3.3 DIRECT INPUT AND OUTPUT COMMUNICATIONS
The UR-series IEEE C37.94 communication modules (modules types 2G, 2H, 76, and 77) are designed to interface with
IEEE C37.94 compliant digital multiplexers or an IEEE C37.94 compliant interface converter for use with direct input and
output applications. The IEEE C37.94 standard defines a point-to-point optical link for synchronous data between a multi-
plexer and a teleprotection device. This data is typically 64 kbps, but the standard provides for speeds up to 64n kbps,
where n = 1, 2,..., 12. The UR-series C37.94 communication modules are either 64 kbps (with n fixed at 1) for 128 kbps
(with n fixed at 2). The frame is a valid International Telecommunications Union (ITU-T) recommended G.704 pattern from
the standpoint of framing and data rate. The frame is 256 bits and is repeated at a frame rate of 8000 Hz, with a resultant bit
rate of 2048 kbps.
The specifications for the module are as follows:.
IEEE standard: C37.94 for 1 × 128 kbps optical fiber interface (for 2G and 2H modules) or C37.94 for 2 × 64 kbps opti-
cal fiber interface (for 76 and 77 modules)
Fiber optic cable type: 50 nm or 62.5 µm core diameter optical fiber
Fiber optic mode: multi-mode
Fiber optic cable length: up to 2 km
Fiber optic connector: type ST
Wavelength: 820 ±40 nm
Connection: as per all fiber optic connections, a Tx to Rx connection is required
The UR-series C37.94 communication module can be connected directly to any compliant digital multiplexer that supports
the IEEE C37.94 standard as shown below.
The UR-series C37.94 communication module can be connected to the electrical interface (G.703, RS422, or X.21) of a
non-compliant digital multiplexer via an optical-to-electrical interface converter that supports the IEEE C37.94 standard, as
In 2008, GE Digital Energy released revised modules 76 and 77 for C37.94 communication to enable multi-ended fault
location functionality with firmware 5.60 release and higher. All modules 76 and 77 shipped since the change support this
feature and are fully backward compatible with firmware releases below 5.60. For customers using firmware release 5.60
and higher, the module can be identified with "Rev D" printed on the module and is to be used on all ends of L60 communi-
cation for two and three terminal applications. Failure to use it at all ends results in intermittent communication alarms. For
customers using firmware revisions below 5.60, it is not required to match the revision of the modules installed.
L60 Line Phase Comparison System
3.3.8 IEEE C37.94 INTERFACE