Specifications

December 2008 Page 22–3
Chapter 22. Sub-Rate Data (SRD-2/SRD-4) Module
22
(microcomputers, protective relays, event
recorders, modems, etc.) from one remote site to
another. Typical devices you can connect to the
module include:
• An RTU
•Amodem you want to connect to fiber
optic cables across DS0
•A"smart switch" that can select the correct
device
• IED (Intelligent Electronic Device) for
retrieval of data
• SEL

pilot relay channel using "mirrored
bits" protocol
• The two RS-485 functions on the SRD-4N
are connected on the interface to permit a
LAN extension for protocols like DNP.
22.2.1 Typical RS-232 Application
Figure 22-5 shows a typical RS-232 application.
Substation A is a conventional installation with a
remote terminal unit (RTU) connected to the
system SCADA line via a modem. Substations B
and C are linked by FOCUS. The RTU in
Substation B is linked to the SCADA line in the
usual manner. Substation C's RTU, however, is
linked through channel A on the FOCUS T1/E1
line to the SCADA line via a second modem at
Substation B. This eliminates the need to extend
the SCADA line to Substation C.
Data terminal equipment (microcomputers,
protective relays, event recorders, etc.) at
Substations B and C are directly linked through
the second RS-232 data circuit, or channel B. Data
and control signals are transparently transmitted
across both circuits. The data transfer is asynchro-
nous only.
22.2.2 Five volt auxiliary output
The RS-232 outputs may be supplied with +5 volt
dc power, suitable for powering small interface
devices. These devices may include RS-232 to F0
or RS-232 to RS-485 converters, similar to the
SEL

2885. Jumpers (0Ω resistors, R3, R4, R5 &
R6) are required on the interface module for each
DB-9 where +5V is desired on pin 1. Please see
Figure 22-8 for the DB-9 pinout.
22.2.3 SRD-2
This version has two RS-232 channels per each of
two DS0s. Therefore the two DS0s in a given DS0
are mapped to the same two end nodes. The
second DS0 may be mapped to the same or
different remote node. Sub-channels A & C are in
one DS0, as are sub-channels B & D. You must
use sub-channel A with either A or B at the remote
end. Similarly, you must use sub-channel C or D
with either C or D at the remote end.
IED
IED
IED
FOCUS FOCUS
RS-485 LAN
FOCUS
IED
IED
Figure 22–6. SRD-4N Typical LAN Extension.