Right, I'm back up to speed now, mostly.
What cable are you connecting the 'cat to the Z with? Since you say you have a 5500, I'm guessing you're using the standard sync cable. Because the sync cable is wired to present the Z as a peripheral (since you'd usually be connecting to a PC) you would need a null-modem in-between to swap the TX and RX lines.
(And remember that the 5500 has TTL serial lines itself )