Longer parallel cable or longer steper wires?

[jvanick]

I'm in the process of finishing up my home-built jgro type CNC machine.

I have the Hobby CNC 4axis Pro board along with their 305oz motors.

I have a large quantity of 18ga control cable for the steppers (however it's not shielded).

due to shop space issues I'd like to place the pc about 20 or so feet away from the cnc machine (also will help keep it less dusty)...

I have a 20ft parallel cable that I found at the office... it's double shielded and is pretty thick... along with the fact that all 25 pins are wired straight through.

so...

should I.

a. use the long parallel cable and mount all the controller stuff under the table?

-or-

b. mount the controller stuff next to the PC and run longer stepper wires?

-or- ... not preferred:

c. split the difference and use maybe a 5 or 10ft parallel cable and mount thhe box in the middle?

Thanks!
-J

[Crevice Reamer]

Hi jvanick. Welcome to the Zone!

Go with the longer parallel cable and keep the motor wires short. Don't run your Home or limit switch wiring alongside the motor wires though.

CR.

[Mariss Freimanis]

CR,

Sorry but it's the other way around.:-) A parallel port cable must not be longer than 6' (2 meters) or crosstalk will become an issue. Our current record holder (nuclear reactor application) uses a 350' (100 meter) motor cable without any problems.

Mariss

[mechatronicsman]

I have to agree with mariss.
Long parallel cable can at worst lead to unwanted stepping or missed steps from noice/crosstalk. On the stepper side however most steppers are driven by a voltage much higher than the stepper voltage rating, and the current is regulated to a fixed level by the stepper driver. Hence even if long motor cables leading high currents will give a voltage drop, and reduce the voltage at the motor coil, the effect should not be appearent on the motors as the current is controlled by the stepper driver and kept at its correct level.

In any case you should use heavy cabling to your motors to reduce the resistance of the cables and hence the voltage drop introduced by them.

[mechatronicsman]

wrt the home/limit switches CR is right, you should not run these along the motor cables. Also using shielded cables for the limit/home switches is a good idea. Connect the shielding to power ground at the control box end.

[steve323]

Originally Posted by mechatronicsman In any case you should use heavy cabling to your motors to reduce the resistance of the cables and hence the voltage drop introduced by them.

Extra wiring resistance is not actually that bad. The motors probably have around 3 ohms of resistance. 40 feet of 22ga wire only adds 0.644 ohms of resistance. Stepper drivers operate by driving current at high voltages. They barely even notice the slight increase in resistance.

Steve

[martinw]

Dear jvanick,

Really bad idea to use long parallel cables

Mariss is right of course.

Best wishes,

Martin

[mechatronicsman]

Originally Posted by steve323 Extra wiring resistance is not actually that bad. The motors probably have around 3 ohms of resistance. 40 feet of 22ga wire only adds 0.644 ohms of resistance. Stepper drivers operate by driving current at high voltages. They barely even notice the slight increase in resistance. Steve

Hi Steve,
you are right the resistance is not that big, but the voltage drop can be significant if the current is large. Driving 5 A in your cables gives a voltage drop of 5*0.644=3,22Volts in each of the cables, i.e. 6,5V in total (back and forth). Of course this might not necessarily lead to a problem, but in any case must be considered.

Usually you would like the highest voltage possible at the motor if driven by a chopping (current regulated) stepper driver (as most stepper drivers are). As steppers are inductive, higher voltages gives faster current rise time (dI/dt), increasing the possible maximal speed of the system.

-bjorn

[jvanick]

I've got 18ga cable for the motors to the controller.

upon all the advice, I'm going to put the controller next to the pc and hook it up with the shortest parallel cable I can find.

on the limit/home switch cabling... can I get away with 28ga shielded+drain since it's not really carrying any current? I have a whole spool of it here at the office that's begging for a home.

[mechatronicsman]

Originally Posted by jvanick I've got 18ga cable for the motors to the controller. upon all the advice, I'm going to put the controller next to the pc and hook it up with the shortest parallel cable I can find. on the limit/home switch cabling... can I get away with 28ga shielded+drain since it's not really carrying any current? I have a whole spool of it here at the office that's begging for a home.

Yes that should be OK. For the limit/home switches the gauge is not important. Only length, placement (i.e. not near any noisy cables as the stepper cables), and preferably shielding.

[neilw20]

Make sure NO CURRENT flows through your shields.
Only connecting the shield at one end (controller) guarantees no current will flow.

NEVER use the shield as a wire connection because you were one short in the cable.
Long parallel cables are a no-no. Fast signal can (and will) get corrupted.

[joeoancea]

there is an other solution, but you need to have 5V power supply. you can use a "buffer " circuit on the long parallel cable, something like 74ls.... they are many of them and i used them and they do their job