EIA (a short name for the RS-232 specification, which it is often
called by) is the bit serial signalling scheme most commonly used for
connecting terminals, modems and other low speed data entry equipment to
computers. It is replacing an older and much less flexible scheme called
20 mA current loop. It is a voltage sensing system that uses signals
ranging from positive 8-10 (logical zero, called mark) to negative 8-10
(logical one, called space) volts, and is suitable for a variety of
signalling schemes such as synchronous or asynchronous, with or without
external clocking, and includes features designed for modem control (which
20 mA lacked). It uses a standard connector (a 25 pin D connector) with a
standard pinout.
	It has become common over the years to use this system for local
connections as well, since to have only one scheme for data entry
connections makes life much simpler, as well that fact that EIA has many
technical advantages. The modem control features are obvious, but they
can also be used, for example, for a computer to detect if local
terminals are turned on. It can run at varying speeds without hardware
modification; most 20 mA systems included filters which had to be
changed as the baud rate was jacked up. It has been designed to be almost
impervious to wiring errors; it is very hard to damage EIA interfaces.
Finally, through the use of simple wiring adaptors any two EIA devices
using a similar signalling scheme can be connected, which is what makes
local use practical. 20 mA had an active and a passive end (usually
terminal and computer), and connecting two like ends together required
an active adaptor.
	Essentially the only use around the lab is for asynchronous
internally clocked operation. In this scheme, the data line is kept in the
spacing, or one condition, and when data is to be sent one bit of start
(mark, or zero) is sent, followed by the data bits (from five to eight, least
significant bit first, with data zero being logical zero), an optional
parity bit (the parity may be even, odd, or unused), followed between
one and two bits of stop (space, or one). The line may then return to
idle or another start bit may appear. The two ends must agree on the
baud rate to within some reasonable tolerance, and often the number of
data bits and parity as well; synchronization is provided by the start
bit.
	It was originally designed as a standard for the interconnection of
modems and computers, and this fact is important to keep in mind if you
wish to understand how it works. It assumed that every connection has
two ends, a DCE (which stands for Data Communication Equipment) and a
DTE (Data Terminal Equipment). All the signal names, as well as how they
work, assumes this configuration. For example, a DTE (computer port) would
connect to a DCE (modem), which would connect over terminal lines to
another DCE (modem) and thence to a DTE (terminal).
	To operate two DTE's (or DCE's) in direct connection, an adaptor
is needed which makes one of the devices look like the appropriate mate
for the other one. In most cases a passive adaptor suffices, and usually
many (if not all) of the special modem control lines may be ignored. The
adaptor for two DTE's is called a null modem, for obvious reasons. Often,
the adaptor function is built into the cable. In the original spec, it
was laid down that ALL DCE equipment would have a female connector, and
ALL DTE equipment would have a male. So a cable that includes a null
modem would have two female (DCE) connectors. If this scheme were followed
religously, then you could always tell what find of interface you had from
the type of connector, and it would follow that if a cable were found that
would connect two pieces of equipment, they should (modulo signalling
scheme, speed and data format) be able to converse. Unfortunately, the
world contains losers, and it is possible to find female connectors
wired as DTE's, etc., so be wary.
	To discuss the signal lines, I will list the common ones and what
they do (omitting ground, which is ubiquitous). Received Data is data from
the DCE, and Transmitted Data is data to the DCE. Data Set Ready indicating
that the DCE is online and in some sense (it varies from unit to unit)
ready to operate. Data Terminal Ready is the same, but for the DTE; for
example auto-answer modems sense this before deciding whether to answer
a call. Ready To Send and Clear To Send are low level flow control; the
first is a signal from the DTE indicating that it has data it wants to
send, and the second is a signal from the DCE indicating that it is
ready to accept data. Ring is a signal from the DCE, as is Carrier Detect;
the function of both of these is obvious. The remainder have to do
with things like external clocking and strange secondary data and
exception channels.
	Most of the lines listed above are usually unused. Ring is little
used now since most modems are auto answer. Carrier Detect is only of
interest on lines that control modems (and sometimes not even there). The
hardware flow control signals are almost never used, except to tie them
on. The status lines are occasionally used to detect the liveness of
the equipment on the other end, but in most local situations this is not
important. Some computer ports do sense Data Set Ready, and will reset
the line in software if it goes off (which is a good way to clear software
hangs of various sorts); others sense it and turn the line off if it is not
present, since it could indicate unplugged equipment, and long open
EIA lines have a tendency to pick up noise that looks like data, presenting
a unnecessary (and often heavy) load on the computer. In general, the only
lines used are Ground and the two Data lines.
	The internal wiring of a null modem should now be obvious: one simply
cross connects the Data lines, the Ready lines, and the Send lines, and ties
each Carrier Detect to either Clear to Send or Data Set Ready. Often, one
only needs to worry with the Data lines.


	A complete list of the EIA pinout (but beware of strange variations
by manufacturers) is as follows:

1	Ground (GND)
2	Transmitted Data (TD)
3	Received Data (RD)
4	Request To Send (RTS)
5	Clear To Send (CTS)
6	Data Set Ready (DSR)
7	Ground
8	Carrier Detect (CARR, CD, CXR)
9	+ Power
10	- Power
11	202 Secondary Transmit
12	202 Secondary Receive
13	Secondary Clear To Send
14	EIA Secondary Transmit
15	Serial Clock Transmit
16	EIA Secondary Receive
17	Serial Clock Receive
18	Unassigned
19	Secondary Request To Send
20	Data Terminal Ready
21	Signal Quality
22	Ring (RD, RING)
23	Signal Rate
24	External Clock
25	Force Busy