TECHNICAL BULLETIN #122 - Rev 1.10 (8/30/96)
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TITLE : Testing an RS-232 Connection (Paper-Clip Test)
PRODUCT : Procomm Plus for Windows All Versions
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No characters appearing on the Terminal screen? Suspect your serial port or
cable? Here's how to test the send data and receive data lines of your serial
port and RS-232 cable.
COMMON SERIAL PORT PROBLEMS
Several things can prevent characters from appearing on your screen when you
type to your modem or to another computer. Check these first before testing
your port.
* Resolve any port conflicts. Each serial device on your computer must
have a unique base port address (BPA) and interrupt request line (IRQ)
assigned to it. For help in resolving IRQ and BPA conflicts, see
technical bulletins #89 (PROCOMM PLUS for Windows) and #90 (PROCOMM PLUS
for DOS).
* Make sure you are using the proper cable for your type of connection. A
PC-to-PC connection requires a "null-modem" cable (see technical bulletin
#87), whereas a PC-to-modem connection requires a standard serial cable
with all lines connected straight through.
* If connecting two PCs, be sure line settings in the communications
programs are identical on both machines (e.g., no parity, 8 data bits, 1
stop bit).
* If connected to a modem, make sure echo is turned on and quiet mode is
turned off in the modem. Type ATE1Q0 <Enter> from your terminal screen,
then type AT <ENTER> key. You should see the "AT" appear on your screen,
followed by an "OK" response from your modem.
* In a PC-to-PC connection with Procomm Plus set to FULL DUPLEX, you will
not see what you type on your terminal screen, but the characters should
appear on the screen of the computer you are connected to.
RS-232 PORT DESCRIPTION
A serial port has either 9 or 25 pins. To locate your serial port, look on
the back of your computer for "male" pins in a "D" shell. These two figures
will give you a better idea about the physical appearance of those ports.
---------------
\ 1 2 3 4 5 /
\ 6 7 8 9 /
\---------/
9 - pin RS232 port
-----------------------------------------
\ 1 2 3 4 5 6 7 8 9 10 11 12 13 /
\ 14 15 16 17 18 19 20 21 22 23 24 25 /
\-----------------------------------/
25 - pin RS232 port
Pin 2 is used to send the data to, and pin 3 to receive data from, another
serial device. By connecting those two you can make a closed circuit that
will echo characters you type back to your Procomm Plus terminal screen. Pin
4 carries the RTS (ready to send) signal and Pin 5 the CTS (clear to send)
signal. RTS and CTS lines are used for Hardware Flow Control and are
discussed below.
PAPER-CLIP TEST
Due to low voltages, there is no risk of electric shock when connecting two
pins on the RS232 port.
1. You will need a metal paper clip (without plastic coating) or a
screwdriver to establish a circuit between pin 2 and pin 3
2. Have your computer running Procomm Plus
3. Turn Hardware Flow Control OFF for the duration of this test:
* Procomm Plus Connections
Options | System Options | Modem Connection |
Modem/Connection Properties
* Procomm Plus 4.x
Options | System Options | Modem Connection |
Modem/Connection Properties
* PROCOMM PLUS for Windows 3.x
Setup | Data | Data Connection | General | Use hardware flow control
4. Set Procomm Plus to FULL DUPLEX mode:
* PROCOMM PLUS Connections for Windows
Setup | Data | Data Options | Advanced | Duplex
* PROCOMM PLUS for Windows 4.x
Setup | Data | Data Options | Advanced | Duplex
* PROCOMM PLUS for Windows 3.x
Setup | Data | Port Settings | Full Duplex
5. Disconnect the serial RS-232 cable from the back of your computer.
6. Connect ONLY pins 2 and 3 with the metal object. (Press it against them
and hold it there).
7. Press an alphabetic key on your keyboard and see if it shows up on your
terminal screen.
If the letter shows up, connect the cable back to the computer and test the
far end of the cable (the one that plugs into the other serial device -- an
RS-232 serial port or a modem) and repeat steps 6 and 7. Serial cables
have "female" connectors, so you will find holes instead of pins. A paper
clip should fit firmly into openings 2 and 3. The positions of those
openings will be reversed from the ones on the "male" side, however. The
following two diagrams illustrates the locations of the send and receive
lines (pin slots 2 and 3) in a serial cable.
---------------
\ 5 4 3 2 1 /
\ 9 8 7 6 /
\---------/
9 - pin female RS232 serial cable
-----------------------------------------
\ 13 12 11 10 9 8 7 6 5 4 3 2 1 /
\ 25 24 23 22 21 20 19 18 17 16 15 14 /
\-----------------------------------/
25 - pin female RS232 serial cable
HARDWARE FLOW CONTROL LINES MUST BE PRESENT IN CABLE
If the test indicates characters are being sent and received from the port
and/or cable properly, the cause of the problem is not in the hardware
component that was tested. But only the SD and RD (send data and receive
data) lines are tested; there could still be problems with other lines in the
cable. The RTS/CTS lines, which carry flow control signals, are primary
suspects. Hardware (RTS/CTS) flow control signals are carried by pins 4 and 5
in your serial port and cable. If these lines are missing in your cable (a
common problem), Procomm Plus may attempt to send characters correctly, will
sense that it does not have permission to send, and will wait for the CTS
signal to be asserted. Those two lines are used in PC-to-modem communications
and if missing or defective, will prevent you from sending characters to your
modem normally.
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revision number to that of the same document on any of our support sites.
This technical bulletin may be copied and distributed freely as long as it
is distributed in its entirety and it is not distributed for profit.
Copyright (c) 1996 Quarterdeck All rights reserved.
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