plc usage

[ceencee]

I want to check if I am on the right track. When the computer outputs the Gcode to drop drill spindles 8,9,10 and 11 and it has been set that that gcode is G98 , does the plc receive this code G98 and sets into motion the dropping of said spindles by activating coils 8,9,10 and 11. Does the plc except Gcode or what does it accept so that it knows when to drop spindles. Im also assuming that this would be a digital input on the plc. Does the plc even care what it gets....as long as the input matches one of its triggers it will set the proper set of events into motion?
Just trying to get my head on straight before i purchase the plc for my router.

Thanks

[Al_The_Man]

I can't remember what control you are using, but in general all a PLC would need is one particular input set to trigger the spindles you want to activate, if this were selective, IOW, if you needed to select particular spindles then a input code such as binary number could be used.
If G98 is passed to the PLC, this is Normally an M function not G, and as such after the PLC completes the task, it sets an Operation Complete bit that signals the CNC to carry on.
Al.

[ceencee]

Thanks Al! These would all be digital inputs and outputs( for the operation completed signal) Now for the 24v magnetic coil pneumatic valves that drop the actual spindle...I assume these are analog outputs.
I am still trying to understand how the pc talks with the plc, what it sends to get the 1 or 0 in the bit box? What is the common or i guess best connection method from pc to plc...USB, parallel port, or the cable that slips my mind at the moment?

[Al_The_Man]

Apart from using descrete inputs there are other methods used such as Modbus etc.
The outputs would be digital, not analogue, PLC's can be obtained with relay or SSR outputs.
In the case of Mach it could be from the parallel port.
The controller would need to set an output bit when issuing the appropriate M code and use an input to the PC via a PLC output to signify when the PLC is done .
Al.