RS232 Information
Floyd
Kling
RS232 interface is arguably the most well
known connector on a personal computer.
Mostly used as the interface to modems, it
is also used for many other serial
connections such has mouse and printer
ports. Well known as "Comm"
(Communications) or "Serial" port.
While EIA RS232 defines the electrical and
signal definitions, internationally the
ITU-T (CCITT) equivalent are V.24 for the
Signal definitions and V.28 for the
electrical characteristics. RS232 and
V.24/V.28 are virtually identical.
The Personal computer RS232 ports are mostly
Asynchronous. It is very
rare to find a PC that uses the Serial port
for synchronous operation. Special
cards and firmware are needed for
synchronous operation. This paper will
indicate full pin definitions for both
Asynchronous and Synchronous operation, but
will focus on Asynchronous operation for the
signal definitions.
(FYI – Your internet connections are
"Asynchronous" i.e. 8 data bits, one start,
one stop = 10 bits per character)
Electrical Characteristics:
Receiving the signal - viewed as the
receiver/input
·
DC resistance: Must be between 3 to
7Kohms
·
Open -circuit voltage: Less than 2 volts.
·
Signals greater than +3VDC are considered
the binary "1" (on) condition
·
Signals less than -3VDC are considered the
binary "0" (off) condition
·
Signals of -3VDC to +3VDC are consider in
transition and not considered a valid level
for determining a state.
Generating the signal - viewed as the
driver/output
·
Not to exceed +/-25VDC (no load)
·
Open-circuit must be greater than
+/-5volts(no load)
·
Must be between +/-5 to15VDC with a load of
3 to 7Kohms.
·
Signals greater than +5VDC are considered
the binary "1" (on) condition
·
Signals less than -5VDC are considered the
binary "0" (off) condition
·
Signals range of -5VDC to +5VDC is consider
in transition and not considered a valid
level for determining a state.
Physical:
The DTE connector (On your PC)
utilizes a 25pin (DB25) Male
connector and the DCE connector (On
the modem) utilizes a Female 25pin
(DB25) connector. Many PC's use DE9
(9pin) connectors (Yes DE-9.
DE-9 connectors have commonly albeit
incorrectly, referred to as DB-9's)
Noteworthy site on connectors and
nomenclature:
http://www.interfacebus.com/Connetor_D-Sub_Manufacturers.html
Signal direction is viewed as the modem sees
the signal. i.e. TXD (Transmit Data)
is the data from the Terminal (PC) "To"
the Modem and out the phone line to the
remote modem. RXD (Receive Data) is
the data from the Modem to your PC.
Signal Definitions - listed by the DB25
(DE9) pin designations:
FG - Frame Ground - Pin 1 (NA but can be
used for Signal Gnd)
Normally not connected but is intended for
connection to "Earth" ground. Not
necessarily the same as "Signal Ground - Pin
7)
TXD - Transmit Data - Pin 2 (Pin 3) <Input
to the modem>
Data from the PC (DTE) to the modem (DCE
eventually sent out the phone line.
RXD - Received Data - Pin 3 (Pin 2) <Output
from the modem>
Data from the Modem (DCE) to the DTE (PC).
This is the data received from the modem.
RTS - Request to Send - Pin 4 (Pin 7) <Input
to the modem>
Signal from the DTE to the Modem indicating
the DTE has data to send. RTS is used
with CTS for 'Hardware flow control'
In the command mode, the modem ignores RTS.
In asynchronous operation, the modem ignores
RTS unless RTS/CTS flow control is selected
by the &Kn command.
In synchronous on-line operation, the modem
can be commanded by the &Rn command to
ignore RTS or to respond to RTS by turning
on CTS after the delay specified by Register
S26.
CTS - Clear to Send - Pin 5 (Pin 8) <Output
from the modem>
CTS true indicates the modem is ready to
accept data from the DTE. In asynchronous
operation, in error correction or normal
mode, CTS is always ON (high) unless RTS/CTS
flow control is selected by the &Kn command.
In synchronous operation, the modem also
holds CTS ON during asynchronous command
state.
The modem turns CTS OFF immediately upon
going off-hook and holds CTS OFF until both
DSR and DCD(Carrier Detect) are ON and the
modem is ready to transmit and receive
synchronous data. The modem can also be
commanded by the &Rn command to turn CTS ON
in response to an RTS OFF-to-ON transition.
DSR - Data Set Ready - Pin 6 (Pin 6) <Output
from the modem>
DSR indicates modem status to the DTE. DSR
OFF (low) indicates that the DTE is to
disregard all signals appearing on the
interchange circuits except Ring Indicator
(RI - pin 22). DSR output is controlled by
the AT&Sn command.
SG - Signal Ground - Pin 7 (Pin 5)
Common for all the interface pins. Pin
7 may, or may not be tied to pin 1.
DCD - Data Carrier Detect - Pin 8 (Pin 1)
(also known as 'RSDL' or "CXR') <Output from
the modem>
DCD output is ON when a carrier is detected
on the telephone line or OFF when carrier is
not detected. Controlled by &C0
command
DTR - Data Terminal Ready - Pin 20 (Pin 4)
<Input to the modem>
The DTR input is turned ON (high) by the DTE
when the DTE is ready to transmit or receive
data. DTR ON prepares the modem to be
connected to the telephone line, and
maintains the connection established by the
DTE (manual answering) or internally
(automatic answering). DTR OFF forces
disconnect under control of the &Dn and &Qn
commands.
DTR also determines if the modem will auto
answer the call based on &Dn and &Qn.
RI - Ring Indicate - Pin 22 (Pin 9) <Output
from the modem>
RI output ON (high) indicates the presence
of a ringing signal on the telephone line.
25Pin DB25 |
9 Pin
DE9 |
Signal |
EIA
Name |
EIA Desig |
CCITT
Desig |
Direction
To-From
Modem |
1 |
|
Shield (Fame Ground) |
FG |
AA |
101 |
|
2 |
3 |
Transmitted Data |
TxD |
BA |
103 |
To |
3 |
2 |
Received Data |
RxD |
BB |
104 |
From |
4 |
7 |
Request To Send |
RTS |
CA |
105 |
To |
5 |
8 |
Clear To Send |
CTS |
CB |
106 |
From |
6 |
6 |
Data Set Ready |
DSR |
CC |
107 |
From |
7 |
5 |
Signal Ground |
SG |
AB |
102 |
|
8 |
1 |
Data Carrier Detect |
DCD |
CF |
109 |
From |
9 |
|
Pos DC Test Voltage |
|
|
- |
From |
10 |
|
Neg DC Test Voltage |
|
|
- |
From |
11 |
|
Unassigned |
|
|
- |
|
12 |
|
Secondary Carrier Detect |
SDCD |
SCF |
122 |
From |
13 |
|
Secondary CTS |
SCTS |
SCB |
121 |
From |
14 |
|
Secondary TxD |
STD |
SBA |
118 |
To |
15 |
|
Transmit Signal Element Timing |
TC |
DB |
114 |
From |
16 |
|
Secondary RxD |
SRD |
SBB |
119 |
From |
17 |
|
Receive Signal Element Timing |
RC |
DD |
115 |
From |
18 |
|
Receiver Dibit Clock |
|
|
- |
|
19 |
|
Secondary RTs |
SRTS |
SCA |
120 |
To |
20 |
4 |
Data Terminal Ready |
DTR |
CD |
108.2 |
To |
21 |
|
Signal Quality Detect |
SQ |
CG |
110 |
To |
22 |
9 |
Ring Indicate |
RI |
CE |
125 |
From |
23 |
|
Data Rate Select |
|
CH/CI |
111/112 |
To |
24 |
|
EXT Transmit CLK. |
(TC) |
DA |
113 |
To |
25 |
|
BUSY/Test Mode |
|
TM |
- |
From |
END