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7.1.3 ASTM Lower Layer
ASTM lower layer receives messages for a transmission request from the upper layer. These
messages are then split into frames and sent to a communication medium to be transmitted
to other parties. ASTM lower layer also constructs frames received from a communication
medium to recreate messages to be transferred to the ASTM upper layer as reception
messages. Configuration and communication procedures for transmission and reception of
frames are explained in the following chapters.
Item Method Explanation
Frame
Configurations
For Middle Frame
<STX> FN text <ETB>
C1 C2 <CR><LF>
For Last Frame
<STX> FN text <ETX>
C1 C2 <CR><LF>
Control character (characters enclosed in <>):
<STX> is control character (HEX 02)
<ETB> is control character (HEX 17)
<CR> is control character (HEX 0D)
<LF> is control character (HEX 0A)
<ETX> is control character (HEX 03)
FN: FN is a single ASCII number. FN indicates the sequence
number for a frame (the frame number modulus 8). Frames of a
single transmission phase are consecutively numbered
beginning with 1, so FN runs from 1 to 7, then continues with 0,
1, and so on.
Text: the data content of a frame (maximum 240 characters).
Records are sub-divided into intermediate (middle) frames with
240 or fewer characters. Text is part of a split message.
C1 and C2: When 1 byte resulting from adding each byte, FN to
<ETB> for the middle frame and FN to <EXT> for the last
frame, is expressed in hexadecimal, the upper character (16
1
) is
C1 and the lower character (16
0
) is C2. Characters used are ‘0’
to ‘9’ or ‘A’ to ‘F’.
Frame Character
Configuration of
Text
Characters other than
<SOH><STX><ETX>
<EOT><ENQ><ACK
>
<DLE><NAK><SYN
>
<ETB><CR><LF>
<DC1><DC2><DC3>
<DC4>
<SOH> is control character (HEX 01)
<EOT> is control character (HEX 04)
<ENQ> is control character (HEX 05)
<ACK> is control character (HEX 06)
<DLE> is control character (HEX 10)
<NAK> is control character (HEX 15)
<SYN> is control character (HEX 16)
<DC1> ~ <DC4> are control characters (HEX 11 ~
14)
Maximum Length
of the Frame
247 characters For one frame, maximum of 240 characters for text, 7 characters
for frame control characters.
Messages equal to or less than 240 characters are transmitted
as one final frame. Messages greater than 240 characters are
split into frames that have character lengths that fall within the
240-character limit. The only or final remaining frame becomes
the last frame and is indicated by <ETX>. All others are
intermediate (middle) frames and are indicated by <ETB>.
Table 7: ASTM Lower Layer Communication Methods
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