The REM-3 isolator is an inline adapter. It plugs into a Type 3 E&M port. The audio lines pass straight through, but both the E lead (PTT) and M lead (COR) are buffered with an optocoupler.
The REM-3 series of isolators are approximately 6" long. The RJ45 plug connects to the routers E&M port. The radio connects to the female RJ45. It makes a Type 3 port, look like a Type 5 port, and uses the signaling battery voltage to make interfacing easy. The REM-3 has the audio lines connected straight through (3+6, 4+5), and it has opto couplers on the E+M leads. The REM-3 and 3R isolators can sink up to 120mA on the E lead (PTT). The REM-3HC isolator can sink up to 2500mA on the E lead (PTT).
Uses include preventing old style Cisco E+M ports from causing the radio to go into transmit while the router is rebooting. Failover Isolation, to allow hot standby routers. Easier interfacing to the E or M leads. Protect the port when experimenting and prototyping.
REM-3-R is electrically the same, except the gender of the connectors are reversed. Router connects to jack.
REM3 (Grey) / REM3R (Black)
The REM-3HC is a higher current version of the REM3
The REM-3HC is used with REM-229