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APP 723: Dec 29,
2000

Download, PDF Format (270K)

Selecting and Using
RS-232, RS-422, and RS-485 Serial Data Standards

Three common serial data standards: RS-232, RS-422, and
RS-485 are described by specification and electrical interface. Cable
termination techniques, use of multiple loads, daisy-chaining of RS-232,
conversion of RS-232 to RS-485, converting RS-485 to RS-232, and RS-232
port powered RS-485 conversions are described.

"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.

Perhaps the
most successful serial-data standard for PC and telecom applications is the
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 often
necessary to communicate between them. This article offers insight into the
different standards (electrical physical-layer specifications), how to
convert from one standard to another standard, and how to combine different
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 the
use of hardware handshaking to control the flow of data.

Figure 1. Typical RS-232 connection.

A typical 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 data
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 transmissions
seldomly exceed 100 feet for two reasons: The difference between transmitted
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 interface,
its drivers are specified for single loads from 3k
to 7k.
Therefore, a daisy-chain scheme is typically implemented for multi-drop
interface applications (Figure 3).

Figure 2. An RS-232 receiver accepts the bipolar input signal (top trace,
CH1) and outputs an inverted TTL/CMOS signal (bottom trace, CH2).

Figure 3. Daisy-chaining allows multiple slave transceivers on a single
RS-232 line.

Daisy-Chain Devices

In a daisy-chain configuration, the RS-232 signal enters through one receiver
and is looped through to a transmitter. This configuration is repeated for
subsequent devices along the data transmission line. Cable breaks are a major
problem with this technique. A break between slave 1 and slave 2, prevents
all downstream devices from transmitting or receiving data. Other multi-drop
RS-232 techniques involve pre-buffering or boosting the RS-232 output drive
(enabling it to drive multiple 5k
inputs in parallel).

To eliminate
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.
When a device is not selected, its input resistance remains in a high
impedance state allowing communication to proceed with other devices along
the shared bus.

Another
solution would be to convert the RS-232 RX and TX signals to an RS-422 signal
(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".

Figure 4. A typical RS-422 system allows as many as 10 slave transceivers
on the differential transmission line.

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 standards
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 applications.
The opposite, however, is not true. The common-mode output range for RS-485
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
for RS-422 drivers and 12k
for RS-485 drivers.

Table 1.
RS-232 Summary of Major Electrical Specifications

Parameter

Conditions

Min

Max

Units

Driver Output Voltage Open Circuit

25

V

Driver Output Voltage Loaded

3k
< RL < 7k

±5

±15

V

Driver Output Resistance Power Off

-2V < V < 2V

300

Slew Rate

4

30

V/µS

Maximum Load Capacitance

2500

pF

Receiver Input Resistance

3

7

k

Receiver Input Threshold:

Output = Mark (Logic 1)

-3

V

Output = Space (Logic 0)

3

V

Table 2. RS-422 Summary of Key Specifications

Parameter

Conditions

Min

Max

Units

Driver Output Voltage Open Circuit

±10

V

Driver Output Voltage Loaded

RL = 100

2

-2

V

Driver Output Resistance

A to B

100

Driver Output Short Circuit Current

Per output to common

150

mA

Driver Output Rise Time

RL = 100

10

% of bit width

Driver Common-Mode Voltage

RL = 100

±3

V

Receiver Sensitivity

VCM < ±7V

±200

mV

Receiver Common-Mode Voltage Range

-7

7

V

Receiver Input Resistance

4

k

Differential Receiver Voltage

Operational

±10

V

Withstand

±12

V

Table 3. RS-485 Summary of Key Specifications

Parameter

Conditions

Min

Max

Units

Driver Output Voltage Open Circuit

1.5

-1.5

6

-6

V

V

Driver Output Voltage Loaded

RL = 100

1.5

-1.5

5

-5

V

V

Driver Output Short Circuit Current

Per output to common

±250

mA

Driver Output Rise Time

RL = 54

CL = 50pF

30

% of bit width

Driver Common-Mode Voltage

RL = 54

±3

V

Receiver Sensitivity

-7V < VCM < 12V

±200

mV

Receiver Common-Mode Voltage Range

-7

12

V

Receiver Input Resistance

12

k

To reduce wiring expense and achieve longer line lengths, RS-485 transceivers
have become a popular standard for use in point-of-sale, industrial, and
telecom applications. Its wider common-mode range also enables longer line
lengths and a higher input resistance per node, allowing more nodes to be
connected to the bus (Figure 5).

Figure 5. Compared with RS-422, the higher input impedance and wider
common-mode range of an RS-485 connection enables longer line lengths.

Differential
RS-485 transmissions (Figure 6) produce opposing currents and magnetic fields
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 terminals
is harmless. By default, the transmission line should be terminated at the
last transceiver on the line (bus).

Figure 6. The opposite-polarity signals on an RS-485 line minimize EMI by
cross-canceling each other's respective magnetic fields. The GND references
on the above scope photo have been shifted (offset) to clearly show the
inverted polarities of the RS-485 output signals.

Figure 7. The choice of termination resistors for a transmission line
depends on the application.

Fail-Safe

Deciding whether you need a termination resistor or not is only part of the
problem in implementing an RS-485 system. Normally, an RS-485 receiver output
is "1" if A > B by +200mV or more, and "0" if B > A
by 200mV or more. In a half-duplex RS-485 network, the master transceiver
tri-states the bus after transmitting a message to the slaves. Then, with no
signal driving the bus, the receiver's output state is undefined, for the
difference between A and B tends towards 0V. If the receiver output (RO) is
"0", the slaves interpret it as a new start bit and attempts to
read the following byte. The result is a framing error, because the stop bit
never occurs. The bus goes unclaimed, and the network stalls.

Unfortunately,
different runs of chips can produce different output signals on RO for a 0V
differential input. The prototype can work perfectly, however certain nodes
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 ensures
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

The MAX3162 is a very unique device for it contains both RS-232 and RS-485
receivers and transmitters. This wide range of communication devices
contained within a single device enables an individual to convert
bi-directionally between RS-232 and RS-485 signals. The circuit in Figure 8
illustrates the MAX3162 configured to bi-directionally convert RS-232 and
RS-485 signals in a point-to-point application.

Figure 8. The above figure utilizes the MAX3162 to convert
bi-directionally between RS-232 and RS-485 signals in a point-to-point
application.

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 signal
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 RTS line
offers a common means for controlling bus direction in circuits that convert
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 that
the system cannot be sure that a byte of data in the UART's transmit buffer
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 direction.

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 ground).
The value of these resistors varies with cable capacitance, but is typically
1k.

Figure 9. The above figure utilizes the MAX3162 to convert
bi-directionally between RS-232 and RS-485 signals in a multi-point
application.

Port-Powered Devices

Many converters from RS-232 to RS-485 are "port-powered
converters", in which the RS-485 power is derived from the RS-232 RTS
line (or sometimes a combination of the RTS and CTS (DTR) lines). Since the
power available from an RS-232 port is limited, the RS-485 launch voltages
are not achieved when using a port-powered converter with for example 100
RS-485 terminations. However, the low receiver threshold (200mV) allows for a
fair margin for error. This technique is acceptable in systems with short
line lengths and without termination resistors across the A-B terminals.

Hot-Swap

When circuit boards are inserted into a hot or powered backplane,
differential disturbances to the data bus can lead to data errors. Upon
initial circuit board insertion, the data communication processor undergoes
its own power-up sequence. During this period, the processor's logic-output
drivers are high impedance and are unable to drive the DE and /RE\ inputs of
the MAX3060E/MAX3080E to a defined logic level. Leakage currents up to
±10mA from the high-impedance state of the processor's logic drivers
could cause standard CMOS enable inputs of a transceiver to drift to an
incorrect logic level. Additionally, parasitic circuit board capacitance
could cause coupling of VCC or GND to the enable inputs. Without
the hot-swap capability, these factors could improperly enable a
transceiver's driver or receiver.

References

1.
RS-422 and RS-485 Application Note, B&B Electronics (This
is a great source of information on RS-232, RS-485 and RS-422 standards and
their practical realizations. http://www.bb-elec.com)

2.
Serial Port Complete, Jan Axelson (This definitive reference
for serial ports includes lots of useful source code written in Visual Basic.
http://www.lvr.com)

More Information

APP 723: Dec 29,
2000

MAX232:

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-- Full (PDF)
Data Sheet (648k)

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Samples

MAX3162:

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-- Full (PDF)
Data Sheet (368k)

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Samples

MAX3232:

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-- Full (PDF)
Data Sheet (400k)

-- Free
Samples

MAX485:

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-- Full (PDF)
Data Sheet (448k)

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Samples

Download, PDF Format (270K)

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