C.2 Connecting the IRIG-B Output
159
C.2.4
Connecting Unmodulated IRIG-B
To drive multiple loads from one clock timing output, make sure they are wired in parallel. Some
call this “Daisy-Chaining”, however the idea is to drive all of these loads in parallel from the single
output. It is much simpler to connect loads to unmodulated IRIG-B than for modulated. This is
because all of the loads are driven at the same voltage and each draws current from the transmission
line.
To determine capacity for Unmodulated IRIG-B, follow these steps:
1. determine the number of loads to be connected to a single clock output
2. determine the impedance (or resistance) of each load
3. divide the drive voltage (5 V) by the resistance of each device
4. sum up all the load currents for the total current for one clock output.
Another method is to determine the lumped impedance of all of the connected IED’s in parallel.
Then, determine the overall current by dividing the drive voltage (5 V) by the computed lumped
impedance value. This current should not exceed 250 mA.
C.2.5
Wire Losses
Another factor affecting the available voltage is the resistive losses through the cabling. Wire has a
certain resistivity associated with it that is determined by its metallic composition, and resistance
determined by the diameter and length. For example, single-strand, 22 AWG (bare, enamel-coated)
copper wire has a resistance of approximately 19.6 ohms per 1000 feet. To compute the loss we
must include both wires in the connection, signal and return. For coaxial cabling, the resistance of
the center conductor is rated differently than the shield. For a twisted pair, both of them should
essentially have the same resistance per cut length. If we use a twisted pair of 22 AWG (copper
as above), then the available voltage (at 100 mA of current) for 500 feet of wire is calculated as
in C.2.
(C.2)
V pp available
= 5
.
0
−
I
×
19
.
6
wire
= 3
.
04
V pp
So, you can see that a considerable amount of the drive voltage is lost with 100 mA of current
and 500 feet of 22 AWG twisted pair transmission line. This level should would most likely be
detected by the decoder in most IED’s using CMOS, but not TTL. To avoid these problems, make
your cable runs as short as possible, to use larger diameter cable, and to carefully distribute the
loads.
C.2.6
Cable Delays
Electromagnetic waves travel at the speed of light (C) in free space or vacuum and a fraction of
that through cabling. The speed of a wave in free space is given in Identity C.3.
(C.3)
C
= 3
×
10
8
meters/second
