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Electronics Tips: RS-232 Interface
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For Information on DB9 Plug See Below
SIGNALS FROM TERMINAL SIGNALS FROM MODEM
--------------------- ------------------
PIN 2 PIN 3
Transmitted Data (TD) Received Data (RD)
Data from terminal. Data from modem
PIN 4 PIN 5
Request to Send (RTS) Clear to Send (CTS)
Tells modem that terminal Tells terminal that it may
wants to send data. now place data on the
transmit data line (PIN 2).
PIN 20 PIN 6
Data Terminal Ready (DTR) Data Set Ready (DSR)
Tells modem that terminal is Tells terminal modem is
connected, powered up and. connected, powered up and
ready ready.
PIN 7 PIN 7
Signal Ground Signal Ground
Common ground reference Common ground reference
for all signal lines. for all signal lines.
PIN 1 PIN 1
Protective Ground Protective Ground
Safety or power line Safety or power line
ground for equipment. ground for equipment.
PIN 24 PIN 8
Transmit Signal Element Timing Received Line Signal Detector
Clock signal from terminal. or Carrier Detect (CD)
Tells terminal that carrier is being
received from computer modem.
PIN 14 PIN 15
Secondary Transmitted Data Transmission Signal Element Timing
Identical in function to PIN 2 Clock signal from modem (used
except it applies only to systems only with synchronous modems.
with full secondary channel
implemented.
PIN 22
Ring Indicator
Signal telling terminal
that phone line is "ringing".
i.e. there is an incoming call.
PIN 19 PIN 17
Secondary Request to Send Received Signal Element Timing
Tells modem to turn on the Clock signal from modem (used
secondary channel carrier used only with synchronous modems).
for HD supervisor operation.
PIN 23 PIN 16
Data Rate Signal Selector Secondary Received Data
Used by Modem/Terminals with Identical in function to PINs 3 and 5
programmable data rate selection. except as they apply only to systems
with full secondary channels implemented.
PIN 13
Secondary Clear to Send
Identical in function to Pins 3 and 5
except as they apply only to systems
with full secondary channels implemented.
PIN 12
Secondary Received Line Dignal Detect
Tells terminal that carrier is
present on secondary channel.
used for HD supervisor operation.
PIN 21
Signal Quality Detector
Used by some modems which incorporate
signal evaluating circuitry to advise
terminal that present signal is poor
and a high error rate is probable.
Pins receiving signal Pins receiving signals
from modem: from terminal
3, 5, 6, 8, 12, 13, 2, 4, 14, 19, 30, 23, 24.
15, 16, 17, 21.
DB9 DB25 PIN PIN NAME DESCRIPTION ------------------------------------------------- 1 8 CD RLSD Carrier Detect 2 3 RD Receive Data 3 2 TD, SD Transmit Data 4 20 DTR Data Terminal Ready 5 7 GND Signal Ground 6 6 DSR Data Set Ready 7 4 RTS Request to Send 8 5 CTS Clear to Send 9 22 RI Ring Indicator
Keep in mind that on many computers, COM1 and COM2 are wired differently, COM1 being DTE, COM2 being DCE. If COM2 is configured as a DCE, a null modem cable with TX and RX reversed will be needed to use it. Some 9 pin to 25 pin adapters, but not all of them, also reverse TX and RX.
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