There are many different comms systems available, many, but not all of which are compatible to some degree. Comms systems are used to allow different departments to communicate and give and receive cues. Also known as 'Talkback' systems or 'Cans' systems. The systems most commonly used in live entertainment are what are known as Party Line systems where everyone on a communications ring can hear and speak to everyone else. More complex matrix systems which allow more structured communications systems to be created are appropriate for large events with many departments and crew where everyone really doesn't need to hear eveyone else.
TecPro / Canford
For replacement signal lamps use 24V 30mA wedge base lamps, T1 1/2 (5mm). RS 655-9766 / Farnell 1139323 / CPC SC00354.
Decended from the original 'Theatre Projects' communications system and sold for a some years as the 'Technical Projects' intercom, manufactured on the Isle of Wight. Since 1986 the Tecpro range has been owned by Canford Audio Ltd. and is manufactured by them in Washington in the North East of England.
The original TecPro logo, a tweaked "Pi" symbol; compare to Production Intercom who use a normal "Pi" symbol.
Originally a licencee of Technical Projects Limited, who produced TecPro equipment for the USA marketplace, identical to that available from Technical Projects for the UK market.
Something happened to Technical Projects Limited, with Canford aquiring the UK rights to the intercom system in 1986, and Production Intercom in the USA being able to carry on producing the same kit, though with modifications, for example the use of an LED signalling lamp rather than the incandescent lamp of the UK variant.
Production Intercom now have many items of intercom equipment not from the original licencee arrangement.
The schematic for the Tecpro BP-1 beltpack can be found here: 
The schematic for the Tecpro power supply can be found here: 
Amongst other things they do some really useful call light beacons, great for getting the attention of people who aren't always 'on cans' all the time.
See in particular their 'Party-line' system.
Headset connector: XLR4
4 Pin headset systems:
- Pin 1 Mic (low)
- Pin 2 Mic (high)
- Pin 3 Headphone (low)
- Pin 4 Headphone (high)
6 Pin headset systems
- Pin 1 Mic (low)
- Pin 2 Mic (high)
- Pin 3 Headphone common
- Pin 4 Chan B Headphone (high)
- Pin 5 Chan A Headphone (high)
- Pin 6 Headphone common
See in particular:
- RTS Two-Wire systems
- Telex BTR wireless systems.
General Useful Information
- A circuit from Brian for additional LED call light boxes with no external PSU required
- Various useful projects/circuits/notes about comms from DMX512.com
Clear-Com, Tecpro, ASL and Stonewood are all broadly compatible although you may find differences in volume between users of different brand belt packs or other types of station in a mixed system.
Telex RTS systems are not directly compatible with the other systems above although they are all technically "two wire" systems in telecommunications parlance. In particular RTS uses a higher DC supply voltage than the others and different audio levels as well.
It is possible to inteface between almost all different types of system using devices called 2:4 wire adaptors. Setting these devices up is not for the faint hearted however.
A useful guide to basic "Cans Etiquette" can be found here (Scroll down!):
- BR thread on comms procedure
- Theatrecrafts.com Stage Management, Backstage Calls/Cans Etiquette page
To which I will add one tip. Use the 'Cocktail Party Effect' to your advantage; when calling someone e.g. on a radio, Always use the 'From' construction, like this:
"LX from Front of House"
or: "You from Me" , "Bill from Fred"
...rather than "Front of House to LX" or "Me to You", "Fred to Bill".
This avoids the otherwise inevitable "who was calling LX?" reply because with the 'From' construction the called party mentally 'tunes in' when they hear their 'name' in time to hear who is calling at the end of the transmission. This opportunity is lost if you use the "To" format, because by the time called party hears their 'name' it's all over. Once you are used to it it becomes quite natural and you can actually omit the word 'from', just keeping the order the same, e.g. "LX, Front of House" (N.B. no offence intended to LX technicians or FOH folk, names and departments picked at random purely for illustration!)
- The 173 page HANDBOOK OF INTERCOM SYSTEMS ENGINEERING  from RTS is very comprehensive