Sounds to me like you're using the wrong flavour of serial cable. Mac serial cables come in two varieties.
The older kind, which I believe may have been used for the original ImageWriter printer (not the II), are straight through, and are not much use because connecting pin 1 to pin 1, 2 to 2, 3 to 3, and so on means the drivers are connected to their counterparts instead of to the opposite receivers and the receivers are connected to their counterparts instead of to the opposite drivers, thus defeating the purpose of using RS-422 ports (same at both ends) instead of the, in 1984 more common, RS-232 (distinguishes upstream ports from downstream ports, hence the existence of null-modem cables to connect two upstream ports directly and why Macs never needed them).
Then there's the more usual kind of cable, which has the transmit pins at each end wired up to the receive pins at the other end. This latter kind is what you should be using, but if you were accidentally using the straight-through kind you would see exactly the results you saw, and possibly damage your serial ports into the bargain due to duelling line drivers.
The two kinds of cable are only distinguishable with an ohmmeter and at least three very steady hands, which was incredibly bad planning on Apple's part. You'd think they would have at least marked them somehow.
I apologize for not answering this before you threw in the towel. I would have if I'd bothered to actually read it at the time.
The older kind, which I believe may have been used for the original ImageWriter printer (not the II), are straight through, and are not much use because connecting pin 1 to pin 1, 2 to 2, 3 to 3, and so on means the drivers are connected to their counterparts instead of to the opposite receivers and the receivers are connected to their counterparts instead of to the opposite drivers, thus defeating the purpose of using RS-422 ports (same at both ends) instead of the, in 1984 more common, RS-232 (distinguishes upstream ports from downstream ports, hence the existence of null-modem cables to connect two upstream ports directly and why Macs never needed them).
Then there's the more usual kind of cable, which has the transmit pins at each end wired up to the receive pins at the other end. This latter kind is what you should be using, but if you were accidentally using the straight-through kind you would see exactly the results you saw, and possibly damage your serial ports into the bargain due to duelling line drivers.
The two kinds of cable are only distinguishable with an ohmmeter and at least three very steady hands, which was incredibly bad planning on Apple's part. You'd think they would have at least marked them somehow.
I apologize for not answering this before you threw in the towel. I would have if I'd bothered to actually read it at the time.