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Data Communications
Asynchronous/Synchronous Decisions
For each desired multipoint data link you must first decide whether to employ asynchronous or
synchronous transmission. The following considerations may be helpful in making this deci-
sion:
• Synchronous transmission is generally more efficient than asynchronous and may, there-
fore, provide better thoughput.
• Synchronous modems operate at higher speeds than asynchronous modems (1200-9600 baud
as compared with 300-1200 baud).
• Asynchronous modems are less expensive than synchronous modems.
• With an asynchronous data link you can use the HP 30037A Asynchronous Repeater to
achieve greater computer/terminal, modem/terminal, and terminal/terminal distances.
• The
async~ronous
daisy-chain cable provides differential signals which give better ((noise"
immunity, thus reducing the number of retransmissions in electrically ((noisy" environ-
ments.
Hardwired/Modem Decisions
Having chosen between asynchronous and synchronous, you must then decide whether to use a
hardwired or modem connection.
A hardwired connection, where feasible, is the cheaper alternative because it eliminates the
use of modems and common carrier (telephone company) lines.
If an RS-232-C network is to be used, a major consideration in selecting which type of
connection to use is the anticipated distance between the first terminal and the computer. Ifthe
first terminal will be located in the vicinity of the computer system you may use a hardwired
connection. RS-232-C specifications limit cable lengths to a maximum of 50 feet (15 meters),
although with an asynchronous hardwired configuration you may use the HP 30037A Asyn-
chronous Repeater to extend this distance.
Another consideration is the desired usage (terminal load) of the particular computer port. If
you wish to connect several groups of terminals in geographically diverse locations to the port,
then you must choose a modem connection with a leased multidrop line. Figure 7-7 illustrates
such a configuration.
7-16
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