I have a pair of ISA 430s that I would like to try as outboard compressors and/or EQs to use during mixing. Can anyone tell me how, if at all, I can do that? Preferably I would leave them set up for preamp duty but also wired in to use when mixing. If having them setup as outboard processors is only possible for either compressor or eq I would chose eq.
It depends a little on which interface you use and what connectivity your box allows for.
Never used the ISA430 but the principle is the same regardless: for mixing, you want to get your signal from the DAW out to the box (via one or two interface line outs, depending if you need to send a mono or stereo signal) and get it back into the DAW from one of the interface line ins.
A few interfaces have specialized "insert points" but most don't, so you use one or two line in/out couple for each box (a couple for mono, two couples for stereo). This means that your interface needs to have line ins/outs - and free ones at that.
As for cabling, ideally your interface exposes line ins/outs as XLR but many use jack sockets instead; when it comes to the outboard box, same thing - many use XLR for balanced line I/O but some boxes expose unbalanced TS. In case you deal with jacks you need to make sure you aren't using taking the signal from a balanced XLR output and using a TRS jack to send it to an unbalanced input as the two channels presented by the TRS will cancel each other and leave only noise.
You also need to make sure that the reference line level in the interface matches the expected reference level for the outboard box (-10dBV for "pro" gear as the ISA, +4dBU for consumer gear... of course to confuse things, -10dBV is actually a higher
voltage aka signal then the +4dBU :) )
Once all that is ready, you need a way to route the signal in your DAW and/or in the interface routing software. That depends on the interface and DAW.
In the DAW, often you have a specific plugin called "external insert" which can compute the actual delay in going thru the outboard (by sending a test signal) and allows you to simply plug the outboard as any other plugin (the sonic result will of course vary depending on the hardware settings, so no recall!).