TPU2000/2000R Modbus/Modbus Plus Automation Guide
33
Calculating the load impedance presented to the IRIG B source generator is illustrated in Figure 4-14. Each IED
load on the IRIG B link presents a parallel impedance to the source. The general equation for parallel impedance
is:
1 = 1 + 1 + 1 + . . .
Z
Total
Z
1
Z
2
Z
3
I
Total
= I
1
+ I
2
+ I
3
+ . . .
This impedance equation simplifies to the form in Figure 4-14 when all IED loads are identical. If the loads are
not identical, the general equation listed above must be used to calculate the load.
1000 ohms = 1 unit load.
Load presented
to the IRIG B
Generator as per
example in Figure 1
1000
Ohms
1000
Ohms
1000
Ohms
Z
total
= 1/(N*(1/1000))
where N = number of DPU/GPU/TPU 2000/R Units.
Z
total
= 1/(3*(1/1000))
Z
total
= 333.33 ohms.
Thus the Source must be capable of driving a 333.33 ohm load.
Z
total
=
Figure 4-13. Load Impedance Calculation
The calculated load impedance for the architecture presented in Figure 4-14 is 333.33 ohms. In this example the
IRIG B receiver/converter must be capable of sending a three milli-amp TTL-level signal to a 333.33 ohm load. If
the source is not matched with the load impedance, IRIG B will not operate correctly.
The cable recommended to connect the IRIG B devices shall have the following characteristics:
Capacitance: less than 40 pF per foot line to shield
Construction: 2-wire
twisted pair shielded with PVC jacket
The maximum lead length of the entire relay is to be no more than 1000 feet. Cable types and vendors
recommended and supported by ABB to interconnect the IRIG B devices are: The required IRIG signal voltage is
5V with a minimum required current drain of 4 mA per device added to the network.
BELDEN 9841, BELDEN YM29560, or equivalent
An example of the terminal to terminal daisychain interconnection of three units is illustrated in Figure 4-15.