Specifications

30 RS-422/485 Application Note
 Copyright B&B Electronics -- Revised October 1997
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH [phone] -- FAX [phone]
Figure 4.7 Isolated node with shunt protection to earth ground
Figure 4.8 Isolated port with ungrounded shunt protection
The method shown in Figure 4.7 is recommended, in this case isolation
protects the circuit from any voltage drops in the earth ground connection. The
shunt devices will prevent a surge from exceeding the breakdown voltage of the
isolators as well as handling any differential surges on the cable. Figure 4.8
illustrates a method recommended for cases where there is no way to make an
earth ground connection. Here, the shunt device’s function is to protect the port
from differential surges, a differential surge will be balanced between conductors
by the shunting device, converted to common mode. The isolation provides
protection from the common mode transient remaining.
Isolated Power
Device
Vcc
Port
Earth Ground
Ground line
Shunting Device
Data Lines Out
Shunting Device
Device
Isolated Power
Vcc
Port
Signal Ground
Data Lines