Specifications

RS-422/485 Application Note 27
© 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]
Vcc
Port
Device
Data Lines
Local Chassis Ground Connection
Ground line
Figure 4.5 RS-485 Device with Signal Ground
Connected to Chassis Ground
Transient Protection using Isolation
Isolation Theory
The most universal approach to protecting against transients is to
galvanically isolate the data port from the host device circuitry. This method
separates the signal reference from any fixed ground. Optical isolators,
transformers and fiber optics are all methods commonly used in many types of
data networks to isolate I/O circuitry from its host device. In RS-422 and RS-
485 applications, optical isolators are most common. An optical isolator is an
integrated circuit that converts the electrical signal to light and back, eliminating
electrical continuity. With an isolated port, the entire isolated circuitry floats to
the level of the transient without disrupting data communications. As long as the
floating level of the circuitry does not exceed the breakdown rating of the
isolators (typically 1000 - 2500 volts) the port will not be damaged. This type of
protection does not attempt to absorb or shunt excess energy so it is not sensitive
to the length of the transient. Even continuous potential differences will not
harm isolated devices. It is important to note that isolators work on common
mode transients, they cannot protect against large voltage differences between
conductors of a data cable such as those caused by short circuits between data
and power circuits.
28 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]
Isolation Devices
Optical isolation can be implemented in a number of ways. If a conversion
from RS-232 to RS-422 or RS-485 is being made, optically isolated converters
are available. Optically isolated ISA bus serial cards can replace existing ports
in PC systems. For systems with existing RS-422 or RS-485 ports, an optically
isolated repeater can be installed. Examples of each of these type devices can be
found in the B&B Electronics Data Communications catalog.
Transient Protection using Shunting
Shunting Theory
Creating one common ground at the host device provides a safe place to
divert surge energy as well as a voltage reference to attach surge suppression
devices to. Shunting harmful currents to ground before they reach the data port
is the job of components such as TVS (often referred to by the trade name
Tranzorb), MOV or gas discharge tubes. These devices all work by “clamping”
at a set voltage, once the clamp voltage has been exceeded, the devices provide a
low impedance connection between terminals.
Since this type of device diverts a large amount of energy, it cannot tolerate
very long duration or continuous transients. Shunting devices are most often
installed from each data line to the local earth ground, and should be selected to
begin conducting current at a voltage as close as possible above the systems
normal communications levels. For RS-422 and RS-485 systems, the voltage
rating selected is typically 6 - 8 volts. These devices typically add some
capacitive load to the data lines. This should be considered when designing a
system and can be compensated for by derating the total line length to
compensate for the added load. Several hundred feet is usually adequate.
To apply these type products correctly they should be installed as close to
the port to be protected as possible, and the user must provide an extremely low
impedance connection to the local earth ground of the unit being protected. This
ground connection is crucial to proper operation of the shunting device. The
ground connection should be made with heavy gauge wire and kept as short as
possible. If the cable must be longer than one meter, copper strap or braided
cable intended for grounding purposes must be used for the protection device to
be effective. In addition to the high frequency nature of transients, there can be
an enormous amount of current present. Several thousand amps typically result
from applications of the combination wave test in the ANSI and IEC
specification.