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Date Code 20011026
Installation
6-5
SEL-321/321-1 Instruction Manual
IRIG-B Input Connection
The SEL-321 Relay accepts a demodulated IRIG-B signal through a rear-panel 9-pin,
subminiature "D" connector labeled “DEMODULATED IRIG-B”. This allows synchronization
with an external time source. See
Communications Ports Specifications
in
Section 2:
Specifications
for connector pinout and additional details.
The IRIG-B signal includes code for time-of-day and day-of-year. It does not include a code to
identify the year. To ensure the device calendar is set to the proper year, set the date using the
DATE command (see
Section 3: Communications
). The relay stores the year for the set date
with the relay settings in nonvolatile memory, so once the date is properly set, the relay will
maintain the proper year even if relay power is cycled off and on. The relay does not maintain
the time, month or day when power is off. If no external IRIG-B input signal is applied, the relay
will start its clock/calendar at 0:00:00 and 1/1/YY at power up.
Where distance between the SEL-321 Relay and the IRIG-B sending device exceeds the cable
length recommended for conventional EIA-232 metallic conductor cables, you can use modems
to provide isolation and to establish communications to remote locations. Unfortunately,
conventional short-haul, fiber optic, and telephone modems do not support IRIG-B signal
transmission, so their use requires that you use some other method to synchronize the time in the
SEL-321 Relay. Special fiber optic modems (like the SEL-2810) are available, however, that
include a channel for the IRIG-B time code; these modems enable you to synchronize more
precisely with devices (such as the SEL-2020) capable of sending IRIG-B time code, even with a
fiber optic communication link.
The actual IRIG-B input circuit is a 330
Ω
resistor in series with an optocoupler input diode.
The input diode forward voltage drop is about 1.5 V. Driver circuits should put approximately
10 mA through the circuit when “on” (this would be accomplished by a 0 - 5 Vdc signal capable
of supplying at least 10 mA when at 5 V). Ensure that the two connector pins are connected with
proper polarity.
The IRIG-B serial data format consists of a one-second frame containing 100 pulses that are
divided into fields. The relay decodes the second, minute, hour, and day fields and sets its
internal time clock accordingly.
When IRIG-B data acquisition is activated, either manually (with the IRIG command), or
automatically, two consecutive frames are taken. The first frame is updated by one second and
then compared with the second frame. If they do not agree, the relay considers the data
erroneous and discards it.
The relay reads the time-code automatically about once every minute. The relay stops IRIG-B
data acquisition ten minutes before midnight on New Year’s Eve every year so the relay clock
may implement the year change without interference from the IRIG-B input. Ten minutes later,
the relay restarts IRIG-B data acquisition.
Communication Circuit Connections
The SEL-321 Relay is equipped with two rear-panel serial communication ports, labeled
“SERIAL PORT 1” and “SERIAL PORT 2”, and one front-panel serial communication port,
labeled “SERIAL PORT 3”. The data connection for each SEL-321 serial communication port
uses EIA-232 standard signal levels in a 9-pin, subminiature "D" connector (see Figure 6.1 for
pin number convention and
Communications Ports Specifications
in
Section 2: Specifications
Summary of Contents for SEL-321
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