Hardware Reference
In-Depth Information
inthecomputer,andtheyexist until theyreach themodem hardware. Iftheendsofamodem
connection use different values for start and stop bits, the connection transmits gibberish in-
stead of usable data.
The use of a single start bit is required in all forms of asynchronous communication, but
some protocols use more than one stop bit. To accommodate systems with different pro-
tocols, communications software products usually enable you to modify the format of the
frame used to transmit each byte. The standard format that describes an asynchronous
communications format is parity/data bits/stop bits. Almost all asynchronous connections
today are therefore abbreviated as N-8-1 (No parity/8 data bits/1 stop bit). The meanings
for each of these parameters and their possible variations are as follows:
Parity —Before error-correction protocols became standard modem features, a simple
parity mechanism provides basic error checking at the software level. Today, this is
almost never used, and the value for this parameter is nearly always set to none. Other
possible parity values you might see in a communications software package are odd,
even, mark, and space.
Data bits —This parameter indicates how many bits are actually carried in the data
portion of the packet (exclusive of the start and stop bits). PCs typically use 8 data
bits, but some types of computers use a 7-bit byte, and others might call for other data
lengths. Communications programs provide this option to prevent a system from con-
fusing a stop bit with a data bit.
Stop bits —This parameter specifies how many stop bits are appended to each byte.
PCs typically use 1 stop bit, but other types of protocols might call for the use of 1.5
or 2 stop bits.
In most situations, you will never have to modify these parameters manually, but the con-
trols are almost always provided. InWindows, forexample, if youopen the Modems con-
trol panel and look at the Connection page of your modem's Properties dialog box, you
will see Data Bits, Parity, and Stop Bits selectors.
Unless you use the Windows HyperTerminal program to establish a direct connection
to another computer via phone lines, you might never need to modify these parameters.
However, if you need to call a mainframe computer to perform terminal emulation, you
mightneedtoadjusttheseparameters.( Terminal emulation meansusingsoftwaretomake
your PC keyboard and screen act like a terminal, such as a DEC VT-100.) Many main-
frame computers use even parity and a 7-bit word length. If your PC is set incorrectly,
you'll see garbage text on your monitor instead of the other system's login or welcome
screen.
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