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DIDS-402-2AMI3
Thus, even though the Display Terminal is primarily designed to provide
a rapid transfer of information to and from the CPU, the conversational mode
employed determines how and when this transfer will occur.
4-5 OPTIONAL OPERATION
Display Terminals are normally designed to perform certain functions and to
operate in a set manner.
However, the operation of any particular Display Ter-
minal may be readily changed by the incorporation of numerous design options.
In the remainder of this chapter where no reference is made to any particular
option, it can be assumed that the Display Terminal operates in the manner
de scribe d r e gar dIe s s of optional s tr apping.
4-5. I
Optional Communication Channel
When the Display Terminal is connected to a particular CPU I/O controller,
an option allows either a 2-wire or 4-wire communication channel to be employed
between the controller and the CPU.
In 2-wire systems, data communication
between the CPU and the Display Terminal is accomplished by means of a
common pair of telephone lines.
In 4-wire systems, transmitted data is trans-
ferred to the CPU by means of one pair of lines while received data is returned
over a second pair of lines.
In either case, the Display Terminal operates in the
half-duplex transmission mode whereby it can either transmit or receive data,
but cannot do both simultaneously.
4-5.2 Optional Conversational Modes
The Display Terminal can operate in either of two conversational modes:
polling or enquiry-response.
In the polling mode, the CPU, by ,issuing either a
read or a write directive, exercises control over all data transfers.
In the
enquiry-response mode, the display terminal operator usually issues an enquiry
and the CPU responds with an answering message.
Since these two modes essen-
tially determine how the display system operates, separate discussions for each
mode are included throughout the remainder of this chapter.
4-5.3 Address Options
The Display Terminal can assume anyone of 6.4 address codes to provide
selective addressing when several Display Terminals are employed by the display
system.
When the broadcast address option is employed, the Display Terminal
is capable of recognizing either its own address or a broadcast address"
When
several Display Terminals are modified
in
this manner, the CPU can issue a
write directive (polling mode) or a response (enquiry-response mode) to display
a data message on all connected Display Terminals simultaneously.
4-5.4 Transmit Options
Normally, two steps are required to initiate transmission from the Display
Terminal to the CPU.
The operator usually types in the desired text and then
performs the following two steps:
4-3
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