Hi Hood
The Dell Optiplex 755 has arrived in the meantime. I installed Mach3.043.022 and ran a few tests. At first sight all seemed to be well. Then I heard a thump from the steppers when I hit a key on the keyboard. I call it the sound of a missed step. It doesn't happen every time but often enough to worry about. And not all keys in all circumstances do it, but the space key to pause a running program is especially bad. When I hit it during a rapid move (G0) ever so often the screech and the lost steps are back!
After a through investigation I found that both the charge pump and the stepper pulses are cut out for some 400 microseconds every time I hit a key. The charge pump monostable is dimensioned at 290 us so it briefly disconnects the stepper drive which makes for the thump when it comes back. Making the monostable less stringent wouldn't resolve the problem because during those 400 us no stepper pulses are coming from the PC anyway. It seems that this is enough for the steppers (running at 20 kHz = 1200 mm/min @ 1000 pulses/mm) to lose track. My suspicion is that Dell has a concurrency problem between USB events (keyboard) and the LPT output. Scanning this forum for "Dell Optiplex" I seem not to be the first one having problems.
Apart of that the PC seems to be amply fast enough: The Windows (XP) task manager never showed more than 3% CPU load when running a reasonably complex G-code. And the PC is very small, just 26 x 9 x 26 cm and fits nicely under the workbench. What a shame!
So it seems there are a couple of options:
- Buy another PC small enough for the workbench, e.g. one of the rare barebones with parallel port. But unless I can try that with Mach3 and the mill before actually buying it I risk another wasted expense.
- Buy a SmoothStepper Ethernet. Electrically elegant, but not from a packaging point of view: There is not enough space left for it in the control electronic box - so I need another box plus power supply plus cabling.
What do you think?
Peter