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App Note 083: Fundamentals
of RS-232 Serial Communications
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Due to it's relative simplicity and low hardware
overhead (as compared to parallel interfacing), serial
communications is used extensively within the electronics industry.
Today, the most popular serial communications standard in use is
certainly the EIA/TIA-232-E specification. This standard, which has
been developed by the Electronic Industry Association and the
Telecommunications Industry Association (EIA/TIA), is more popularly
referred to simply as "RS-232" where "RS" stands for "recommended
standard". In recent years, this suffix has been replaced with
"EIA/TIA" to help identify the source of the standard. This paper
uses the common notation of "RS-232" in its discussion of the
topic. |
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The official name of the EIA/TIA-232-E standard
is "Interface Between Data Terminal Equipment and Data Circuit-Termination
Equipment Employing Serial Binary Data Interchange." Although the name may
sound intimidating, the standard is simply concerned with serial data
communication between a host system (Data Terminal Equipment, or "DTE")
and a peripheral system (Data Circuit-Terminating Equipment, or "DCE").
The EIA/TIA-232-E standard which was introduced in 1962 has been
updated four times since its introduction in order to better meet the
needs of serial communication applications. The letter "E" in the
standard's name indicates that this is the fifth revision of the standard.
RS-232 Specifications
RS-232 is a
"complete" standard. This means that the standard sets out to ensure
compatibility between the host and peripheral systems by specifying 1)
common voltage and signal levels, 2) common pin wiring configurations, and
3) a minimal amount of control information between the host and peripheral
systems. Unlike many standards which simply specify the electrical
characteristics of a given interface, RS-232 specifies electrical,
functional, and mechanical characteristics in order to meet the above
three criteria. Each of these aspects of the RS-232 standard is discussed
below.
Electrical Characteristics
The
electrical characteristics section of the RS-232 standard includes
specifications on voltage levels, rate of change of signal levels, and
line impedance.
The original RS-232 standard was defined in 1962. As this was before
the days of TTL logic, it should not be surprising that the standard does
not use 5 volt and ground logic levels. Instead, a high level for the
driver output is defined as being +5 to +15 volts and a low level for the
driver output is defined as being between -5 and -15 volts. The receiver
logic levels were defined to provide a 2 volt noise margin. As such, a
high level for the receiver is defined as +3 to +15 volts and a low level
is -3 to -15 volts. Figure 1 illustrates the logic levels defined by the
RS-232 standard. It is necessary to note that, for RS-232 communication, a
low level (-3 to -15 volts) is defined as a logic 1 and is historically
referred to as "marking." Likewise a high level (+3 to +15 volts) is
defined as a logic 0 and is referred to as "spacing."
Figure
1. RS-232 Logic Level Specifications
The RS-232 standard also limits the maximum slew rate at the driver
output. This limitation was included to help reduce the likelihood of
cross-talk between adjacent signals. The slower the rise and fall time,
the smaller the chance of cross talk. With this in mind, the maximum slew
rate allowed is 30 V/ms. Additionally, a maximum data rate of 20k
bits/second has been defined by the standard. Again with the purpose of
reducing the chance of cross talk.
The impedance of the interface between the driver and receiver has also
been defined. The load seen by the driver is specified to be 3k
to 7k.
For the original RS-232 standard, the cable between the driver and the
receiver was also specified to be a maximum of 15 meters in length. This
part of the standard was changed in revision "D" (EIA/TIA-232-D). Instead
of specifying the maximum length of cable, a maximum capacitive load of
2500 pF was specified which is clearly a more adequate specification. The
maximum cable length is determined by the capacitance per unit length of
the cable which is provided in the cable specifications.
Functional Characteristics
Since
RS-232 is a "complete" standard, it includes more than just specifications
on electrical characteristics. The second aspect of operation that is
covered by the standard concerns the functional characteristics of the
interface. This essentially means that RS-232 has defined the function of
the different signals that are used in the interface. These signals are
divided into four different categories: common, data, control, and timing.
Table 1 illustrates the signals that are defined by the RS-232 standard.
As can be seen from the table there is an overwhelming number of signals
defined by the standard. The standard provides an abundance of control
signals and supports a primary and secondary communications channel.
Fortunately few applications, if any, require all of these defined
signals. For example, only eight signals are used for a typical modem.
Some simple applications may require only four signals (two for data and
two for handshaking) while others may require only data signals with no
handshaking. Examples of how the RS-232 standard is used in some "real
world" applications are discussed later in this paper. The complete list
of defined signals is included here as a reference, but it is beyond the
scope of this paper to review the functionality of all of these signals.
Table 1. RS-232 Defined Signals
Circuit
Mnemonic |
Circuit Name* |
Circuit Direction |
Circuit Type |
| AB |
Signal Common |
— |
Common |
BA
BB |
Transmitted Data (TD)
Received Data (RD) |
To DCE
From DCE |
Data |
CA
CB
CC
CD
CE
CF
CG
CH
CI
CJ
RL
LL
TM
|
Request to Send (RTS)
Clear to Send (CTS)
DCE Ready
(DSR)
DTE Ready (DTR)
Ring Indicator (RI)
Received Line
Signal Detector** (DCD)
Signal Quality Detector
Data Signal
Rate Detector from DTE
Data Signal Rate Detector from
DCE
Ready for Receiving
Remote Loopback
Local
Loopback
Test Mode
|
To DCE
From DCE
From DCE
To DCE
From DCE
From
DCE
From DCE
To DCE
From DCE
To DCE
To DCE
To
DCE
From DCE
|
Control |
| DA |
Transmitter Signal Element Timing from DTE |
To DCE |
|
DB
DD |
Transmitter Signal Element Timing from DCE
Receiver Signal
Element Timing From DCE |
From DCE
From DCE |
Timing |
SBA
SBB |
Secondary Transmitted Data
Secondary Received Data |
To DCE
From DCE |
Data |
SCA
SCB
SCF |
Secondary Request to Send
Secondary Clear to
Send
Secondary Received Line Signal Detector |
To DCE
From DCE
From DCE |
Control |
*Signals with abbreviations in parentheses are the eight most
commonly used signals.
**This signal is more commonly referred to as
Data Carrier Detect (DCD).
Mechanical Interface
Characteristics
The third area covered by RS-232 concerns the
mechanical interface. In particular, RS-232 specifies a 25-pin connector.
This is the minimum connector size that can accommodate all of the signals
defined in the functional portion of the standard. The pin assignment for
this connector is shown in Figure 2. The connector for DCE equipment is
male for the connector housing and female for the connection pins.
Likewise, the DTE connector is a female housing with male connection pins.
Although RS-232 specifies a 25-position connector, it should be noted that
often this connector is not used. This is due to the fact that most
applications do not require all of the defined signals and therefore a
25-pin connector is larger than necessary. This being the case, it is very
common for other connector types to be used. Perhaps the most popular is
the 9-position DB9S connector which is also illustrated in Figure 2. This
connector provides the means to transmit and receive the necessary signals
for modem applications, for example. This will be discussed in more detail
later.
Figure
2. RS-232 Connector Pin Assignments
Practical RS-232 Implementation
Most
systems designed today do not operate using RS-232 voltage levels. Since
this is the case, level conversion is necessary to implement RS-232
communication. Level conversion is performed by special RS-232 IC's. These
IC's typically have line drivers that generate the voltage levels required
by RS-232 and line receivers that can receive RS-232 voltage levels
without being damaged. These line drivers and receivers typically invert
the signal as well since a logic 1 is represented by a low voltage level
for RS-232 communication and likewise a logic 0 is represented by a high
logic level. Figure 3 illustrates the function of an RS-232 line
driver/receiver in a typical modem application. In this particular
example, the signals necessary for serial communication are generated and
received by the Universal Asynchronous Receiver/Transmitter (UART). The
RS-232 line driver/receiver IC performs the level translation necessary
between the CMOS/TTL and RS-232 interface.
Figure
3. Typical RS-232 Modem Application
The UART just mentioned performs the "overhead" tasks necessary for
asynchronous serial communication. For example, the asynchronous nature of
this type of communication usually requires that start and stop bits be
initiated by the host system to indicate to the peripheral system when
communication will start and stop. Parity bits are also often employed to
ensure that the data sent has not been corrupted. The UART usually
generates the start, stop, and parity bits when transmitting data and can
detect communication errors upon receiving data. The UART also functions
as the intermediary between byte-wide (parallel) and bit-wide (serial)
communication; it converts a byte of data into a serial bit stream for
transmitting and converts a serial bit stream into a byte of data when
receiving.
Now that an elementary explanation of the TTL/CMOS to RS-232 interface
has been provided we can consider some "real world" RS-232 applications.
It has already been noted that RS-232 applications rarely follow the
RS-232 standard precisely. Perhaps the most significant reason this is
true is due to the fact that many of the defined signals are not necessary
for most applications. As such, the unnecessary signals are omitted. Many
applications , such as a modem, require only nine signals (two data
signals, six control signals, and ground). Other applications may require
only five signals (two for data, two for handshaking, and ground), while
others may require only data signals with no handshake control. We will
begin our investigation of "real world" implementations by first
considering the typical modem application.
RS-232 in Modem Applications
Modem
applications are one of the most popular uses for the RS-232 standard.
Figure 4 illustrates a typical modem application utilizing the RS-232
interface standard. As can be seen in the diagram, the PC is the DTE and
the modem is the DCE. Communication between each PC and its associated
modem is accomplished using the RS-232 standard. Communication between the
two modems is accomplished via telecommunication. It should be noted that
although a microcomputer is usually the DTE in RS-232 applications, this
is not mandatory according to a strict interpretation of the standard.
Figure
4. Modem Communication Between Two PC'S
Many modem applications require only nine signals (including ground).
Although some designers choose to use a 25-pin connector, it is not
necessary since there are only nine interface signals between the DTE and
DCE. With this in mind, many have chosen to use 9- or 15-pin connectors
(see Figure 2 for 9-pin connector pin assignment). The "basic nine"
signals used in modem communication are illustrated in Figure 3. Note that
with respect to the DTE, three RS-232 drivers and five receivers are
necessary. The functionality of these signals is described below. Note
that for the following signal descriptions, "ON" refers to a high RS-232
voltage level (+5 to +15 volts) and "OFF" refers to a low RS-232 voltage
level (-5 to -15 volts). Keep in mind that a high RS-232 voltage level
actually represents a logic 0 and a low RS-232 voltage level refers to a
logic 1.
Transmitted Data (TD): One of two separate data signals. This
signal is generated by the DTE and received by the DCE.
Received Data (RD): The second of two separate data signals.
This signals is generated by the DCE and received by the DTE.
Request to Send (RTS): When the host system (DTE) is ready to
transmit data to the peripheral system (DCE), RTS is turned ON. In simplex
and duplex systems, this condition maintains the DCE in receive mode. In
half-duplex systems, this condition maintains the DCE in receive mode and
disables transmit mode. The OFF condition maintains the DCE in transmit
mode. After RTS is asserted, the DCE must assert CTS before
communicationcan commence.
Clear to Send (CTS): CTS is used along with RTS to provide
handshaking between the DTE and the DCE. After the DCE sees an asserted
RTS, it turns CTS ON when it is ready to begin communication.
Data Set Ready (DSR): This signal is turned on by the DCE to
indicate that it is connected to the telecommunications line.
Data Carrier Detect (DCD): This signal is turned ON when the DCE
is receiving a signal from a remote DCE which meets its suitable signal
criteria. This signal remains ON as long as the a suitable carrier signal
can be detected.
Data Terminal Ready (DTR): DTR indicates the readiness of the
DTE. This signal is turned ON by the DTE when it is ready to transmit or
receive data from the DCE. DTR must be ON before the DCE can assert DSR.
Ring Indicator (RI): RI, when asserted, indicates that a ringing
signal is being received on the communications channel.
The signals described above form the basis for modem communication.
Perhaps the best way to understand how these signals interact is to give a
brief step by step example of a modem interfacing with a PC. The following
steps describe a transaction in which a remote modem calls a local modem.
- The local PC monitors the RI (Ring Indicate) signal via software.
- When the remote modem wants to communicate with the local modem, it
generates an RI signal. This signal is transferred by the local modem to
the local PC.
- The local PC responds to the RI signal by asserting the DTR (Data
Terminal Ready) signal when it is ready to communicate.
- After recognizing the asserted DTR signal, the modem responds by
asserting DSR (Data Set Ready) after it is connected to the
communications line. DSR indicates to the PC that the modem is ready to
exchange further control signals with the DTE to commence communication.
When DSR is asserted, the PC begins monitoring DCD for indication that
data is being sent over the communication line.
- The modem asserts DCD (Data Carrier Detect) after it has received a
carrier signal from the remote modem that meets the suitable signal
criteria.
- At this point data transfer can began. If the local modem has
full-duplex capability, the CTS (Clear to Send) and RTS (Request to
Send) signals are held in the asserted state. If the modem has only
half-duplex capability, CTS and RTS provide the handshaking necessary
for controlling the direction of the data flow. Data is transferred over
the RD and TD signals.
- When the transfer of data has been completed, the PC disables the
DTR signal. The modem follows by inhibiting the DSR and DCD signals. At
this point the PC and modem are in the original state described in step
number 1.
RS-232 in Minimal Handshake
Applications
Even though the modem application discussed above
is simplified from the RS-232 standard in terms of the number of signals
needed, it is still more complex than the requirements of many systems.
For many applications, two data lines and two handshake control lines are
all that is necessary to establish and control communication between a
host system and a peripheral system. For example, an environmental control
system may need to interface with a thermostat using a half-duplex
communication scheme. At times the control systems may desire to read the
temperature from the thermostat and at other times may need to load
temperature trip points to the thermostat. In this type of simple
application, five signals may be all that is necessary (two for data, two
for handshake control, and ground).
Figure 5 illustrates a simple half-duplex communication interface. As
can be seen in this diagram, data is transferred over the TD (Transmit
Data) and RD (Receive Data) pins and handshake control is provided by the
RTS (Ready to Send) and CTS (Clear to Send) pins. RTS is driven by the DTE
to control the direction of data. When it is asserted, the DTE is placed
in transmit mode. When RTS is inhibited, the DTE is placed in receive
mode. CTS, which is generated by the DCE, controls the flow of data. When
asserted, data can flow. However, when CTS is inhibited, the transfer of
data is interrupted. The transmission of data is halted until CTS is
reasserted.
Figure
5. Half-Duplex Communication Scheme
RS-232 Application Limitations
As
mentioned earlier in this paper, the RS-232 standard was first introduced
in 1962. In the more than three decades since, the electronics industry
has changed immensely and therefore there are some limitations in the
RS-232 standard. One limitation, the fact that over twenty signals have
been defined by the standard, has already been addressed—simply do not use
all of the signals or the 25-pin connector if they are not necessary.
Other limitations in the standard are not necessarily as easy to correct,
however.
Generation Of RS-232 Voltage
Levels
As we saw in the section on RS-232 electrical
characteristics, RS-232 does not use the conventional 0 and 5 volt levels
implemented in TTL and CMOS designs. Drivers have to supply +5 to +15
volts for a logic 0 and -5 to -15 volts for a logic 1. This means that
extra power supplies are needed to drive the RS-232 voltage levels.
Typically, a +12 volt and a -12 volt power supply are used to drive the
RS-232 outputs. This is a great inconvenience for systems that have no
other requirements for these power supplies. With this in mind, RS-232
products manufactured by Dallas Semiconductor have on-chip charge-pump
circuits that generate the necessary voltage levels for RS-232
communication. The first charge pump essentially doubles the standard +5
volt power supply to provide the voltage level necessary for driving a
logic 0. A second charge pump inverts this voltage and provides the
voltage level necessary for driving a logic 1. These two charge pumps
allow the RS-232 interface products to operate from a single +5 volt
supply.
Maximum Data Rate
Another limitation
in the RS-232 standard is the maximum data rate. The standard defines a
maximum data rate of 20k bits/second. This is unnecessarily slow for many
of today's applications. RS-232 products manufactured by Dallas
Semiconductor guarantee up to 250k bits/second and typically can
communicate up to 350k bits/second. While providing a communication rate
at this frequency, the devices still maintain a maximum 30V/ms maximum
slew rate to reduce the likelihood of cross-talk between adjacent signals.
Maximum Cable Length
A final
limitation to discuss concerning RS-232 communication is cable length. As
we have already seen, the cable length specification that was once
included in the RS-232 standard has been replaced by a maximum load
capacitance specification of 2500 pF. To determine the total length of
cable allowed, one must determine the total line capacitance. Figure 6
shows a simple approximation for the total line capacitance of a
conductor. As can be seen in the diagram, the total capacitance is
approximated by the sum of the mutual capacitance between the signal
conductors and the conductor to shield capacitance (or stray capacitance
in the case of unshielded cable).
As an example, let's assume that the user has decided to use
non-shielded cable when interconnecting the equipment. The cable mutual
capacitance (Cm) of the cable is found in the cable's specifications to be
20 pF per foot. If we assume that the input capacitance of the receiver is
20 pF, this leaves the user with 2480 pF for the interconnecting cable.
From the equation in Figure 6, the total capacitance per foot is found to
be 30 pF. Dividing 2480 pF by 30 pF reveals that the maximum cable length
is approximately 80 feet. If a longer cable length is required, the user
would need to find a cable with a smaller mutual capacitance.
Figure
6. Interface Cable Capacitive Model Per Unit Length
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