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SD Channel Design Requirements
The SD (signal detect) channel is designed for a transceiver output that has a DC coupled
differential output with 100-1000 mV amplitude. If a transceiver with LVTTL/LVCMOS output is
used, the signal needs to be conditioned using a few resistors, to obtain desired signal levels for
the brick.
Each line is pulled to GND by a 1.27 kΩ resistor on the brick.
Even if the transceiver has a single ended output and the other line is at a fixed reference
potential, it is recommended to route SDN and SDP side by side all the way to the signal
conditioning resistors. This will give the interference, collected by the transmission line, common
mode characteristics, and it can thus be ignored by the differential input, instead of becoming a
differential mode interference that would corrupt the signal.
Below is a figure describing three different ways to connect a transceiver output to an SD
channel of the brick:
Brick
Trans-
ceiver
Trans-
ceiver
Trans-
ceiver
Brick
Brick
a: Differential
LVPECL output
b: Single ended LVPECL output
If the output is active low:
Connect the output to SDN and
the 820 Ω resistor to SDP .
c: LVCMOS output. If the output is
active low, connect the 1.8 kΩ
resistor to SDN instead.
Fig. 13
In case a and case b, additional pull-down resistors will be required if the LVPECL outputs require
a certain bias current (> 1 mA) to function.
Anybus
®
CompactCom
™
B40 Modbus Serial User Manual
SCM-1202-130 1.1 en-US