405
Long Transmissions The
largest block of data that can be transmitted as a single frame is 131 charac
-
ters.
A command or response of 132 characters or more must therefore be di
-
vided into more than one frame before transmission. When a transmission is
split, the ends of the first and intermediate frames are marked by a delimiter
instead of a terminator.
As each frame is transmitted, the receiving node waits for the delimiter to be
transmitted.
After the delimiter has been transmitted, the next frame will then be
sent.
This procedure is repeated until the entire command or response has been
transmitted.
The following diagram shows an example of host link communica
-
tions addressed to a PC.
Delimiter
Text
Unit no.
Header code
FCS
Delimiter
Frame1
(command)
Text
FCS
Delimiter
Terminator
FCS
Text
End code
Header code
Unit no.
Frame2 (command)
Frame (response)
Delimiter
Text
FCS
Terminator
Frame3 (command)
Host
computer
PC
Precautions for Long Transmissions
When
dividing commands such as WR, WL, WC, or WD that execute write op
-
erations,
be careful not to divide into separate frames data that is to be written
into
a single word. As
shown in the illustration below
, be sure to divide frames so
that they coincide with the divisions between words.
@
FCS
↵
00WD
FCS
: ↵
Frame
1
Node
no.
Header
code
Data
One word of data
Data
from the same word is not divided.
Frame
3
Terminator
Data
Delimiter
One word of data
Data
from the same word is not divided.
FCS (Frame Check Sequence) When
a frame is transmitted, an FCS is placed just before the delimiter or termi
-
nator
in order to check whether any data error has been generated. The FCS is
8-bit data converted into two ASCII characters. The 8-bit data is the result of an
EXCLUSIVE
OR performed on the data from
the beginning of the frame until the
end
of the text in that frame (i.e., just before the FCS). Calculating the FCS each
Command and Response Formats
Section 11-2
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