A-19
A2
framing errors, generates a Bus SRQ message for each data error and
indicates the error by setting the bit 3 in the Standard Event Status
Register.
Although serial data can be transmitted over any medium, most of
today's computer systems use metallic cable. To ensure compatibility,
the manufacturers have adopted interface standards so that they are
electrically compatible. The more popular standards are:
RS-232
Most popular standard for office machines and
computer systems.
RS-422 and
New high speed standard with noise
RS-485
improvements over RS-232 for longer
distances.
Devices employing the same interface standard can usually be
connected together but the user must verify each devices signal
requirements before plugging them together.
When data must be transmitted over long distances, it is typically sent
over the phone company's direct dial network (DDN) as shown in
Figure A-8. Modems are used to convert the serial data bits into tones
that will pass through the telephone system's 300 to 3000 Hz voice
band. For low baud rates, up to 1200 Hz, the modems convert the bits
into two tones (frequency switched keying) that the receiving modem
recognizes and converts back to data bits. These low speed modems
are referred to by the telephone company's designations, i.e.: Type
103 (300 baud) and Type 212 (1200 baud). Higher data rates require
more complex modulation techniques and the modems are referred to
by their CCITT specification i.e., V22.
With asynchronous characters, the receiver normally uses the start bit
to synchronize its internal clock. However, some devices, such as the
higher speed modems, require the data bits to be synchronized with
their clock. These units are referred to as synchronous modems (not
the same as synchronous data characters) and they will supply the
clock signals to both the transmitting and receiving device.
