View Full Version : 3.3 V vs. 5 V TTL on parallel ports

07-16-2005, 06:55 PM
I've got my CNC router (the one Andrew built!) running off of a very old PC. I have a much newer one I would like to use, but when I tried it I got odd results running the machine. It would only move in 1 direction in all 3 axes. This happened with MDI, jogging, and running .nc files. I tested the voltage coming from the parallel port on the newer PC and it's 3.3 Volts. My controllers need 5V TTL so my guess is that's why I can't get it to change direction.

Is there any way to fix this other than buying a PCI parallel port expansion card in the hopes that it is 5 Volt TTL? Can anyone recommend a particular make/model of ppt expansion card that they know to be 5 Volt TTL? Or is there anything I can put in between the new ppt and toe controller to do a step-up DC-DC conversion?


07-16-2005, 10:59 PM
The PCI card is probably your only option. A post on Benny's Mach2 forum says this one is known to work. http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=5220247403&category=3666

07-17-2005, 05:30 AM
What you could try is to put a diode, like the 1n4001, between the ground of the controller and the ground of the parallel port. This should give a voltage drop of 1,2 volt and give you a voltage potential of 4,5 volt.

printer port ground _____________ diode______________controller ground


(I hope you can read my ascii schematic :)).

07-17-2005, 10:49 AM
Thanks for the replies! Gerry, that's just the info I need, I think I'll go that route. And the price is right too, CompUSA wants $30. for a ppt card!


07-28-2005, 03:28 PM

I got a ppt card from the source you suggested and it works great! Now I can use the newer/faster PC to run my router! Thanks!