Emax

In my experience with comms between an embedded system and a modern PC, 
your biggest problem is likely to be the timings.

Not timing between edges on the data (and control lines? does emu gear 
implement these? I don't have my emax service manual right now) but 
timing at more of the "transport" level - between data packets. 
especially requests, and acks. Possibly even timing between bytes...

I haven't done anything with RS422 since my first proper job, 9 years 
ago (eeeek!) but RS422 is pretty much the same as RS232, in terms of 
signalling, only lower level and balanced. As far as voltage goes, there 
is a lot of margin in the design between what a transmitter should aim 
for and what a receiver should act on.

Anyway - I've done a lot with RS232 lately, and can tell you that USB 
converters on a PC for RS232 can fail with older embedded gear, because 
the embedded gear was written when computer interrupts were a lot 
tighter, and serial comms were not also piled on top of a USB interface 
with its own hold-ups and data negotiation crap..... they could expect 
an ack to a query a lot faster than they get now...... and so their 
timeouts are pretty intolerant of sloppy responses, and with USB 
interfaces especially, they can very easily fail.

if you wanted to try something on a PC, have a go with a PCI (or PCMCIA 
or expresscard if you have a laptop) card that does RS422. they'll be 
more expensive than the USB equivalent, but you won't get the faffing 
about related to the USB layer that is almost definitely the thing 
killing your comms attempts.

Also - no need to buy a digital storage CRO here - if you want to 
analyse the integrity of serial data, you can see a lot with just an old 
analogue CRO - looking at the integrity of the "eye" as long stream of 
data goes past. If you want to analyse the actual data, then you're much 
better off with a logic analyser. a cheap USB logic analyser would do 
exactly what you want, and you'd be able to look really in depth at the 
timings, at all of bit level, byte level, and packet level, if you get 
one with a half decent amount of memory.