SMC1500 stepper motor controller

... or using only 2kB of the ATmega664P's flash.

Recently I bought a small CNC mill/hot wire foam cutter. Originally this thing seems to be first made around 1989 in West-Germany, yes, its design is from the time that there were still two Germanies :-).

The mill was connected via an amplifier box to an ISA plugin card. When I bought the it the ISA card was gone but the owner had invested in a stepper motor controller card and some software. He never found the time to put everything together to make a working machine, so he decided to sell it.

I happily bought the machine, brought it home and started investigating the electronics.

The SMC1500

The stepper motor controller card that came with the mill was a SMC1500 from Emis GmbH. This card is a little bit non-standard compared with normal stepper motor controllers. With this card you choose an axis to modify and configure the currents and the direction of the currents of the motor coils. By doing this in the correct pattern you let the motor turn. One drawback is that you must choose which motor signals you modify so you can't change two motors at the same time. Another is that standard CNC software (EMC2 in my case) expects a step/dir interface where you set a direction and spit out pulses to let the motor turn.

There is an additional board available (SMC1500z) with a microcontroller that converts step/dir signals to signals for motor coils. Conrad sells these boards for about 50 euros, which is a reasonable price, in my opinion. However, I'm an electronics engineer, I like hacking and I should be able to make one myself.

The SMC1500 consists of a 5V regulator, six NJM3770AD3 H-bridge chips for the motors which are controlled by three 74HC573 latches and a sort of address decoder in a GAL chip.

New electronics

To be able to control all motors I decided to remove the latches and GAL and to build a simple circuit that controlled the motors. A simple whole-step controller could be made with a four-step state machine. I designed a circuit around a 4013 double D-flip flop and a 4070 quad-xor chip that would go through the four states and generated the correct output for the motordriver chips, inspired by this page.

Because I controlled the motor with full steps the motors weren't as strong as I expected, so I decided to drop the discrete logic steppercontroller and designed a simple microcontroller circuit with an ATmega664P I had lying around.

• View schematic (PNG)
• Download schematic (KiCad; right-click -> Save as...)

These schematics are available under the terms of the Creative Commons Attribution-ShareAlike 3.0 Unported license.

Software

The code for the microcontroller checks the input pins for a pulse on the step input of every axis and then steps through a table with motorstates so that the motors make steps in the indicated direction. After 10 seconds of no pulses for any axis the microcontroller turns off the holding current to avoid unneccesary heating-up of the motors.

The definitive code uses only 2kB of the available 64, so I could have used almost any microcontroller with enough pins.

• Download code (AVR-GCC)

This software is licensed under GNU GPL version 2.0.

Result

That's it; everything works as expected, I can now control my mill with EMC2 and start to learn how to engrave and mill nice things.