Fun With Pin 1
Last Updated: 10th January 2010
The image below appears on my page about converting my laser cutter to use CNC with Mach 3. There I discuss some of the apparent possibilities and explain that the red and yellow wires on J4 were added by me with the intention of using U1 to control the laser beam. My intention was to use pin 1 of the parallel port to provide a signal to switch the laser on and off. However this didn't work and this document explains why not.
Why Pin 1?
Well for one thing, the second of the videos over at Mach 3 talks about designating pin 1 as Output 1 and using it to control a spindle on a mill. It's also the case that when setting up my Sieg Micro Mill for 4-axis control I'd wired four controllers to pins 2-9. Even though I didn't intend to use pin 1 to control the spindle on my mill you can probably see how, I got the idea in my head that pin 1 was the best candidate for switching the laser.
Why It Didn't Work?
I spent/wasted a lot of time on this because my first thought was that maybe it was simply the case that my parallel port couldn't source enough power to drive the optocoupler. Not all parallel ports are born equal; as you will find if you opt to do some further reading on the subject. Thus I began investigating other optocouplers in a bid to find one that would work with my port.
In a way I was lucky that, after observing that the controllers from RoutOutCNC used H11L1 chips, I bought some H11L1s, had no joy, and spoke to Mike (at RoutOutCNC) about it. Mike suggested that I try a spare pin in the range 2-9 and hey presto, it worked.
As you can probably tell, I'm a little out of my depth here, however it seems that pins 2-9 are for the port's data register, while pin 1 is part of the "control register". As such pin 1 is meant for a different purpose and it would appear that its capabilities are different. While pins 2-9 can drive an optocoupler it would seem that pin 1 can not.
So How Can Pin 1 Control a Spindle?
Obviously this is irrelevant in terms of converting a laser cutter but my experience left me wondering why the Mach 3 video (and other sources) suggest using pin 1 to control a spindle. Surely that would also need to be optocoupled?
The answer is that it does, and in those circumstances we'd need a breakout board that taps into the computer's power supply, either internally or via the USB port, to buffer pin 1 such that it can drive an optocoupler.
In a situation where we wanted pins 2-9 to be employed in giving us 4 axes of movement we would need such a board and using pin 1 to control the spindle would be entirely sensible. That's why it's suggested in places such as the Mach 3 video even though it's entirely inappropriate for use in controlling a laser cutter with a laptop using a non-buffered breakout board. In this situation the solution is simple: don't use pin 1. Doh!