CNC plotter 1800 x 80cm, first build, questions and queries.

[Doddy]

Just re-reading the pdf, "That is the reason why this card has two power connections. One power connection is power powering the circuit that interacts with the PC, the other connection is for powering the circuit that interacts with your CNC system." - that confirms my assumption that the two power-supplies need to be provided (and I'll say without reservation that they can share the same PSU source - the only reference to the PC are the ground pins on the parallel interface which will be commoned with the 0V on the PC supply on the board). So I'm happy with the PSU wiring I proposed.

It's interesting that without that wiring the board scavenges the low-level supply voltage (1.8V as you measure) - it's unclear to me exactly where that's sourced from - wouldn't have thought from a powered-off PC, but it's if the two halves of the board ("PC" and "Output") are galvanically isolated as the pdf suggests then there's little other option. It is in part explained with the turning-the-pc-on-extinguishes, as the PC boots if it is resetting the parallel port signalling - clearly something on powering the PC changes something that drives the LED on the board, and similarly leaving the PC powered off extinguishes the LED eventually - so perhaps there is some residual supply on the parallel port when "off". An interesting observation, nothing more.

So, you wired the supply and the outputs pulsed and the output LEDs blinked?, so you've probably now bridged the PC-side signalling to the output-side drive, that's triggered the outputs, but the output status is now inhibited. Okay, that supports the PC supply theory, now the question is what has the board done to determine to inhibit the output

If you have +5V on the PC +5V and the main board +5V and also on the EN input - then the board should, from what I've read, operate. Dean (JazzCNC) refers to a charge-pump - from what I understand the charge-pump is implemented separately off board and signalled through the EN input (which corroborates the advice to pull this high) - so I don't see any other required signalling to operate the board (the linked PDFs in the manual (http://cnc4pc.com/Tech_Docs/E_STOP_N_SCHP.pdf) suggest throwing a charge-pump output from the PC onto output pin 17 - but without an external charge-pump circuit this should not be necessary.

At this stage, given the above measured voltages (PC+5V, +5V, EN) I expect the board to function. You ask whether it's time to think of a new BOB?, your call - you can pick them up for very little cost, or pay rather more for something as robust as this C1 board and test by substitution. Clearly from my perspective the board function as described is at odds with what I expect, but I might be missing a trick here (to be honest, there's not a lot to get wrong with a BoB, so I'd be surprised). My instinct is that the board is at some level functional - I'll offer to test the board for you to a level that I can diagnose the behaviour of each major functional block - but I don't know how that sits with your time-frame - it might be quicker just to get a BoB ordered in the post).

The only other minor concern I have is the 3V3 operation of the parallel port. The "old" 5V interface is normally enabled by configuring the parallel port to EPP mode in the computer's BIOS, but in the absence of any charge-pump circuit this doesn't explain the inhibiting of the outputs, and as stated earlier the 74ACT's should operate sufficiently well with 3.3V logic. So I'm not going to dwell too much on that.