RS-422/485 Application Note 29
© Copyright B&B Electronics -- Revised 2506
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax 353-91-792445
Connecting Signal Grounds
Since a local ground connection is required at each node implementing shunt
type protection, the consequences of connecting remote grounds together must
be considered. During transient events a high voltage potential may exist
between the remote grounds. Only the impedance in the wire connecting the
grounds limits the current that results from this voltage potential. The RS-422
and RS-485 specification both recommend using 100 ohm resistors in series with
the signal ground path in order to limit ground currents. Figure 4.6 illustrates the
ground connection recommended in the specification.
Figure 4.6 Signal Ground Connection between two nodes
with 100 ohm resistor
Shunting Devices
There are two types of shunting devices to choose from. The least expensive
type is single stage, which usually consists of a single TVS device on each line.
Three stage devices are also available. The first stage of a three-stage device is a
gas discharge tube, which can handle extremely high currents, but has a high
threshold voltage and is too slow to protect solid state circuits. The second stage
is a small series impedance which limits current and creates a voltage drop
between the first and third stage. The final stage is a TVS device that is fast
enough to protect solid state devices and brings the clamping voltage down to a
safe level for data circuits.
Combining Isolation and Shunting
Installing a combination of both types of protection can offer the highest
reliability in a system. Figures 4.7 and 4.8 illustrate two means of implementing
this level of protection.
30 RS-422/485 Application Note
© Copyright B&B Electronics -- Revised 2506
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax 353-91-792445
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 Ou
Shunting Device
Device
Isolated Power
Vcc
Port
Signal Ground
Data Lines
(96 pages)
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