Specifications

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Communication Overview
31
COM
RS-422
Balanced Transmission
The RS-422 protocol greatly expands the practical possibilities
of the serial bus. It provides a mechanism by which serial data
can be transmitted over great distances (to 4,000 feet) and at
very high speeds (to10 Mbps). This is accomplished by splitting
each signal across two separate wires in opposite states, one
inverted and one not inverted. The difference in voltage between
the two lines is compared by the receiver to determine the logical
state of the signal. This wire configuration, called differential
data transmission or balanced transmission, is well suited to noisy
environments. With RS-232 communication, which is unbalanced
transmission and uses only one wire, signal degradation can take
place if there is a difference in ground potential between the
transmitting and receiving ends of the cable. With balanced
transmission, this potential difference will affect both wires equally,
and thus not effect their inverse relationship. Twisted pairs of
wire, which ensure that neither line is permanently closer to a
noise source than the other, are often used to best equalize
influences on the two lines. Errors can be caused by high noise
levels affecting one side of the receiver to a different extent than
the other. To combat this, each receiver is generally grounded.
Data
Data
Data +
Data-
R
t
Transmitter
Receiver
(Balanced Transmission)
Errors in balanced transmission systems such as RS-422 can also
be caused by signal reflections. As data transfer speeds increase
and travel over longer distances, the signal can be reflected back
from the far end of the wire. To combat this, termination resistors
are placed at the far end of the cable which make the cable
appear electrically as if it is infinitely long--infinitely long lines
don't have ends, and thus can't reflect from one end to the
other. These termination resistors will differ depending on the
protocol used. For RS-422 a 100 ohm resistor is placed at the
receiving device.
Full-Duplex, Half-Duplex, and Multipoint Systems
An RS-232 based system allows only two devices to communicate.
With RS-422 a master can use one communication line to converse
with up to 10 slaves. With that many parties wanting to talk, a
mechanism for controlling the conversation must be implemented.
Two such mechanism's exist: Full-Duplex and Half-Duplex. In
Half-Duplex operation communication can take place in only
one direction at a time. This is an obvious solution to a balanced
system employing only one set of wires--if signals were coming
in both directions at once, they would conflict with each other.
However, even systems in which different pairs of wire are used
for transmission and reception can still operate in Half-Duplex
mode. In such a case, each data exchange is predicated upon
the previous one, and will not take place without proper
handshaking.
In Full-Duplex systems, transmission and reception can occur at
the same time. Thus Point A can send information to Point B
while at the same time receiving data from Point B. Full-Duplex
operation becomes especially important in systems where
a single master is communicating with multiple receivers.
With a Full-Duplex configuration, Point A can send data
to Point B while receiving data from Point C.
The nature of a Full-Duplex system lends this configuration
to a network of multiple devices communicating with each
other using a single data line. However, the RS-422 protocol
does not permit this type of communication. With RS-422 there
can be only one driver, though there can be up to 10 receivers.
Thus, only the master Point A can communicate directly with
points B through K. If Point C wants to transmit data to Point H,
it must go through Point A to do it.
RS-485
The True Multidrop Network
RS-485 is an upgraded version of the RS-422 protocol that was
specifically designed to address the problem of communication
between multiple devices on a single data line. It is a balanced
transmission system that is virtually identical to RS-422 with the
important addition of the ability to allow up to 32 devices to
communicate using the same data line. Thus any Point A through
Point FF can directly communicate with each other, taking on
the role of master and slave as needed. This is achieved with
tristatable drivers which are usually controlled by a programmable
handshake line to ensure that only one device acts as a driver at
any one time.
In such a system, the RS-485 line cannot be thought to have a
beginning and an end, because communication can be initiated
from any point on the line. Thus, terminating resistors must be
placed at both ends of the RS-485 wire to achieve the infinite
line illusion. For RS-485, 120 ohm resistors are placed at the
two furthest points of the communication link.