Advantech TPC-1881WP Series, User Manual

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TPC-XX81WP
User Manual
38
D.1 Notice
1. RS-422/485 terminal impedance calculation:
In order to improve transmission speed for big data and provide steady signal in long
distance transmission of RS422/485, we'll add a terminal impedance that matches
transmission line's impedance.
Another key for correct signal transmission is that the Vfsb of receptor must be
+/-0.2V. If Vfsb is under 0.2V, then it'll come out abnormal transmission result. The
terminal impedance is the most important condition to maintain Vfsb above 0.2V.
The RS422/485 I/O of TPC-1581WP has been added pull high & pull low resistor, as
shown in following diagram:
When using RS422/485 port of TPC-1581WP/1881WP, please apply below formula
to calculate terminal impedance to meet the standard.
V
fsb

0.2 V
V
fsb
=0.2V, Vcc=5V, R
a
=R
b
=4.7K
Ω
V
fsb

Vcc ( R
c
/ ( R
a

R
c

R
b
))