For the most part, all PC computers, even laptops are sold with at least
one
As the name implies, serial device communicate serially. That is, each byte is split into 8 bits, which are sent one after the other. At the pyhsical level, what this means is that voltage on the transmit pin (line) is toggled to indicate the bits: negative is a 1 bit, and a positive voltage is a 0 bit. On the receiving end, the serial port accept the data on the receive pin and recognizes the voltage and therefore whether the bit is a 0 or 1.
Agreeing on the speed at which the voltage is toggled, is an import apect of the communication. The speed of the change determine how many bits per second can be sent, which is thus the speed of the serial port or serial device. A mouse will communicate with speeds between 1,200 and 9,600 bps, whereas modems can reach as high as 115,200 bps.
Deciding on speed is not the only thing used with serial communication to data arrives correctly. In some cases, like with modems, additional pins are used to say that one side is read to send the data (using the RTS pin) and the other side says that they are clear to send (CTS). Here a voltage of +12 is used for the respective signal. Other kinds of devices uses the pins data terminal ready (DTR) and data set ready (DSR). This has the same functionality, but just uses different pins. Typically, serial terminal use the DTR and DSR pins
Regardless of what pair of pins are used, this mechanism is used to ensure that data only flows when the appropriate side is ready. That is, it is used to control the flow and such a mechanism is thus called “flow control”. Since it the hardware that is used to control the flow, using pins like this is logically called hardware flow control. In other cases, such as when there is no phyiscal connection, you can use “software flow control”, which uses special “control characters”. For example, pressing the control key (CTRL) and the the “s” key (CTRL-S) you can stop the flow. Pressing CTRL-Q, you can start it up again.
With modems, you will typically find that a couple of other pins are used as well: ring indicator (RI) pin and the carrier detect (CD) pin. These indicate (respectively) an incoming call and that a the carrier signal has been detected.
A serial board is an
Parity is the mechanism by which single-bit errors can be detected during
transmission. The number of bits set to one is counted and based on whether
even or odd
Serial communication parameters must be agreed upon by both ends. These
parameters are often referred to in triples, such as 8-1-N (read as
eight-one-none). In this instance, there are eight data bits, 1
One key element of a serial board is the Universal Asynchronous
Receiver-Transmitter, or UART. The transmitter portion takes a
byte of parallel data written by the serial driver to the card and transmits it
one bit at a time (serially). The receiver does just the opposite: It takes the
serial bits and converts them into parallel data that is sent down the
Originally, Linux only provided drivers for standard serial ports;
intelligent boards are often installed to allow many more logins (or other
connections) to the system. The most significant difference is that intelligent
serial boards (often referred to as smart serial boards) have a built-in
In the newer versions, you can find drivers for commercial multiport boards, such as the Stallion Easy IO, which allows you to quickly extend the number of serial ports on your system as the drivers are built in. Stallion is very supportive of the Linux world and even advertises the fact that their boards run on Linux.
In addition, intelligent serial boards can better
Serial board performance is also increased by intelligent boards. Because
signals are buffered and sent in large chunks, there is less overhead on a
per-character basis. With non-intelligent boards, single characters are often
transmitted, so the per-character overhead is much larger. In fact, most
non-intelligent boards generate and
It is possible to obtain supported serial boards that have multiple ports.
Although such boards have multiple UARTs, they do not have the performance of
intelligent boards, though they do provide a low-cost alternative. For a
discussion on the
The pin assignments mentioned above and the commincation protocol (among other thinfs) is known as the RS-232 standard. RS-232 is easily effected by electrical noise, and thus has some limits the length and speed at which it can communicate. The longer the cable the slower the connection must be to ensure communication is not disrupted.
Originally designed to connect mainframe computers to modems, the RS-232
standard is used exclusively for
Two types of connections are used: DB25 (with 25 pins) and DB9 (with 9 pins). Although they both serve the same basic function, the numbering of the pins is slightly different. Table 0-3 lists the main pins of the DB25 connector, their functions, and a mnemonic that is commonly used to refer to them. Table 0-4 lists the pins for the DB9 connector. The physical layout of the pins is shown in Figure 0-13.
| Pin | Function | Mnemonic |
| 2 | Transmit | TXD or TX |
| 3 | Receive | RXD or RX |
| 4 | Request to send | RTS |
| 5 | Clear to send | CTS |
| 6 | Data set ready | DSR |
| 7 | Signal ground | GND |
| 8 | Carrier detect | CD |
| 20 |
Data | DTR |
| 22 | Ring indicator | RI |
| Pin | Function | Mnemonic |
| 1 | Carrier detect | CD |
| 2 | Receive | RXD or RX |
| 3 | Transmit | TXD or TX |
| 4 |
Data | DTR |
| 5 | Signal ground | GND |
| 6 | Data set ready | DSR |
| 7 | Request to send | RTS |
| 8 | Clear to send | CTS |
| 9 | Ring indicator | RI |
Figure – The Physical Layout of Pins on Serial Cables
Note that on a DB25 connector, pin 1 is chassis ground, which is
different from
To communicate properly, the
One side indicates that it has data by sending a
Problems arise when connecting other types of devices. Some devices, such as
printers, are themselves
If you have a
If the