9
Chapter 4. Communication
distinction code as that of the received message to the response message and
returns it. It is convenient to use X (58H) in messages the first time and use x (78H)
for resent messages in order to distinguish them.
z ETX
ETX indicates the end of the application layer.
z Checksum
This value is for checking whether something abnormal (e.g., electromagnetic
noise) has caused the message content to change during transmission. The
checksum is expressed as two hexadecimal characters.
• How to create a checksum
(1) Add up the hex codes from STX to ETX in increments of 1 byte.
(2) Take the two’s complement of the addition result.
(3) Convert it to character codes.
Example:
The example below is based on a message that includes the following elements.
Device address : 10 (0AH)
Subaddress : 00 (fixed)
Device distinction code : X (58H)
1. Add the character codes byte by byte in the message from STX through ETX.
The lowest order byte of the sum is 76H.
2. Take the two’s complement of the sum.
The result is 8AH (8A (hex)).
3. Convert the characters of the result to character codes. Use this as a checksum.
The characters “8A” are converted to their character codes 38H and 41H.
z CR/LF
This indicates the end of the message. Immediately after LF is received, the device is
ready to receive and process another message.
02H
58H30H30H
0DH30H
30H 41H
30H31H2CH
52H 53H
03H
32H2CH57H
31H
41H
38H
0AH
STX
X00
CR0
0 A
01,
R S
ETX
2,W
1
A
8
LF
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