Designing RS-232 Linksment with different connections to determine what type of null modem a link
requires.
:
If you lack a breakout box, you can monitor lines with a voltmeter. If you’re not
sure whether a connector is wired as a DTE or DCE, a voltmeter is all you need
to identify which of pins 2 and 3 is the data input, and which is the data out-
put.
Measure on a port that is powered, but idle (not currently transmitting or
receiving). On the connector, measure the voltage from pin 2 to signal ground
(pin 5 on a 9-pin connector). Also measure from pin 3 to signal ground. On an
idle port, an output should be a negative voltage of at least -5V. An open, or
unconnected, input is typically near 0V and in any case should measure
between -3V and +3V.
For example, on a DB-9 connector, if pin 2 has a negative voltage, the interface
is a DCE, and if pin 3 has a negative voltage, the interface is a DTE.Figure 5-8: Breakout boxes often include LEDs that show the signal states and
jumpers or switches to enable different wiring configurations. This breakout box is
from B&B Electronics Manufacturing Company (www.bb-elec.com).