The SC821 is used to debug serial communication between, say, a uC and a PC. I don't know if it only works with RS232 voltage levels or whether it also works with TTL levels. In most cases, when I use RS232, it is at the RS232 voltage levels because I use a USB->RS232 Serial converter cable. I also have USB->TTL Serial converter cables for 3.3 and 5V.
It's a fairly expensive tool, I would hang on to it. Forty years ago it would have been in very high demand! Time moves on...