My Denford Triac 3 axis milling machine is fitted with Namur type sensors for XYZ Datums/Homes and mechanical switches for overtravels.
The Namur sensors are 2-wire devices and, after doing a bit of research, I understand that they signal their condition by allowing a different amount of current to flow over them. This current is very small being, on this device, only 0.3mA. This small amount of current flow allows them to be used in ATEX (Hazardous) environments etc.
Before I stripped the machine for Mach 3 conversion all three detectors worked everytime and I never had an overtravel event. This, and the problem of replacement cost for 3-wire devices, as focused my attention on keeping them for the conversion.
Unfortunatley, these Namur devices normally need an amplifier which takes the current difference and does the actual switching and these things are more expensive than the 3-wire devices I am trying to avoid buying.
So, with a bit of careful hacksaw work I have cut the Datum section out of the huge original Denford motherboard (15"x16") and am left with what, hopefully, is the bit I need in working order (yet to be tested!).
Take a look at the attached wiring drawing and the bit I am left with is in the bottom left hand corner. I have then removed IC8 to allow me to use pins 2, 17, 4, & 15 for the Dtaums and spindle speed monitoring. As you will see the supply voltage to that side of the circuit is 5V which is lucky, whilst the 12V on the sensor side can be supplied from the origiinal Denford transformer.
Now I think have already taken one step too far as I have also removed the resistor decade RP2 but that can be replace with the same or differnt value.
So, I know it might not be conventional practice to hacksaw up a circuit board, but I would appreciate some advise on my 'project' and, more importantly, how do i now wire it to the PC? To be honest I never really undertood when regarding 'inputs' to the parallel port, whether the PC supplys the 5V or it has to come from elsewhere. I would appreciate guidance on this as well as I don't have a 5V supply from my existing transormer, only 7V.

thanks in advance ...Sweep

The 12v volts could be obtained from the PC if necessary, The 12v looks like it is zener'd down to 9v to feed the sensors.
The RP2 is a resistor pack that consists of 10k pull up resistors. It does not look like you will need them if you are not using IC8.
The sensors are driving BC184 NPN open collector, I am not sure of the option of the jumpers at the input to IC4-7, what are these four IC's?
If you connect the 12v common to your PC common or BOB common,  you should be able to go directly from the transistor collector to your input.
You could also supply the 5vdc from the PC, if you were to need it.
If you notice, the OVI & OV, which are 12v and 5v supply commons, are both presently shown connected via chassis common
If they are two wire devices, it looks like they are set up to source, and input to the sink input of the BC184's.

 

Thanks for your reply NosmoKing. I have tried to respond to your points as best as I can.
1) I can take the 12v from one tapping of the transformer that is powering the stepper motors and other original equipment. This feed is, therefore, unchanged from the original machine.
2) I thought I might need these to 'pull up' the inputs to the PC as it was my intention to link the PC inputs to pins 2, 17, 4 & 15 on the circuit board where once was IC8. That is, as IC8 has been removed I can easily solder in sode wires.
3) There are no jumpers available for IC's 4 to 7. I have attached a info sheet for these MCT6 devices but they are pretty simple things.
4) I thought I would keep the 12v away from the PC as both 12V and 0V can be accomodated on the original board.
5) I have also attached a photo of the board which should give you a better idea. You will be able to see where IC8 has been removed from, which is where I intend soldering in my new connections. You can also see where resistors RP2 has been removed.

What I am really looking for is an idiots guide to wiring this thing into the parallel port; i.e. "pin 2 of where IC8 was removed from goes to, say, pin 13 on the parallel port" etc

thanks ...Sweep

Yes after a closer look, I see the MCT6 is a dual opto, and gives you the isolation from the 12vdc.
For going direct to the parallel port input, you most likely will need to replace the 10k pull ups, you should be able to go directly from the open collector outputs of the MCT6's, pin 7 & 6 corresponding to each sensor input,  to your desired port input,  if using the PC for 5v then your reference will already be in place, if using an external 5v supply, then the 5v PC port common should be connected to the external 5v supply common (0V).
Also I may have been mistaken on the previous post, OVI and 0V are most likely isolated from each other.
N.