The UARTs above the 16550 are used in various types of high-speed and multiport
adapter cards and devices, such as ISDN and DSL modems or four-serial port cards.Synchronous Communications
Synchronoustransmissionsarecoordinatedtoacommonclock,whichfixesthelengthof
the interval between data blocks. The transmitting device synchronizes its clock to that
of the receiving device and sends the clocking signal right along with the data. The com-
municating devices complete one operation before beginning the next, which involves
acknowledgementsthatadatablockisreceivedandcorrectbeforethenextblockissent.RS-232 Communications
The RS-232-C standard (the official name of this standard is the EIA/TIA-232-E standard
or the “Interface Between Data Terminal Equipment and Data Circuit Termination
Equipment Employing Serial Binary Data Interchange” standard) defines the protocol
used by two devices to communicate remotely over a serial connection. EIA is the Elec-
tronics Industry Association and TIA is the Telecommunications Industry Association
Recommended Standard. A protocol establishes the rules that the devices must follow to
carry out a communications session. Under the RS-232 standard, when a modem (or an-
othercommunicationsdevice)isattachedtoaserialport,itisdesignatedasdatacommuni-
cations equipment(DCE), and the PC is designated asdata terminal equipment(DTE). The
importance of these designations is that under the RS-232 standard, the DTE initiates and
controls some parts of the transmission, and the DCE initiates and controls others. The of-
ficial name of DCE equipment isdata circuit-termination equipment, but in common usage
it is referred to as data communication equipment.
ThepinsandwiresintheserialportandconnectorcarrysignalsbetweentheDTEand
DCE to create what amounts to a conversation between the two devices. The signals are
actually low-voltage charges of DC (direct current) power that flow from one device to
the other, where it is detected and interpreted based upon which wire the signal is on (see
Table 19-1). RS-232 communications prescribe a signal series that is followed by devices
to establish what is called a handshake. The DTE controls some of the pins (and wires) of
the serial connection, and the DCE controls the remainder. During the handshake pro-
cess, the DTE uses its pins to communicate requests, status, and acknowledgements to
the DCE. The DCE responds to the DTE and sends its own set of requests and acknowl-
edgements over its pins.
The sequence of signals that flows between the DTE and DCE in RS-232 communica-
tions is as follows:- The DTE (PC) sends a signal on the Data Terminal Ready (DTR) wire,
indicating that it is ready to communicate. - The DCE (modem) acknowledges the DTR signal by sending a signal over
the Data Set Ready (DSR) wire to indicate that it too is ready to communicate.
(^488) PC Hardware: A Beginner’s Guide