Question:  I work for a small college TV station and we recently purchased some JVC GY-HM790 cameras and RM-HP250AU camera control units for our studio. Will these units interface into our existing Clear-Com Party-Line system, namely the MS-704 main station?  
Answer:  Yes, the RM-HP250AU CCU will interface directly into the MS-704, the CCU has options for Clear-Com and RTS 2-wire intercom connections.  Typically we recommend that only one camera CCU is interfaced into a Clear-Com 2-wire PL system in this manner, the audio will be acceptable under these circumstances.  If the user attempts to bridge in more than one CCU using the 2-wire interface on the PL line the impedance will be too low and the therefore so will be the audio on the PL line.  If multiple CCUs are to be deployed it is best to convert them to 4-wires to achieve best audio compatibility between the camera CCUs and the wired PL system. 
The information below is taken from the JVC intercom interfacing document:
The RM‐HP250AU can be connected to either a two wire balanced or a two wire unbalanced intercom system. The table below lists common intercom systems, audio type (balanced or unbalanced), system impedance and manufacturer.
Intercom System    Audio type    Termination Spec           Manufacturer
RTS™                   Unbalanced           200Ω              Telex Communications, Inc.
Clear-Com             Unbalanced           200Ω              Clear-Com Intercom Systems
Audiocom®            Balanced              300Ω              Telex Communications, Inc.
The RM-HP250AU has three terminals that are used to interface with balanced and unbalanced two wire audio intercom systems. These terminals are labelled G (ground) C (cold) H (hot) (see picture below).








Connecting the RM-HP250AU to an unbalanced 2-wire Clear-Com intercom system
Step 1
Make sure that S301 on the RM-HP250AU main board is set to the unbalanced audio position (RTS) which is the factory default position. To access the S301, the top cover of the CCU will need to be removed. Only remove the cover when the power cord is disconnected to prevent danger of electrical shock. Once the S301 switch is properly set reinstall the top cover to the CCU unit.
Step 2
When connecting to a 2 wire Clear-Com audio system, wire the Clear-Com three pin microphone XLRs to the RM-HP250AU as shown below.









Clear-Com Party-line Intercom wiring:
Pin 1 - Ground (G)
Pin 2 - +30Vdc (not used)
Pin 3 - Audio   (H)