Typical hardware associated with control of CNC machines using the parallel port with stepper motors. The description includes full circuit diagrams that have been implemented in the design. - FabRap

Go to content

Main menu:

Typical hardware associated with control of CNC machines using the parallel port with stepper motors. The description includes full circuit diagrams that have been implemented in the design.

Dabula Kahle

The computer used for control of the router is a second hand Dell Optrix GX620 with 120 Gigabyte hard drive.  The drive capacity is such that I have been able to set the machine up as a dual boot system.  This allows me to use Mach3 and Linux CNC one the single machine.  During the boot up process you select the operating system you want to use.  In setting up the system you have the option of selecting which Windows files you want included in the Ubuntu operating system.  This make file transfer very easy.  The Dell is connected to my main computer with a WiFi link.  The Dell has some disadvantages as the power saving mode slows the machine down.  It just barely passes the latency test.  Also it uses an integrated parallel port device that operates on 3.3 Volt.  This is not ideal.

The Dell is used in conjunction with a Compaq display and it sole use is for the CNC router.  All design and preparation is undertaken on my main computer.  The WiFi link is only enabled when needed.  This speeds up my main machine.

The control circuits are powered by a home built 1 Amp 5 Volt power supply.  When this power supply is switched it triggers a solid state relay to switch on the power to a 36 Volt 350 Watt power supply for the motors and motor drivers.  This was done to make the powering up process as easy as possible.

The 36 Volt 350 Watt power supply is a standard Chinese switch mode power supply.  These are a common denominator for most of the Chinese offerings.  This power supply is adequate to allow each motor to operate at it's specified 3 Amp current.  24 Volt was for the additional start up torque.  The voltage is well within the specifications of the motor drivers.

The parallel port is protected from the motor transients by using an Opto Isolation board.  This board if the main interface between all control and signal circuits of the router.  The output circuits include an enable, direction and step signal for each motor plus a relay switch for the spindle.  The signal for the spindle is once again interfaced via a solid state relay.  The input signal includes a home and limit signal for each axis plus an input for an emergency stop button.

It is unfortunate that the Chinese put such emphasis on cost saving to the extent that they have use high speed CMOS circuits(74HC) for the Opto board. The HC technology used allows a leakage of the PPT output into the Opto board.  I have not found out how this occurs, but it is clear that the 5 Volt PSU power on LED lights up if the when the port is connected and the 5 Volt power is switched off.  I have also read of this occurrence from other users on the Internet.  This occurrence is cured when using AHCT technology.

Furthermore with most modern Personal Computers technology has changed to 3 Volt logic.  This also applies to the parallel ports.  The positive switching voltages for the 74HC14 is 3.15 worst case whereas for the 74AHCT14 it is 2.1 Volt.  The negative switching for the 74HC14 is worst case 2.45 Volts whereas for the 74AHCT14 it is 1.7 Volt.  The 74AHCT14 is therefore a far better choice of device.

The motor drivers used are the DQ542MA drivers from Wantai Motor.  I have found these to be well made and reliable.  The drivers are capable of supplying up to 4.5 Amperes to the motor.  Currently I am under supplying each motor with only 2.8 Ampere when the motor is specified for 3.5 Ampere.  The motor still has sufficient power to drive all machining operations but tends to run cooler.

Care must be taken to ensure that each driver is supplied separately and directly from the power supply.  The power must not be supplied via a daisy chain to the motors.  This is because the motors are highly inductive and a daisy chain would allow one motor to feed back power to it's neighbor driver with the effect of exceeding the driver's safe limits.

The motors used for the router are the 28BYGH501 motors from Wantai Motor.  These are NEMA23 motors with a holding torque of 1200 grams per centimeter.  The motor shafts are extended on both sides of the motor allowing for a hand wheel to be fitted to each Axis.

Below is a schematic diagram for the Driver connections as well as the schematic diagram of the router wiring.

 
 
Back to content | Back to main menu