 RS-232
Connections That WORK! -
Connecting Devices or Converters
Connecting two devices using RS-232 sounds
simple, but nearly every day we help a customer get a converter,
isolator or other RS-232 device working by helping correct
the RS-232 cabling connections. This FAQ will help you troubleshoot
and correct such problems.
Usually inputs are connected to inputs
and outputs to outputs. People don't realize that there are
two types of RS-232 ports, DTE and DCE type, and that the
signal names and pin numbers are the same, but signal flow
is opposite! The pin labeled Tx can be input, and Rx the output.
The two ports types are complementary,
the Output signals on a DTE port are Inputs
to a DCE port, and Output signals on a DCE
port are Inputs to a DTE port. The signal names match
each other and connect pin for pin. Signal flow is in the
direction of the arrows. (see figures below)
How Can I Identify DTE or DCE Type Connections?
What devices have DTE type RS-232
ports? A DTE device is "Data Terminal Equipment", this includes
Computers, Serial Printers, PLC's, Video Cameras, Video Recorders,
Video Editors, and most devices which are not used to extend
communications. Think COMPUTER for DTE.
What devices have DCE type RS-232 ports?
A DCE device is "Data Communications Equipment", this includes
devices intended to plug directly into a DTE port, PDA cables,
Modems and devices that extend communications like a modem,
such as RS-422, RS-485, or Fiber Optic converters or Radio
Modems. Think MODEM for DCE.
Rule of Thumb: When connecting a DTE
device to a DCE device, match the signal names. When connecting
two DTE or two DCE devices together, use a Crossover cable.
(TD crosses to RD, RTS to CTS, DTR to DSR as shown in Modem
to Modem connections. (see 9PMMNM) The cable for two computers
(DTE) also simulates modem connections to CD/DSR, so it is
commonly called a "Null Modem" cable. (see
232DTE or
9PMMNM).
Are My Devices Wired
As DTE or DCE? - How to Check
-
Use Rule of Thumb
- If the device plugs into the computer serial port
and works normally, the device is wired as DCE (or the
connection cable is a crossover type that makes it work
as a DCE). If the device connects to the computer port
using a "null modem" crossover cable, it is wired as
DTE.
-
Use RS-232 Line
Tester - A quick and easy way to determine the DTE/DCE
port type is to use a RS-232 line tester such as the
9PMTT. The tester
can show the signal state of any active RS-232 data
lines using LED's lighting Red or Green. Active data
lines are output from a device, they may be either High
or Low.
Just plug the tester
into either of the two devices, see which lines are
lit, unplug it, then plug it in to the other device,
see which lines are lit. (see figures).
If the same light
(TD or RD) is lit, use a crossover cable or null modem
connector that swaps the connections for pins #2 and
#3 and other pins as needed.
If the device is "port
powered" check the active side, then plug in the port
powered device and see if other (TD or RD) LED is lit.
If not, try swapping the leads with a null modem cable,
see if the other LED now lights. If not, you may not
have enough voltage on the handshaking lines of the
port to steal power from.
-
Use a DC Voltmeter
- Technicians with a DC voltmeter can use it to measure
the DC level from signal ground (pin#5 on DB9, pin#7
on DB25) on the connector to pin #2 or pin #3. When
the unit is powered and not sending data, the output
line will have a DC voltage of minus polarity, 3 volts
to 11 volts will be typical. The other pin will have
little or no voltage. For example, we measure -11 volts
on pin#2 of a DB9 connector and the line is labeled
RD or Rx, then the device is wired as DCE. If we measure
the voltage on pin#3, it is DTE.
Measure pin #2 and
pin
#3 to ground (pin # 5 - DB9) (pin #7 -- DB25) on the
on the cable from the first device, the on the device
you want to connect. If the cable and device have voltage
on the same pin, you need to use a crossover or null
modem connector that swaps pins #2 & #3 and the other
pins. (For DB9 see model 9PMMNM, for DB25 see
232DTE)
Electrically active
handshaking lines will be negative when not asserted
or positive when asserted. (for reference, see line
tester figures). Active handshaking lines can be found
by measuring each pin for voltage. Output lines will
have voltage. On a DTE, DTR and RTS will have voltage
if used. On a DCE, DSR and CTS will have voltage, and
if a modem with CD (Carrier Detect) and RI (Ring Indicator)
these last two will be low until Ring is detected or
a Carrier connection is made. If handshaking lines don't
have voltage when the device is powered on and ready,
the device doesn't output them, they may be looped back,
RTS to CTS and DTR to DSR. You can turn off the device
power and measure for continuity (zero ohms) between
pins to confirm if they are looped back.
Other RS-232 Connection
Problems
- Handshaking lines RTS and CTS
not interconnected, DTR and DSR not interconnected.
Swap as needed.
- Programs may use the RTS/CTS
connection to check that a device is ready to receive
data and respond. If there is No CTS connection, the
program will never send data, but wait a long time or
timeout with an error. The RTS line may need to be looped
back to the CTS input. Data errors can occur if the
device actually requires handshaking.
- Programs may also use the DTR/DSR
line connection to check that a cable is connected or
that the device is turned on. If there is No DSR signal,
the DTR line may need to be looped back to the DSR input.
Some devices use DTR handshaking.
- Each signal required for unit
operation must be carried through by the isolator, modem
or RS-422 or fiber optic converter. The primary "2 Channels"
for RS-232 are Receive & Transmit. There are 2 data
flow control channels, RTS and CTS. If these are missing,
data is lost, characters missing, or files scrambled.
- Connections to Telephone Modem/FAX
modem - Make sure CD & RI lines are connected.
Recommended Accessories
for Connections
232CAM - DB9F to DB25M conversion cable - 6 ft. (1.8
m)
232CAMS - DB9F
to DB25M conversion/strain relief cable - 6 inches (15 cm)
232CAMR - DB9M
to DB25F conversion/stain relief cable - 12 inches (30 cm)
RS232 Null Modem Connectors
232DTE - DB25F to DB25M - 25 pin female/male
9PMMNM - DB9M to
DB9M - 9 pin male/male
RS232 Line Testers
9PMTT - DB9F to DB9M - 9 pin female/male
232BOB1 - Breakout
Box DB25F to DB25M with switches & jumpers
Jumper Boxes
Please refer to our catalog
or website for jumper boxes for DB9, DB25, DB9/25, M/F,
F/F, & M/M and DB9 or DB25 to RJ11/RJ12 or RJ45 connectors.
For more information on
serial ports, we recommend Serial
Port Complete and C
Programmer's Guide to Serial Communication.
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