Hardware manual

means off (-12 v). They also sell a "RS-232 Serial Jumper Box" Cat. #276-1403. This permits connecting the
pins anyway you choose. Both these items are under the heading of "Peripheral hookup helpers".
Unfortunately, they are not listed in the index to the printed catalog. They are on the same page as the D type
connecters so look in the index under "Connectors, Computer, D-Sub". A store chain named "Active
Components" may have them.
Measuring voltages
Any voltmeter or multimeter, even the cheapest that sells for about $10, should work fine. Trying to use other
methods for checking voltage is tricky. Don't use a LED unless it has a series resistor to reduce the voltage
across the LED. A 470 ohm resistor is used for a 20 ma LED (but not all LED's are 20 ma). The LED will
only light for a certain polarity so you may test for + or - voltages. Does anyone make such a gadget for
automotive circuit testing?? Logic probes may be damaged if you try to use them since the TTL voltages for
which they are designed are only 5 volts. Trying to use a 12 V incandescent light bulb is not a good idea. It
won't show polarity and due to limited output current of the UART it probably will not even light up.
To measure voltage on a female connector you may plug in a bent paper clip into the desired opening. The
paper clip's diameter should be no larger than the pins so that it doesn't damage the contact. Clip an alligator
clip (or the like) to the paper clip to connect up. Take care not to touch two pins at the same time with any
metal object.
Taste voltage
As a last resort, if you have no test equipment and are willing to risk getting shocked (or even electrocuted)
you can always taste the voltage. Before touching one of the test leads with your tongue, test them to make
sure that there is no high voltage on them. Touch both leads (at the same time) to one hand to see if they shock
you. Then if no shock, wet the skin contact points by licking and repeat. If this test gives you a shock, you
certainly don't want to use your tongue.
For the test for 12 V, Lick a finger and hold one test lead in it. Put the other test lead on your tongue. If the
lead on your tongue is positive, there will be a noticeable taste. You might try this with flashlight batteries
first so you will know what taste to expect.
16.2 Serial Monitoring/Diagnostics
A few Linux programs will monitor the modem control lines and indicate if they are positive (1) or negative
(0). See section Serial Monitoring/Diagnostics
16.3 (The following subsections are in both the Serial and
Modem HOWTOs)
16.4 Serial Port Can't be Found
There are 3 possibilities:
Your port is disabled since both the BIOS and Linux failed to enable it. It has no IO address.1.
Your port is enabled and has an IO address but it has no ttyS device number (like ttyS14) assigned to
that address so the port can't be used. As a last resort, you may need to use "setserial" to assign a ttyS
2.
Serial HOWTO
Breakout Gadgets, etc. 59