BID 710 was line interface. The 710 was used to interface 610/700 setups to the Bruin system or British Telecom landlines. When connected to Bruin, the 710 would interface to a TTVF 4/12 which converted telegraph signals to voice frequencies. Used as single unit it, provided 4 teleprinter channels over a phone line (or BID200, normally Channel 6 but after the data mod it was CH 5 ) When used with a combining box, it with three TTVF units you could get 12 channels of telegraph over a speech channel. The 610/700/710/TTVF setup was also used to connect to VHF radio.
It should be noted that there was a unit that interconnected the BID devices in the secure area with the outside world. Known as an ITL, (Isolator Transmission Line), it consisted of screened relays that resulted in only a magnetic coupling between the two areas. Later models used an opto isolator. ITL was not assigned a BID designator.
A separate telegraph earth was also used in the secure area. All this along with the transition to low
voltage switching (rather than the original teletype +/-80V) minimized any radiation (TEMPEST signature) of plain text from the secure area.