Hardwired Connections; Modem Connections - HP 2624 Manual

Data Communications
POINT-TO-POINT
(ASYNCHRONOUS)
HARDWIRED
(FULL DUPLEX)
I
I
HALF DUPLEX
I
I
MODEM
I
MAIN
CHANNEL
REVERSE
CHANNEL
FULL DUPLEX
Fi~re 7-1. Point-to-Point Decision 'free
A major consideration in selecting which type of connec-
tion to use is the
anticipa~ed
distance between the terminal
and the computer. If the terminal will be located in the
vicinity of the computer system, you may use a hardwired
connection. RS232C specifications limit cable lengths to a
maximum of 50 feet (15 meters).
Another consideration is the desired availability of the
particular computer port. If you wish to have it available
(at different times) to terminals in diverse and/or varying
locations, then you should choose a modem connection with
dial-up capability.
Hardwired Connections
If you have chosen a point-to-point hardwired connection,
the only decision that remains to be made is the type of
cable to be used. The available cables are summarized in
tables 7-1 and 7-2. As noted in figure 7-1, an HP 2624A
hardwired connection is always full duplex (but the HP
2624A can support half duplex hardwired).
MODEM Connections
If you have chosen a point-to-point modem connection, you
must now decide what type of modem to get. As noted in
figure 7-1, point-to-point as supported by the HP 2624A
always employs the asynchronous transmission protocol.
You may, however, use synchronous modems provided that
both the terminal and the host computer are configured for
external Xl clocking.
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, half duplex main chan-
nel, or half duplex reverse cha,nnel) 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 be-
tween half and full duplex:
• Half duplex modems are less expensive.
• Full duplex data links are more efficient (because there
are no "line turnarounds") and may therefore provide
better throughput.
If you choose half duplex, then you must make one more
decision: whether to use main channel or reverse channel
line control (protocol). All half duplex modems offer main
channel; most offer reverse channel as an option. The first
thing to consider, of course, is which protocol your host
computer supports. If it will support both, then the follow-
ing factors may be helpful in making this decision:
• The reverse channel option usually adds a little to the
cost of the modem.
• Reverse channel control is more efficient than main
channel because it uses a separate physical control line
for triggering "line turnarounds" instead of ASCII con-
trol cQdes embedded within the data stream.
• Reverse channel tends to be more widely used in the U.S.
whereas main channel tends to be more popular in
Europe.
Having defined- the desired modem characteristics (full
duplex, half duplex main channel, or half duplex reverse
channel), you then select an appropriate cable and
asynchronous modem using tables 7-1, 7-2, and 7-3 as a
guide. Note that the designation "dialed/leased" in table 7-3
refers to the type of telephone company facilities you will
be using. If you plan to make the connection with the
remote computer by dialing over the public telephone net-
work, then the designation "dialed" applies. If your ter-
minal 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 "leased" applies.
Table-7 -1. Port # 1 Data Communications Cables
Gable No. HP Part No. Description
13222C 13222-60003 RS232C DATA COM
Female RS-232C 25-pin connector.
Length: 6.6 feet (2 meters)
7-2
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