CGIMUNICATIONS
6. 7, 4.1
6,7,4,2
6,7.4,4
VER.V09F
SECTION 6.7
This is transmitted in,r.1ediately following an ITB, ETB or ET)(.
The receiver checks this value and finds no error if they are
equal.
CRC-16 uses the polynoniinal (x
16
+x
1
5+x
2
+1) as the di visor
constant. The 16
bit remainder is sent as two eight bit
characters. This is also sometimes referred to as the BCC
(Block Check Char ac te r ).
Line Bid
For either HUNTER or base computer to transmit a message, the
line must first be seized. This requires sending an enquiry
(ENQ) character. A positive acknowledgement permits the start
of a message,
LINK CONTROL CHARACTERS
The data link is controlled by the use of the following control
characters:
SYN (32H) - Synchronous Idle
This character is used to establish synchronisation. Two
contiguous SYNs are required for synchronisation, They may also
be used as pad characters, although never used as such by
HUNTER. Syncs will be ignored after synchronisation has been
established except as part of CRC,
STX (02H) - Start of Text
This character is used as the start of text preceeding all text
blocks,
It is not part of the CRC accumulation.
ETB (26H) - End of Transmission Block
The ETS character indicates the end of a block of data
characters.
A block check character is sent immediately
following an ETB. A reply is required after an ETll (ACKO, NAK,
etc).
ITB (1F1l) - End of Intermediate Transmission Block
ITS is also called IUS or US (Unit Separator). This enhances
the throughput due to not having to go through a line
turnaround. The CRC accumulation is reset after an ITB. It is
not necessary to send an STX after the ITS.
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