7i76e P1 P2 expansion port questions.

Hi,

I'm planning on getting the Masa 7i76e card and also convert over from Mach to LinuxCNC so I'm testing the waters with a live CD and went through the pin configuration. I'm selecting all kinds of features to see if the I/O will be enough for what I want to do eventually and although I can implement everything, I'm running short on I/O .

RTFM and it seems that P1 and P2 is still available for input or output but with the following 3 caveats:

1.Output High of only 2.9V
2.Input can tolerate 5V
3.When card starts all pins are pulled high so hardware needs to compensate for that.

So, it seems a small custom PCB with opto isolators and voltage dividers and an active high "output enable" function driven by field IO or a Active low "output enable" driven by P1/P2 itself should work?

I also see there is some hackery going on with the serial RS422 expansion protocol which might also enable custom expansion boards to be added this way, remotely and with better noise immunity?

Are my two conclusions above correct?

I would be inclined to use, and this is what I'm going to use on 7i92, 74LVC245 for inputs and 74LS540 or 74LS541 for outputs.

The 74LVC245s work on 3.3v supply but are fine, designed for, up to 5v input.
The 74LS540/541 work on a 5 volt supply and work with 3.3v logic inputs.

Both chips have active low OE pins.

I have used these devices for the baords I used with my BeagleBone Black when running machinekit controlling my mill. They would be suitable for use with the 7i92 but the header signals on my boards don't match the 7i92 headers.

For opto's for inputs this page offers some good info:
www.anderswallin.net/2006/08/optoisolato...mesa-5i20-servocard/

You can use any standard parallel port breakout board on the expansion connectors
(the outputs will swing to 5V but do not have much drive above 3V or so)

Because of the limited drive above 3V and the default high state of the GPIO pins, if you use opto-isolators
they should be driven in sinking mode (anode to +5V through current limit resistor cathode to GPIO pin)
This will insure that they are off during power up/down cycles and that you have full 5V drive)
SSRs should be connected the same way