Dial-Up Operation; Dial-Up Operation (With Checking) - IBM 2740-1 System Reference Manual

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Figure 6. Point-to-Point Operation (With Checking Special Feature)
operation. The basic difference is that now message
transmission is on a broadcast basis (Figure 7).
Thus, all terminals on this line, in a ready condition
and in a standby status, switch to a receive status
when the (B) signal from the initiating terminal is
received. All terminals now in receive status
accept and print all transmitted m essages, until an
EOT signal is sent from the transmitting terminal,
or until terminal runs out of paper (paper presence
contact not locked out). At this time, all terminals
switch to a standby status.
NOTE: The checking special feature is not available for broad­
cast operation.

Dial-Up Operation

This type of operation requires that each terminal
have the dial-up adapter special feature installed.
This feature permits the terminal to be inter­
connected to com m on-carrier switched networks by
means of com m on-carrier provided data sets. Refer
to Dial-Up Adapter in the Special Features, Model 1
section for a detailed explanation.
Dial-up is essentially the same as point-to-point
operation except for the type of communication
facilities employed. Since a dial-up operation
connects the terminal to a line accessible to the
general public, this line connection must be dropped
when message transmission is completed.
a private line, however, the connection to the te r­
minal is permanently assigned).
The dial-up line connection is dropped by use of
the Dial Disconnect key at one of the terminals (or
if the status is lost). With the automatic disconnect
feature (supplied by the common ca rrier when the
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data set is installed or by a 2740 Model 1 special
feature on IBM World Trade machines), operating
this Dial Disconnect key at one term inal disconnects
both ends of the line (Figure 8). Refer to Dial
Disconnect Key and Dial Connect Light under Trans­
mission Controls and Indicators, Model 1 in this
section of the manual.

Dial-Up Operation (with checking)

This operation (Figure 9) is sim ilar to the pre­
viously described dial-up operation, except that
both terminals must have the checking special
feature (VRC— Vertical Redundancy Checking, and
LRC— Longitudinal Redundancy Checking) in addi­
tion to the dial-up adapter special feature installed.
Provided with the checking feature is the EOB key
and the Restart key and light (refer to Transmission
Controls and Indicators, Model 1 in this section of
the manual for a detailed description of these items).
With checking installed, each transmitted char­
acter is checked for parity (an odd number of bits)
as it arrives at the receiving terminal (VRC). Any
character in erro r is not printed. Instead, a hyphen
character (or an underscore if in upshift) is printed
in its place, and message reception continues. A
different character is printed on IBM World Trade
machines. Also, at the end of each message block,
the total message block is checked by an LRC com­
parison at the receiving terminal.
If either a VRC erro r or an LRC erro r is de­
tected in a message block, a hyphen character is
printed at both terminals (following the LRC com­
parison). Also, the transmitting terminal keyboard
is locked (while the receiving terminal keyboard r e ­
Principles of Operation, Model 1
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