCD display. Simplifying the setup by removing additional components can help isolate this type of issue. - **Other Reasons** These are less common but can be worth considering, especially when dealing with Heisenbug errors: