Hardware manual

The DTR and DSR Pins
Just like RTS and CTS, these pins are paired. For DTE-to-DTE connections they are likely to cross over.
There are two ways to use these pins. One way is to use them as a substitute for RTS/CTS flow control. The
DTR pin is just like the RTS pin while the DSR pin behaves like the CTS pin. Although Linux doesn't support
DTR/DSR flow control, it can be obtained by connecting the RTS/CTS pins at the PC to the DSR/DTR pins at
the device that uses DTR/DSR flow control. DTR flow control is the same as DTR/DSR flow control but it's
only one-way and only uses the DTR pin at the device. Many text terminals and some printers use DTR/DSR
(or just DTR) flow control. In the future, Linux may support DTR/DSR flow control. The software has
already been written but it's not clear when (or if) it will incorporated into the serial driver.
The normal use of DTR and DSR (not for flow control) is as follows: A device asserting DTR says that its
powered on and ready to operate. For a modem, the meaning of a DTR signal from the PC depends on how
the modem is configured. Negating DTR is sometimes called "hanging up" but it doesn't always do this. One
way to "hang up" (negate DTR) is to set the baud rate to 0 using the command "stty 0". Trying to do this from
a "foreign" terminal may not work due to the two-interface problem. See Two interfaces at a terminal. For
internal modem-serial_ports it worked OK with a port using minicom but didn't work if the port was using
wvdial. Why?
19.5 Preventing a Port From Opening
If "stty -clocal" (or getty is used with the "local" flag negated) then a serial port can't open until DCD gets an
assert (+12 volts) signal.
20. Voltage Waveshapes
20.1 Voltage for a Bit
At the RS-232 serial port, voltages are bipolar (positive or negative with respect to ground) and should be
about 12 volts in magnitude (some are 5 or 10 volts). For the transmit and receive pins +12 volts is a 0-bit
(sometimes called "space") and -12 volts is a 1-bit (sometimes called "mark"). This is known as inverted logic
since normally a 0-bit is both false and negative while a one is normally both true and positive. Although the
receive and transmit pins are inverted logic, other pins (modem control lines) are normal logic with a positive
voltage being true (or "on" or "asserted") and a negative voltage being false (or "off" or "negated"). Zero
voltage has no meaning (except it usually means that the unit is powered off).
A range of voltages is allowed. The specs say the magnitude of a transmitted signal should be between 5 and
15 volts but must never exceed 25 V. Any voltage received under 3 V is undefined (but some devices will
accept a lower voltage as valid). One sometimes sees erroneous claims that the voltage is commonly 5 volts
(or even 3 volts) but it's usually 11-12 volts. If you are using a EIA-422 (RS-422) port on a Mac computer as
an RS-232 (requires a special cable) or EIA-423 (RS-423) then the voltage will actually be only 5 V. The
discussion here assumes 12 V.
Note that normal computer logic normally is just a few volts (5 volts was once the standard) so that if you try
to use test equipment designed for testing 3-5 volt computer logic (TTL) on the 12 volts of a serial port, it
may damage the test equipment.
Serial HOWTO
The DTR and DSR Pins 73