80
Timing, IRIG-B and Pulses
9.4.1
Attaching Cables to Screw Terminals
Strip the insulation back to expose about 6 mm (1/4 in) of bare wire. DO NOT tin with solder. Insert the
stripped wire into the terminal and tighten the set screws clockwise to secure.
9.4.2
How Far Can I Run IRIG-B Cabling?
Factors to consider for long digital-signal cable runs: (1) resistive losses, (2) electromagnetic interference,
(3) propagation delays, and (4) installation and maintenance costs.
When a length of cable is laid from point A to point B there are two parts: one outgoing and one return.
For coaxial cable, the resistance is different for the center conductor than for the outer conductor, or shield.
For twisted pair wires, both outgoing and return wires will essentially be the same. As a simple example, to
connect an IRIG-B signal to a device 100 feet away from the clock, you must account for resistive losses in
200 feet of wire.
Two useful documents for details on distributing IRIG-B signals over long distances and maintaining
accuracy:
(1) Application Note 101, Distributing Timing Signals in a High EMI Environment.
(2)
IRIG-B Time Code Accuracy, IED and System Design Issues.
Both are available on this web page:
http://www.arbiter.com/resources/documentation.php
9.4.3
Synchronizing Multiple IED’s From One Clock Output
In many installations, master clock signals are distributed from one output to multiple devices. This method
makes more efficient use of the clock synchronizing capability since the clock drivers are designed to handle
multiple loads. The exact number of possible loads must be determined from the input impedance of each
connected IED.
9.4.4
Connecting Unmodulated IRIG-B
To drive multiple loads from one unmodulated IRIG-B output, make sure that the loads are wired in parallel.
A common term for this is “Daisy Chaining”, however the idea is to drive all of these loads in parallel from
a single output. It is simple to connect loads using unmodulated IRIG-B because all of the loads are driven
at the same voltage. Connect loads to one output until reaching the driver limit (dependent on specific
output).
To determine load current imposed on one Unmodulated IRIG-B output:
1. Determine the impedance or resistance (
R
dev
) of each IED load (check device manuals).
2. Calculate the load current for each device (
I
dev
= 5
V
÷
R
dev
).
3. To get the total current, sum up all the load currents for all devices connected to the output in
question.
I
devT
=
I
dev
1
+
I
dev
2
+
. . .
+
I
devN
,
where T = Total for N devices.
Unmodulated Example
If the input impedance of an IED is 5 kΩ, determine the device current (I) as seen in Calculation 9.1:
(9.1)
I
=
V
÷
R
dev
= 5
V olts
÷
5000 Ω = 0
.
001
A
(1
mA
)
Connecting ten of the same IED’s to the same output makes the total current draw: 10
×
0.001 A =
0.01 A (10 mA).
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 combined parallel impedance of all
devices. This current should not exceed 75 mA.
