Hardware manual

Don't confuse this type of framing with the framing used for a packet of bytes on a network. The serial port
just frames every byte. For a network, many bytes are framed into a packet (sometimes called a frame). For a
network frame, instead of a start bit, there is a sequence of bytes called a header. On a network that uses serial
ports (with modems), a report of a frame error usually refers to a multi-byte frame and not the serial port
frame of a single byte.
20.5 How "Asynchronous" is Synchronized
The RS-232 serial port as implemented on PC is asynchronous which in effect means that there is no "clock"
signal sent with "ticks" to mark when each bit is sent.. There are only two states of the transmit (or receive)
wire: mark (-12 V) or space (+12 V). There is no state of 0 V. Thus a sequence of 1-bits is transmitted by just
a steady -12 V with no markers of any kind between bits. For the receiver to detect individual bits it must
always have a clock signal which is in synchronization with the transmitter clock. Such a clock would
generate a "tick" in synchronization with each transmitted (or received) bit.
For asynchronous transmission, synchronization is achieved by framing each byte with a start bit and a stop
bit (done by hardware). The receiver listens on the negative line for a positive start bit and when it detects one
it starts its clock ticking. It uses this clock tick to time the reading of the next 7, 8 or 9 bits. (It actually is a
little more complex than this since several samples of a bit are normally taken and this requires additional
timing ticks.) Then the stop bit is read, the clock stops and the receiver waits for the next start bit. Thus async
is actually synchronized during the reception of a single byte but there is no synchronization between one byte
and the next byte.
21. Other Serial Devices (not async RS-232)
21.1 Successors to RS-232
A number of EIA (or RS) standards have been established for higher speeds and longer distances using
twisted-pair (balanced) technology. Balanced transmission make possible higher speeds, and can be a
thousand times faster than unbalanced RS-232. For a given speed, the distance (maximum cable length) may
be many times longer with twisted pair. But PC's, prior to about 2004??, kept being made with the
quasi-obsolete RS-232 since it works OK with modems and mice since the cable length is short.
High speed serial ports (over 460.8 kbps) will often support both RS-232 and EIA-485/EIA-422
(RS-485/RS-422) modes . (Note that for non-RS-232 I've used the "EIA" designation instead of the more
commonly used "RS" but they both mean the same thing.) At such high speeds RS-232 is not of much use
(except for a very short cable).
21.2 EIA-422-A (balanced) and EIA-423-A (unbalanced)
EIA-423 is just like the unbalanced RS-232 except that the voltage is only 5 volts. Since this falls within
RS-232 specs it can be connected to a RS-232 port. Its specs call for somewhat higher speeds than the RS-232
(but this may be of little help on a long run where it's the unbalance that causes interference). Since EIA-423
is not much of an improvement over RS-232, it is seldom used except on old Mac computers.
EIA-422 is twisted pair (known as "balanced" or "differential) and is (per specs) exactly 100 times as fast as
EIA-423 (which in turn is somewhat faster than RS-232). Apple's Mac computer used it prior to mid-1998
with its RS-232/EIA-422 port. The Mac used a small round "mini-DIN-8" connector and named these serial
Serial HOWTO
20.4 Forming a Byte (Framing) 75