I have precisely the cable linked in the first post and was able to connect two SE/30s running 6.0.8 and 7.0.1 directly through localtalk.

I was unable to connect a Classic running 6.0.8, an SE running 7.0.1, or a Quadra 700 running 7.6.1 to either SE/30 or each other in any combination.

Are damaged serial ports common in older Macs or am I overlooking something simple? Is there a software test that can be performed to check the health of printer/modem ports?