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App Note 083: Fundamentals of RS-232 Ser=
ial
Communications
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=
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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 Associati=
on
and the Telecommunications Industry Association (EIA/TIA), is more popu=
larly
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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=
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The
official name of the EIA/TIA-232-E standard is "Interface Between Da=
ta
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 sys=
tem
(Data Circuit-Terminating Equipment, or "DCE").
The EIA/TIA-232-E st=
andard
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 t=
hat
this is the fifth revision of the standard.
RS-232
Specifications
RS-232 is a "complete" standard. This means that the standard s=
ets
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 t=
he
electrical characteristics of a given interface, RS-232 specifies electri=
cal,
functional, and mechanical characteristics in order to meet the above thr=
ee
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 li=
ne
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 b=
eing
+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 provi=
de 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 l=
ogic
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."
 <=
/o:p>
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 cro=
ss
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 l=
oad
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 ju=
st
specifications on electrical characteristics. The second aspect of operat=
ion
that is covered by the standard concerns the functional characteristics of
the interface. This essentially means that RS-232 has defined the functio=
n 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 def=
ined
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 ot=
hers
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 her=
e 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 D=
efined
Signals <=
o:p>
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Circuit
Mnemonic
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Circuit Name*
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Circuit Direction
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Circuit Type
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|
=
AB
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=
Signal
Common
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=
—
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=
Common
|
|
=
BA
BB
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=
Transmitted
Data (TD)
Received Data (RD)
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=
To DCE
From DCE
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=
Data
|
|
=
CA
CB
CC
CD
CE
CF
CG
CH
CI
CJ
RL
LL
TM
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=
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
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=
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
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=
Control
|
|
=
DA
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=
Transmitter
Signal Element Timing from DTE
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=
To DCE
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=
|
|
=
DB
DD
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=
Transmitter
Signal Element Timing from DCE
Receiver Signal Element Timing From DCE
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=
From DCE
From DCE
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=
Timing
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|
=
SBA
SBB
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=
Secondary
Transmitted Data
Secondary Received Data
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=
To DCE
From DCE
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=
Data
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|
=
SCA
SCB
SCF
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=
Secondary
Request to Send
Secondary Clear to Send
Secondary Received Line Signal Detector
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=
To DCE
From DCE
From DCE
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=
Control
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*Signals with
abbreviations in parentheses are the eight most commonly used signals.
**This signal is more commonly referred to as Data C=
arrier
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 connec=
tor
housing and female for the connection pins. Likewise, the DTE connector i=
s 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 typ=
es
to be used. Perhaps the most popular is the 9-position DB9S connector whi=
ch
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. <=
/p>
<=
/o:p>
Figure 2. RS-232 Connector Pin Assignments
Practical
RS-232 Implementation
Most systems designed today do not operate using RS-232 voltage levels. S=
ince
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 require=
d by
RS-232 and line receivers that can receive RS-232 voltage levels without
being damaged. These line drivers and receivers typically invert the sign=
al
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 necess=
ary
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.
<=
/o:p>
Figure 3. Typical RS-232 Modem Application
The UART just mentio=
ned
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 t=
he
host system to indicate to the peripheral system when communication will
start and stop. Parity bits are also often employed to ensure that the da=
ta
sent has not been corrupted. The UART usually generates the start, stop, =
and
parity bits when transmitting data and can detect communication errors up=
on
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 elementa=
ry
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 precisel=
y.
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, req=
uire
only nine signals (two data signals, six control signals, and ground). Ot=
her
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&qu=
ot;
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 standa=
rd.
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 mod=
ems
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.
<=
/o:p>
Figure 4. Modem Communication Between Two PC'S <=
/p>
Many modem applicati=
ons
require only nine signals (including ground). Although some designers cho=
ose
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 p=
in
assignment). The "basic nine" signals used in modem communicati=
on
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 sign=
als
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). K=
eep
in mind that a high RS-232 voltage level actually represents a logic 0 an=
d a
low RS-232 voltage level refers to a logic 1.
Transmitted Data =
(TD): One of two separate data sig=
nals.
This signal is generated by the DTE and received by the DCE. <=
/span>
Received Data (RD=
): The second of two separate d=
ata
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 m=
ode.
In half-duplex systems, this condition maintains the DCE in receive mode =
and
disables transmit mode. The OFF condition maintains the DCE in transmit m=
ode.
After RTS is asserted, the DCE must assert CTS before communicationcan
commence.
Clear to Send (CT=
S): 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 (D=
SR): This signal is turned on by =
the
DCE to indicate that it is connected to the telecommunications line.
Data Carrier Dete=
ct
(DCD): This =
signal
is turned ON when the DCE is receiving a signal from a remote DCE which m=
eets
its suitable signal criteria. This signal remains ON as long as the a
suitable carrier signal can be detected.
Data Terminal Rea=
dy
(DTR): DTR
indicates the readiness of the DTE. This signal is turned ON by the DTE w=
hen
it is ready to transmit or receive data from the DCE. DTR must be ON befo=
re
the DCE can assert DSR.
Ring Indicator (R=
I): 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 exa=
mple
of a modem interfacing with a PC. The following steps describe a transact=
ion
in which a remote modem calls a local modem.
1.=
The
local PC monitors the RI (Ring Indicate) signal via software. =
2.=
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.=
3.=
The
local PC responds to the RI signal by asserting the DTR (Data Terminal Re=
ady)
signal when it is ready to communicate.
4.=
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.
5.=
The
modem asserts DCD (Data Carrier Detect) after it has received a carrier
signal from the remote modem that meets the suitable signal criteria.
6.=
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.
7.=
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 an=
d 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 diagra=
m,
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 (Cle=
ar
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 CT=
S is
inhibited, the transfer of data is interrupted. The transmission of data =
is
halted until CTS is reasserted.
<=
/o:p>
Figure 5. Half-Duplex Communication Scheme
RS-232
Application Limitations
As mentioned earlier in this paper, the RS-232 standard was first introdu=
ced
in 1962. In the more than three decades since, the electronics industry h=
as
changed immensely and therefore there are some limitations in the RS-232
standard. One limitation, the fact that over twenty signals have been def=
ined
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, howev=
er.
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 d=
rive
the RS-232 voltage levels. Typically, a +12 volt and a -12 volt power sup=
ply
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 th=
is
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 ca=
ble
allowed, one must determine the total line capacitance. Figure 6 shows a
simple approximation for the total line capacitance of a conductor. As ca=
n 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 shi=
eld
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 interconn=
ecting
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 f=
or
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 reve=
als
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 mu=
tual
capacitance.
<=
/o:p>
Figure 6. Interface Cable Capacitive Model Per Unit Length
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