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Data Communications
Hardwired Connections
If you have chosen a multipoint hardwired connection, you must decide if you want to use a
Daisy Chain or DSN Data Link connection. The following information may be helpful in
deciding between them:
• The Daisy Chain allows the computer system to perform group functions, such as polling the
group rather than polling each terminal individually. This can result in better response
time, as the computer system can be made aware that the user has entered data sooner.
• The DSN Data Link provides a higher degree of noise immunity. In addition it allows you to
power down a terminal or remove it from the link without disturbing data on the line. This is
useful if you will be moving terminals around a lot, for example, from one office to another.
Modem Connections
Ifyou have chosen a multipoint modem connection you must now decide what type of modem to
get.
If you are going to be communicating with an existing modem at a remote computer site, then
you must choose the same type of modem (full duplex or half duplex) as already exists at the
remote computer site.
If you are choosing the modems for both ends of the line, then the following factors may be
helpful in deciding between half and full duplex:
• Half duplex modems are less expensive.
• Full duplex data links are more efficient (because there are no ttline turnarounds") and may
therefore provide better throughput.
• Multipoint is a half duplex protocol in that data can travel in only one direction at a time.
However, there are many line turnarounds due to polling, so it may be advantageous to
minimize the line turnaround time by choosing a full duplex modem.
• Note that if you select half duplex modems they do not have to include the reverse channel
option. The HP multipoint protocol does NOT use the reverse channel, even ifit is physically
present.
Having defined the desired modem characteristics (full duplex or half duplex), you then select
the appropriate cables and modems using tables 7-2 and 7-3 as a guide. Note that the
designation ttdialed/leased" in table 7-2 refers to the type of telephone company facilities you
will be using. If you plan to make the connection to the remote computer by dialing over the
public telephone network, then the designation ttdialed" applies. If your terminal will be
connected to the remote computer over a set of leased telephone company lines (that is, you will
always be communicating over the same physical telephone lines), then the designation
ttleased" applies.
Note that if you wish to establish a multidrop configuration (see figure 7-7), then you must use
a leased line.
7-18
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