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Introduction
The purpose of this application note is to describe the main
elements of an RS-422 and RS-485 system. This application
note attempts to cover enough technical details so that the
system designer will have considered all the important aspects
in his data system design. Since both RS-422 and RS-485 are
data transmission systems that use balanced differential signals,
it is appropriate to discuss both systems in the same application
note. Throughout this application note the generic terms of
RS-422 and RS-485 will be used to represent the EIA/TIA-422
and EIA/TIA-485 Standards.
Data Transmission Signals
Unbalanced Line Drivers
Each signal that transmits in an RS-232 unbalanced data transmission
system appears on the interface connector as a voltage with
reference to a signal ground. For example, the transmitted
data (TD) from a DTE device appears on pin 2 with respect
to pin 7 (signal ground) on a DB-25 connector. This voltage
will be negative if the line is idle and alternate between
that negative level and a positive level when data is sent
with a magnitude of ±5 to ±15 volts. The RS-232 receiver typically
operates within the voltage range of +3 to +12 and -3 to -12
volts as shown in Figure 1.1.
Figure 1.1 RS-232 Interface Circuit
Balanced Line Drivers
In a balanced differential system the voltage produced by the
driver appears across a pair of signal lines that transmit only
one signal. Figure 1.2 shows a schematic symbol for a balanced
line driver and the voltages that exist. A balanced line driver
will produce a voltage from 2 to 6 volts across its A and B
output terminals and will have a signal ground (C) connection.
Although proper connection to the signal ground is important,
it isn't used by a balanced line receiver in determining the
logic state of the data line. A balanced line driver can also
have an input signal called an "Enable" signal. The purpose
of this signal is to connect the driver to its output terminals,
A and B. If the "Enable" signal is OFF, one can consider the
driver as disconnected from the transmission line. An RS-485
driver must have the "Enable" control signal. An RS-422 driver
may have this signal, but it is not always required. The disconnected
or "disabled" condition of the line driver usually is referred
to as the "tristate" (Note
1) condition of the driver.
Note 1: The term
"tristate" comes from the fact that there is a third output
state of an RS-485 driver, in addition to the output states
of "1" and "0".
Figure 1.2 Balanced Differential Output Line Driver
Balanced Line Receivers
A balanced differential line receiver senses the voltage state
of the transmission line across two signal input lines, A and
B. It will also have a signal ground (C) that is necessary in
making the proper interface connection. Figure 1.3 is a schematic
symbol for a balanced differential line receiver. Figure 1.3
also shows the voltages that are important to the balanced line
receiver. If the differential input voltage Vab is greater than
+200 mV the receiver will have a specific logic state on its
output terminal. If the input voltage is reversed to less than
-200 mV the receiver will create the opposite logic state on
its output terminal. The input voltages that a balanced line
receiver must sense are shown in Figure 1.3. The 200 mV to 6
V range is required to allow for attenuation on the transmission
line.
Figure 1.3 Balanced Differential Input Line Receiver
EIA Standard
RS-422 Data Transmission
The EIA Standard RS-422-A
entitled "Electrical Characteristics of Balanced Voltage Digital
Interface Circuits" defines the characteristics of RS-422
interface circuits. Figure 1.4 is a typical RS-422 four-wire
interface. Notice that five conductors are used. Each generator
or driver can drive up to ten (10) receivers. The two signaling
states of the line are defined as follows:
- When the "A" terminal of the driver
is negative with respect to the "B" terminal, the line is
in a binary 1 (MARK or OFF) state.
- When the "A" terminal of the driver
is positive with respect to the "B" terminal, the line is
in a binary 0 (SPACE or ON) state.
Figure 1.4 Typical RS-422 4 Wire Network
Figure 1.5 shows the condition of the
voltage of the balanced line for an RS-232 to RS-422 converter
when the line is in the "idle" condition or OFF state. It
also shows the relationship of the "A" and "B" terminals of
an RS-422 system and the "-" and "+" terminal markings used
on many types of equipment. The "A" terminal is equivalent
to the "-" designation, and the "B" terminal equivalent to
the "+" designation. The same relationship shown in Figure
1.5 also applies for RS-485 systems. RS-422 can withstand
a common mode voltage (Vcm) of ±7 volts. Common mode voltage
is defined as the mean voltage of A and B terminals with respect
to signal ground.
Note: Under 'idle' conditions it is possible to determine
which terminal is 'A' and which is 'B'
Figure 1.5 - Relationship Between EIA Standard 'A' and 'B'
Terminals on RS-422 or RS-485 Device and '+' and '-' Identification
Convention
EIA Standard
RS-485 Data Transmission
The RS-485 Standard permits
a balanced transmission line to be shared in a party line
or multidrop mode. As many as 32 driver/receiver pairs can
share a multidrop network. Many characteristics of the drivers
and receivers are the same as RS-422. The range of the common
mode voltage Vcm that the driver and receiver can tolerate
is expanded to +12 to -7 volts. Since the driver can be disconnected
or tristated from the line, it must withstand this common
mode voltage range while in the tristate condition. Some RS-422
drivers, even with tristate capability, will not withstand
the full Vcm voltage range of +12 to -7 volts.
Figure 1.6 shows a typical two-wire multidrop
network. Note that the transmission line is terminated on
both ends of the line but not at drop points in the middle
of the line. Termination should only be used with high data
rates and long wiring runs. A detailed discussion of termination
can be found in Chapter 2 of this application note. The signal
ground line is also recommended in an RS-485 system to keep
the common mode voltage that the receiver must accept within
the -7 to +12 volt range. Further discussion of grounding
can be found in Chapter 3 of this application note.
Figure 1.6 Typical RS-485 Two Wire Multidrop Network
An RS-485 network can also be connected
in a four-wire mode as shown in Figure 1.7. Note that four
data wires and an additional signal ground wire are used in
a "four-wire" connection. In a four-wire network it is necessary
that one node be a master node and all others be slaves. The
network is connected so that the master node communicates
to all slave nodes. All slave nodes communicate only with
the master node. This network has some advantages with equipment
with mixed protocol communications. Since the slave nodes
never listen to another slave response to the master, a slave
node cannot reply incorrectly to another slave node.
Figure 1.7 Typical RS-485 Four Wire Multidrop Network
Tristate
Control of an RS-485 Device using RTS
As discussed previously, an RS-485 system must have a driver
that can be disconnected from the transmission line when a
particular node is not transmitting. In an RS-232 to RS-485
converter or an RS-485 serial card, this may be implemented
using the RTS control signal from an asynchronous serial port
to enable the RS-485 driver. The RTS line is connected to
the RS-485 driver enable such that setting the RTS line to
a high (logic 1) state enables the RS-485 driver. Setting
the RTS line low (logic 0) puts the driver into the tristate
condition. This in effect disconnects the driver from the
bus, allowing other nodes to transmit over the same wire pair.
Figure 1.8 shows a timing diagram for a typical RS-232 to
RS-485 converter. The waveforms show what happens if the VRTS
waveform is narrower than the data VSD. This is not the normal
situation, but is shown here to illustrate the loss of a portion
of the data waveform. When RTS control is used, it is important
to be certain that RTS is set high before data is sent. Also,
the RTS line must then be set low after the last data bit
is sent. This timing is done by the software used to control
the serial port and not by the converter.
When an RS-485 network is connected in
a two-wire multidrop party line mode, the receiver at each
node will be connected to the line (see Figure 1.6). The receiver
can often be configured to receive an echo of its own data
transmission. This is desirable in some systems, and troublesome
in others. Be sure to check the data sheet for your converter
to determine how the receiver "enable" function is connected.
Note 1 - Voltage here is determined by other devices on the
line
Note 2 - All peak values of voltages are approximate
Figure 1.8 - Timing Diagram for RS-232 to RS-485 Converter
with RTS Control of RS-485 Driver and Receiver
Send Data Control
of an RS-485 Device
Many of B&B Electronics' RS-232 to RS-485 converters and RS-485
serial cards include special circuitry, which is triggered
from the data signal to enable the RS-485 driver. Figure 1.9
is a timing diagram of the important signals used to control
a converter of this type. It is important to note that the
transmit data line is "disabled" at a fixed interval after
the last bit, typically one character length. If this interval
is too short, you can miss parts of each character being sent.
If this time is too long, your system may try to turn the
data line around from transmit to receive before the node
(with the Send Data converter) is ready to receive data. If
the latter is the case, you will miss portions (or complete
characters) at the beginning of a response.
Note 1 - Voltage here is determined by other
devices on the line
Note 2 - This timing interval detremined by components in
timing ciruit. The start of this interval is determined by
the leading edge of each data bit
Note 3 - All peak values of voltages are approximate
Figure 1.8 - Timing Diagram
for RS-232 to RS-485 Converter with
Send Data (SD) Control of RS-485 Driver and Receiver
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