Command a Step and Dir Controller

What is a step/dir driver?

Many high-current stepper motor drivers — such as those used with large observatory focusers and dome rotators — use a simple two-signal interface: a step pulse line that advances the motor one microstep per pulse, and a direction line that sets the rotation direction. The Seletek generates these signals natively, allowing it to command any step/dir compatible driver without needing any additional hardware.

This mode is particularly useful when the motor current exceeds what the Seletek's internal driver can deliver, or when you already have an existing external driver for your motor.

Wiring

Connect the Seletek step/dir output lines to the corresponding inputs on the external driver:

• STEP — connect to the Step or CLK input on the external driver.
• DIR — connect to the Direction or DIR input on the external driver.
• GND — connect the grounds together to establish a common reference.

The Seletek outputs are 3.3 V logic. Check that your external driver accepts 3.3 V signals on its step/dir inputs — most modern drivers do, but some older models require a level-shifting circuit.

Do not connect the Seletek motor output terminals (the ones normally driving a motor coil) when using step/dir mode — only the signal lines are used.

Software configuration

In the Seletek software, open the Configuration panel and select the motor port you have wired. Choose Step & Dir as the output mode. Set the following parameters:

• Microstep resolution — match the microstepping setting on your external driver (e.g., 1/8, 1/16). Both must match for the position counts to be consistent.
• Step pulse width — the minimum pulse duration your driver requires. Most drivers need at least 1–2 µs; the default of 10 µs is safe for virtually all hardware.
• Maximum speed — set a speed that your driver and motor can handle reliably. Start low and increase until motion becomes consistent.