CNC Control “Signals”

Now that the components of a CNC controller system have been identified, let’s look closer into CNC control though what we will be referring to as command signals.

The computer is responsible for generating these control signals. It interprets a language such as G-code into the signals. More will be discussed on the computer and software in the CNC software section.

The “step and direction” signal format describes itself. There are two commands sent to each driver. The step signal and the direction signal. As stated in the controller component section , the signals generated by the computer are 5V DC supplied to the controller via the communication (DB25) or the serial (DB9) port. The communication signals are a square wave form known as Transistor-to-Transistor Logic (TTL) signals. This signal is essentially a series of small pulses ranging from 0V to +5V that represent 0’s and 1’s in a binary computer language.

This signal is a form of a Pulse-Width Modulated (PWM) signal where the duration of the pulse is varied to indicate information. The duration of the pulse determines the binary code, either “0” or “1”, as communicated by the computer and interpreted by the motor driver. Each motor driver is different, that is why it is important to understand the command signals and CNC control. Let’s look at a popular stepper motor driver for example the Gecko G201.

The product manual specifies that the minimum step pulse “0” time is 0.5 uS (half a microsecond). The minimum step pulse “1” time is 4uS. Refer to the image below for clarification.

The step signal and the direction signal will be sent simultaneously. The step signal may sit at “0” if no steps are commanded. However, the direction signal continuously streams a direction until the direction changes.

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