Figure 6-14. Circuitry Topology
IED IED
Termination
Resistor
+
IED
-
GridTime™
3000
Port
-
+
Termination Resistor Selection
TTL Signals
When outputting TTL signals:
• DCLS or Modified Manchester IRIG-B
• User-defined pulses
• DCF77
It is recommended that a single termination resistor is used to dampen signal overshoot and reflections that result
from the sharp edges of the signal. The termination resistor must meet the following requirements:
• its resistance should be approximately the same as the characteristic impedance of the bus cable, which should
be in the cable’s datasheet.
• it should have a sufficient power rating. Use
Power = Voltage
2
/Resistance
to calculate the maximum power
dissipation required, with 5 V as the Voltage term (all TTL signals from the ports are 0-5 V), and the resistance
value of the resistor as the Resistance term.
AM-IRIG
When outputting AM IRIG-B signals it is recommended that a termination resistor is used to ensure end devices
receive the correct voltage levels.
R
term
= (((R
s
× V
req
)/(V
out
- V
req
)) - 1/R
l
)
-1
If R
term
comes out negative, the bus is overloaded. Refer to the
6.6.2.3.1. Cable Recommendations
.
Fixed Frequency
Loading and Cable Length Recommendations
The BNC ports are rated to supply 150 mA of current, more current than this will trigger a high current alarm. Ensure
the combined maximum current draw of all devices doesn’t exceed 150 mA. The maximum expected current draw of
each end device can be added to calculate the combined maximum current draw.
Earth Cables
For STP, earth shield in a way that avoids earth loops.
Installation
©
2022 Microchip Technology Inc.
and its subsidiaries
Manual
DS00004572A-page 43
