RS-232 Connections That WORK! - Connecting Devices or Converters

Connecting two devices using RS-232 sounds pretty basic and simple, but almost every day we
assist a customer to get a converter, isolator or similar RS-232 device working by helping correct
the RS-232 connections. This FAQ will help you correct RS-232 connection problems.
The most common problems are inputs connected to inputs and outputs to outputs. This
happens because 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!
How Do We Sort Out DTE and DCE Port Connections?
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)
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 DCE devices together, they need a Crossover Cable to route the outputs
of each to the inputs of the other. The cable for two computers simulates modem connections, so
it is commonly called a "Null Modem" cable. (see 232DTE or 9PMMNM adapters)
Are My Devices Wired As DTE or DCE? - How to Check
1. 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.
2. 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 pins
#2 and #3 and the other pins.
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.
3. Use DC Voltmeter - The least convenient but still effective method is to use a DC voltmeter
and measure the DC level from signal ground on the connector to pin #2 and 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.
Measure pin #2 and pin #3 to ground (pin # 5 - DB9) (pin #7 -- DB25) on the two devices you
want to interconnect. If both devices 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 may be negative when not asserted or positive when
asserted. (for reference, see line tester figures) You can confirm what handshaking lines are
active by measuring each pin for voltage. The lines shown as outputs 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
1. Handshaking lines RTS and CTS not interconnected, DTR and DSR not interconnected.
Swap as needed.
2. Many programs 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
3. Many programs 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.
4. 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.
5. 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.

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