The ICM can use the logic level voltages that are present on the ICM (+5 and +12). There is LSCOM and INCOM which are the common rails of the limit switch inputs and the general purpose inputs. I usually tie +24 volts from an external supply to these common terminals. I do NOT recommend using the logic level voltages on mechanical switched circuits. So 24v is pretty much mandatory. This sets the circuit up for the switches to ground the circuit. Each of the wires tied to an input or limit is then tied to one side of the switch and the other end of the switch is tied to ground. Either normally open or normally closed switches can be used. However, for safety reasons, I would recommend that the limit switches be the normally closed type.
The output wiring depends on if the ICM is opto-isolated or not. If it is, it will have a -OPTO label on it. You would then supply OUTPWR and OUTGND. Read the Galil documentations for examples of wiring the output circuits. i usually use the outputs to drive solid state relays regardless of if the ICM is -OPTO or not. Then you can use the logic level voltages from the ICM to control the output SSRs and let the SSRs switch what ever voltages you need.
The general inputs and outputs (8 each on your 1842 board) are not used for anything special on the Galil. They are merely there for machine I/O duty. There is one exception, however, and that is the case of probing. Inputs 1-4 also serve as the high speed position latch triggers. So if you want to use a probe, do not use these inputs for other purposes. The triggers are PER Galil axis. Input 1 is for axis A, input 2 is for axis B, etc... So if you are using Galil axes A, B, and C to drive Mach axes X, Y, and Z, then you will want to reserve inputs 1, 2, and 3 for your probe inputs. Each input should be wired up to the probe so that a probe strike activates all of the inputs at the sane time.
Steve