Specifications

RS-422/485 Application Note 31
© 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 [phone]
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax [phone]
Special Consideration for Fault Conditions
Data systems that could be exposed to short circuits to power conductors
require an extra measure of protection. In these cases its recommended to add a
fuse type device in addition to shunting type suppression, as shown in Figure 4.9.
When a short circuit occurs, the shunt suppression will begin conducting, but
shunting by itself cannot withstand the steady state currents of this type of surge.
A small enough fuse value should be chosen so that the fuse will open before the
shunt device is damaged. A typical fuse value is 125 mA.
Figure 4.9 Fused port protection
Choosing the right protection for your system
While it is hard to predict what type and level of isolation is correct for a
system, an educated guess should be made based on the electrical environment,
physical conditions and cost of failures in downtime and repair costs. Systems
connected between two power sources, such as building to building, office to
factory floor, or any system covering long distances should require some level of
transient protection. Table 4.1 is a comparison of transient protection
techniques.
Device
Vcc
Earth Ground
Signal Ground
Data Lines
125 mA Fuse
32 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 [phone]
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax [phone]
Table 4.1 Comparison of Protection Techniques
Optical Isolation Shunting
Requires no ground reference Must have low impedance ground path
Adds no loading to data lines Presents additional capacitive loading to data
lines
Higher complexity Lower complexity, uses passive components
Effective on common mode
transients
Effective on both common and differential
mode transients
Not dependent on installation quality Can be improperly installed by user
Requires an external power source No power required
Not affected by long term or
continuous transients
Subject to damage by long duration transients