An optional system expansion device.
Allows an ECU-224 desk controller to be connected to an ECU-4, 8 or 16 SigTEL controller to simplify call handling on larger SigTEL applications.
Mounts inside an ECU-4, 8 or 16 (can also be used to network the ECU-4, 8 or 16 to another controller if required).
Not compatible with ECU-8NT controllers.