(Much of this section is now found in Serial-HOWTO.) Text terminals on Unix-like systems (and on PC's) are usually connected to an asynchronous 232 serial port of a computer. It's usually a RS-232-C, EIA-232-D, or EIA-232-E. These three are almost the same thing. The original RS prefix became EIA (Electronics Industries Association) and later EIA/TIA after EIA merged with TIA (Telecommunications Industries Association). The EIA-232 spec provides also for synchronous (sync) communication but the hardware to support sync is almost always missing on PC's. The RS designation is obsolete but is still in use. EIA will be used in this article.
The serial port is more than just a physical connector on the back of a computer or terminal. It includes the associated electronics which must produce signals conforming to the EIA-232 specification. The standard connector has 25 pins, most of which are unused. An alternative connector has only 9 pins. One pin is used to send out data bytes and another to receive data bytes. Another pin is a common signal ground. The other "useful" pins are used mainly for signalling purposes with a steady negative voltage meaning "off" and a steady positive voltage meaning "on".
The UART (Universal Asynchronous Receiver-Transmitter) chip does most of the work. Today, the functionality of this chip is usually built into another chip.
At the EIA-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 terminals 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 port on a Mac computer as an EIA-232 (requires a special cable) or EIA-423 then the voltage will actually be only 5 V. The discussion here assumes 12 V. There is much confusion about voltages on the Internet.
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.
The transmit pin (TxD) is held at -12 V (mark) at idle when nothing is being sent. To start a byte it jumps to +12 V (space) for the start bit and remains at +12 V for the duration (period) of the start bit. Next comes the low-order bit of the data byte. If it's a 0-bit nothing changes and the line remains at +12 V for another bit-period. Then comes the next bit, etc. Finally, a parity bit may be sent and then a -12 V (mark) stop bit. The line remains at -12 V (idle) until the next start bit. Note that there is no return to 0 volts and thus there is no simple way (except by a synchronizing signal) to tell where one bit ends and the next one begins for the case where 2 consecutive bits are the same polarity (both zero or both one).
A 2nd stop bit would also be -12 V, just the same as the first stop bit. Since there is no signal to mark the boundaries between these bits, the only effect of the 2nd stop bit is that the line must remain at -12 V idle twice as long. The receiver has no way of detecting the difference between a 2nd stop bit and a longer idle time between bytes. Thus communications works OK if one end uses one stop bit and the other end uses 2 stop bits, but using only one stop bit is obviously faster. In rare cases 1 1/2 stop bits are used. This means that the line is kept at -12 V for 1 1/2 time periods (like a stop bit 50% wider than normal).
Characters are normally transmitted with either 7 or 8 bits (of data). An additional parity bit may (or may not) be appended to this resulting in a byte length of 7, 8 or 9 bits. Some terminal emulators and older terminals do not allow 9 bits. Some prohibit 9 bits if 2 stop bits are used (since this would make the total number of bits too large: 12 bits total).
The parity may be set to odd, even or none (mark and space parity may be options on some terminals). With odd parity, the parity bit is selected so that the number of 1-bits in a byte, including the parity bit, is odd. If a such a byte gets corrupted by a bit being flipped, the result is an illegal byte of even parity. This error will be detected and if it's an incoming byte to the terminal an error-character symbol will appear on the screen. Even parity works in a similar manner with all legal bytes (including the parity bit) having an even number of 1-bits. During set-up, the number of bits per character usually means only the number of data bits per byte (7 for true ASCII and 8 for various ISO character sets).
A "mark" is a 1-bit (or logic 1) and a "space" is a 0-bit (or logic 0). For mark parity, the parity bit is always a one-bit. For space parity it's always a zero-bit. Mark or space parity only wastes bandwidth and should be avoided when feasible. "No parity" means that no parity bit is added. For terminals that don't permit 9 bit bytes, "no parity" must be selected when using 8 bit character sets since there is no room for a parity bit.
In serial transmission of bytes via EIA-232 ports, the low-order bit is always sent first. Serial ports on PC's use asynchronous communication where there is a start bit and a stop bit to mark the beginning and end of a byte. This is called framing and the framed byte is sometimes called a frame. As a result a total of 9, 10, or 11 bits are sent per byte with 10 being the most common. 8-N-1 means 8 data bits, No parity, 1 stop bit. This adds up to 10 bits total when one counts the start bit. One stop bit is almost universally used. At 110 bits/sec (and sometimes at 300 bits/sec) 2 stop bits were once used but today the 2nd stop bit is used only in very unusual situations (or by mistake since it seemingly still works OK that way).
The conventional EIA-232 serial port is inherently low speed and is severely limited in distance. Ads often read "high speed" but it can only work at high speed over very short distances such as to a modem located right next to the computer. All of the wires use a common ground return so that twisted-pair technology (needed for high speeds) can't be used without additional hardware. However some computers have more modern interfaces. See Successors to EIA-232.
It is somewhat tragic that the RS-232 standard from 1969 did not use twisted pair technology which could operate about a hundred times faster. Twisted pairs have been used in telephone cables since the late 1800's. In 1888 (over 110 years ago) the "Cable Conference" reported its support of twisted-pair (for telephone systems) and pointed out its advantages. But over 80 years after this approval by the "Cable Conference", RS-232 failed to utilize it. Since RS-232 was originally designed for connecting a terminal to a low speed modem located nearby, the need for high speed and longer distance transmission was apparently not recognized.
A number of EIA standards have been established for higher speeds and longer distances using twisted-pair (balanced) technology. Balanced transmission can sometimes be a hundred times faster than unbalanced EIA-232. For a given speed, the distance (maximum cable length) may be many times longer with twisted pair. But PC-s keep being made with the "obsolete" EIA-232 since it works OK with modems connected to slow telephone lines, and it works OK with mice.
One exception is Apple's Mac computer with its EIA-232/EIA-422 GeoPort which provides twisted-pairs (balanced) for transmit and receive. It uses a small round "mini-DIN" connector. It also provides conventional EIA-232 but only at 5 volts (which is still legal EIA-232). However, due to the fact that Macs cost more than PC's, they are seldom used as a host computer for terminals. Some newer terminals use EIA-423 but this is just like the unbalanced EIA-232 and can be connected to a EIA-232 port. This EIA-423 is only 5 volts, but the specs call for higher speeds than the EIA-232 (which will be of no help on a long run where it's the unbalance that causes interference).
EIA-485 is also balanced and is used as a "bus" like ethernet and USB. One device connected to it is the "master" and polls the "slaves" for input. Since many computers may share the twisted pair its sometimes called "multidrop". Another twisted pair is used for traffic from the master to the slaves. The output voltage on the pins must be tristate with the third state being open circuit to permit other units to use the "bus". It's been claimed that protocols are not standardized for multidrop and that's bad news. See http://www.hw.cz/english/docs/rs485/rs485.html
EIA-530-A (balanced but can also be used unbalanced) at 2Mbits/s (balanced) was intended to be a replacement for EIA-232 but few have been installed. It uses the same 25-pin connector as EIA-232.
The High Speed Serial Interface ( HSSI = EIA-612/613) uses a 50-pin connector and goes up to about 50 Mbits/s but the distance is limited to only several meters. The Universal Serial Bus (USB) is being built into PCI chips. It is 12 Mbits/s over a twisted pair with a 4-pin connector (2 wires are power supply) but it also is limited to short distances of at most 5 meters (depends on configuration).
For a text terminal, the EIA-232 speeds are fast enough but the usable cable length is often too short. Balanced technology could fix this. The common method of obtaining balanced communication with a text terminal is to install 2@ line drivers in the serial line to convert unbalanced to balanced (and conversely). They are a specialty item and are expensive if purchased new.
Per EIA-232 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 clocks 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 line for a 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 often 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.
Asynchronous (async) means "not synchronous". In practice, an async signal is what the async serial port sends and receives which is a stream of bytes each delimited by a start and stop bit. Synchronous (sync) is most everything else. But this doesn't explain the basic concepts.
In theory, synchronous means that bytes are sent out at a constant rate one after another in step with a clock signal tick. There is often a separate wire or channel for sending the clock signal. Asynchronous bytes may be sent out erratically with various time intervals between bytes (like someone typing characters at a keyboard).
There are borderline situations that need to be classified as either sync or async. The async serial port often sends out bytes in a steady stream which would make this a synchronous case but since they still have the start/stop bits (which makes it possible to send them out erratically) its called async. Another case is where data bytes (without any start-stop bits) are put into packets with possible erratic spacing between one packet and the next. This is called sync since the bytes within each packet must be transmitted synchronously.
Did you ever wonder what all the unused pins are for on a 25-pin connector for the serial port? Most of them are for use in synchronous communication which is seldom implemented on PC's. There are pins for sync timing signals as well as for a sync reverse channel. The EIA-232 spec provides for both sync and async but PC's use a UART (Universal Asynchronous Receiver/Transmitter) chip such as a 16450, 16550A, or 16650 and can't deal with sync. For sync one needs a USART chip or the equivalent where the "S" stands for Synchronous. Since sync is a niche market, a sync serial port is likely to be quite expensive.
Besides the sync part of the EIA-232, there are various other EIA synchronous standards. For EIA-232, 3 pins of the connector are reserved for clock (or timing) signals. Sometimes it's a modem's task to generate some timing signals making it impossible to use synchronous communications without a synchronous modem (or without a device called a "synchronous modem eliminator" which provides the timing signals).
Although few serial ports are sync, synchronous communication does often take place over telephone lines using modems which use V.42 error correction. This strips off the start/stop bits and puts the date bytes in packets resulting in synchronous operation over the phone line.
Block mode is seldom used with Linux. In block mode when one types at a terminal, the results are saved in the terminal memory and are not sent just yet to the host computer. Such terminals often have built-in editing capabilities. When the user presses certain keys (such as the send key) what has been saved in the terminal memory is sent to the host computer. Now the Linux editors vi and emacs, react instantly to pressing certain keys but in the above situation such keys will be pressed and nothing will happen since nothing is sent when a key is pressed. Thus using a block mode terminal will not allow the use of such interactive programs. The old IBM mainframe interface uses block mode (see IBM Terminals so many IBM terminals are block-mode only and also synchronous (see Section Synchronization & Synchronous).
Block mode may itself have various sub-modes such as "page" (a page at a time) and "line" (a line at a time). Some terminals have both block transmission modes and conventional character modes and may be switched from one mode to another. Async terminals which have block modes include HP2622A, VT130, VT131, VT330, VT340, and Visual500. Many later model terminals can emulate block mode. Block modes may include a forms capability where the host computer sends a form to the terminal. Then the user fills it out and hits the send key which sends only the data in the form back to the host computer. The form itself (not the data) is displayed on the screen in protected fields which don't get transmitted to the host.
Block mode takes a great deal of load off the host computer, especially if the host computer's hardware is designed for block modes (as IBM mainframes were). In character mode every character typed is sent immediately to the serial port and usually causes an interrupt at the host computer. The host that receives the byte must stop whatever it is doing and fetch that character from the port hardware. Even with UART's that have FIFO hardware buffers, the hardware timeout is normally only the transmission time of 3 bytes so that an interrupt is usually issued for every character typed.
In true block mode a long block of characters is received using only one interrupt. If block mode is used with conventional async FIFO serial ports, an interrupt is needed only every 14 bytes since they have 16-byte hardware buffers. Thus much of the load and overhead of interrupt handling is eliminated and the computer has more time to do other tasks when block mode is used.
A significant savings for block mode occurs if the terminal is connected to its host via a network. Without block mode, every character (byte) typed is sent in its own packet including all the overhead bytes (40 in a TCP/IP packet as used on the Internet). With block mode, a large number of characters are sent in a single packet.
(Note: The first book covers much more than just EIA-232.)
See Serial Software for Linux software for the serial ports including getty and port monitors.