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DIDS-402-2AM13
response mode is that display terminal operators can gain immediate access to the
CPU since the operators are responsible for initiating a11 message transfers.
A
disadvantage is that all display terminal positions have equal standing and there-
fore important message transfers may be held up while the CPU is processing a
subordinate request.
The polling mode eliminates this often unwanted feature
by placing the CPU in direct control of the display system.
A disadvantage of
the po11ing mode which must be considered, however, is that the CPU uses
valuable time while issuing polling directives to elicit or display information.
4-7.1 Enquiry-Response
In the enquiry-response conversational mode, all data transfers are initi-
ated by display terminal operators (enquiry) and are answered or acknowledged
by the CPU (response).
However, the CPU can access the Display Terminal at
any time, since a constant receive condition exists.
Messages received in this
manner are not solicited by the operator and are therefore termed unsolicited
responses.
4-7.2 Polling
In the po11ing conversational mode, all data transfer is under the positive
control of the CPU.
In
display systems that employ the po11ing mode, the CPU
periodically issues one of two directives: read or write.
The read directive is issued by the CPU to read the data message from the
addressed Display Terminal.
The Display Terminal receiving the read directive
immediately responds to the po11 by transferring the message currently on its
screen to the CPU.
If a Display Terminal receives a read directive and has no
data available for transfer, a 'no busines's
I
report is issued to the CPU.
The write directive is issued by the CPU when it has a message for transfer
to the addressed Display Terminal.
(In systems employing the broadcast addre ss
option, either one or all display terminals in the system can receive the message
simultaneously. )
4-8 CHARACTER REPRESENTATION
The Display Terminal uses the American Standard Code for Information
Interchange (ASCII) for the coding of digital information.
ASCII primarily involves
representing alphabetic letters, numerals, symbols or control codes in the form
of 7-bit binary characters.
In
addition to the 7 bits used for representing some form of information,
each character contains its own parity, start, and stop bits.
The parity bit is
used to check the validity of each received character.
Each character trans-
mitted or received contains an even number of I' s, thus the Display Terminal
operates as an even parity device.
Start and stop bits are added to the extreme
ends of each character to provide asynchronous transmission.
In
asynchronous
transmission, data characters may be transmitted one at a time, or in groups;
initializing or terminating message characters are not
requi~ed
for data syn-
chronization.
4-7
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