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Selecting and U=
sing
RS-232, RS-422, and RS-485 Serial Data Standards
"The great thing about standards is there are so many=
to
choose from." This statement was made at a recent conference on fiber
optics, and it holds true for electrical-interface standards as well.
Serial-data standards tend to evolve separately, within particular
industries, thus we have more standards than we should. Perhap=
s the
most successful serial-data standard for PC and telecom applications is t=
he
RS-232. Similarly, the RS-485 and RS-422 are among the most successful
standards for industrial applications. These standards are not directly
compatible, however for purposes of control and instrumentation it is oft=
en
necessary to communicate between them. This article offers insight into t=
he
different standards (electrical physical-layer specifications), how to
convert from one standard to another standard, and how to combine differe=
nt
standards within the same application. The RS=
-232
link was initially intended to support modem and printer applications on =
IBM
PCs, however it now enables a variety of peripherals to communicate with =
PCs.
The RS-232 standard was defined as a single-ended standard for increasing
serial-communication distances at low baud rates (< 20kbps). Over the
years the standard has changed as necessary to accommodate faster drivers
like the MAX3225E, which offers 1Mbps data-rate capability. For RS-232
compliance, a transceiver such as the MAX3225E must meet the electrical
specifications listed in Table 1. A typical connection (Figure 1) shows t=
he
use of hardware handshaking to control the flow of data.
A typi=
cal RS-232
signal (Figure 2, CH1) swings positive and negative; note the relative
location of the 0V trace markers on the left axis. Although the RS-232 da=
ta
is inverted, an overall translation from TTL/CMOS to RS-232 and back to
TTL/CMOS restores the data's original polarity. Typical RS-232 transmissi=
ons
seldomly exceed 100 feet for two reasons: The difference between transmit=
ted
levels (±5V) and receive levels (±3V) allows for only 2V of
common-mode rejection, and the distributed capacitance of a longer cable =
can
degrade slew rates by exceeding the maximum specified load (2500pF). Since
the RS-232 was designed as a point-to-point rather than multi-drop interf=
ace,
its drivers are specified for single loads from 3k
Daisy-Chain De=
vices To eli=
minate
the problems associated Daisy-Chain networks Maxim developed the
MAX3322E/MAX3323E, which are specifically designed to be configured in
multi-drop applications. These unique devices employ a logically switched
input resistance of 5k Another
solution would be to convert the RS-232 RX and TX signals to an RS-422 si=
gnal
(see Table 2). RS-422 is a differential standard that allows transmission
over much greater distances. The higher input resistance of RS-422 inputs,
combined with their higher drive capability, allows a connection of up to=
10
nodes (Figure 4). Another advantage of RS-422 is the separate transmit and
receive paths, for which no direction control is needed. Any necessary
handshaking between devices can be performed with either software (XON/OFF
handshaking) or hardware (a separate set of twisted pairs). The MAX3162
provides an economical way to translate between RS-232 and RS-422 signals.
For more detail, refer to the section titled "RS-232/RS-485
Protocol Translators".
RS-422=
and
RS-485 transceivers are often confused with each other; one is assumed to=
be
a full-duplex version of the other. However the electrical differences in
their common-mode ranges and receiver-input resistances suit these standa=
rds
for different applications. Since the RS-485 meets all of the RS-422
specifications (Table 3), RS-485 drivers can be used in RS-422 applicatio=
ns.
The opposite, however, is not true. The common-mode output range for RS-4=
85
drivers is -7V to +12V, where as the common-mode range for RS-422 drivers=
is
only ±3V. The minimum receiver-input resistance is 4k Tab=
le 1.
RS-232 Summary of Major Electrical Specifications
Differ=
ential
RS-485 transmissions (Figure 6) produce opposing currents and magnetic fi=
elds
along each segment (wire) of a twisted-pair cable; thus minimizing the
emitted electromagnetic interference (EMI) via cross-canceling of the
opposing fields around each wire. For transmissions through a long cable =
or
at high data rates, the cable appears as a transmission line and should be
terminated with the cable's characteristic impedance. This aspect of the
RS-485 connection causes confusion. Does the line need to be terminated, =
and
if so how should it be terminated? If the designer is not the end user,
should these questions be left for the installer to figure out? For most
RS-485 transceivers, the data sheet indicates a simple choice between no
termination, and a simple point-to-point termination when the cable acts =
as a
transmission line (Figure 7). A termination resistor across the A-B termi=
nals
is harmless. By default, the transmission line should be terminated at the
last transceiver on the line (bus).
Fail-Safe Unfort=
unately,
different runs of chips can produce different output signals on RO for a =
0V
differential input. The prototype can work perfectly, however certain nod=
es
will fail in a later production run. To solve this problem, bias the bus =
as
shown in Figure 7 (Multi-Drop/Fail-Safe Termination). Biasing the bus ens=
ures
that the receiver output remains "1" when the bus is tri-stated.
Or, you can use "True Fail-Safe" receivers like those of the
MAX3080 (5V) and MAX3070 (3V) families. These devices ensure an RO output=
of
"1" in response to a 0V differential input by changing the
receiver's threshold to -50mV. RS-232/RS-485 Protocol Translators
Figure=
9
shows the MAX3162 configured as an RS-232/RS-485 multi-point protocol
translator. The direction of translation is controlled through the RTS si=
gnal
R1IN. The single-ended RS-232 receiver input signal is translated to a
differential RS-485 transmitter output. Similarly, a differential RS-485
receiver input signal is translated to a single-ended RS-232 transmitter =
output.
RS-232 data received on R2IN is transmitted as an RS-485 signal on Z and =
Y.
RS-485 signals received on A and B are transmitted as an RS-232 signal on
T1OUT. The RT=
S line
offers a common means for controlling bus direction in circuits that conv=
ert
from RS-232 to RS-485. This line on the RS-232 port controls whether the
RS-485 transceiver acts as a transmitter or a receiver (Figure 9). Note t=
hat
the system cannot be sure that a byte of data in the UART's transmit buff=
er
has been transmitted unless the system monitors the RS-485 driver input (=
DI).
That is, the system must either allow for a fixed time delay or actively
monitor the DI input before using the DE input to change the bus directio=
n. Other
direction-control techniques include using a microcontroller and driving =
the
DE input with data while pulling the A-B lines apart (connecting a pull-up
resistor from A to 5V and connecting a pull-down resistor from B to groun=
d).
The value of these resistors varies with cable capacitance, but is typica=
lly
1k
Port-Powered D=
evices Hot-Swap References 1.
RS-422 and RS-485 Application Note, B&B Electronics (T=
his
is a great source of information on RS-232, RS-485 and RS-422 standards a=
nd
their practical realizations. http://www.bb-elec.com) 2.
Serial Port Complete, Jan Axelson (This definitive referen=
ce
for serial ports includes lots of useful source code written in Visual Ba=
sic.
http://www.lvr.com)
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