Property Management System - Communication Interface | Architecture |
The control codes used for the message texts are:
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Control Code | Hexadecimal | Function | |
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STX | 02 | Indicates the start of a block. | |
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ETX | 03 | Indicates the end of a block. | |
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EOT | 04 | Indicates the end of transmission of a block or release of the | |
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| data link by the sender. | |
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ENQ | 05 | Used to query other party for expected response. | |
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ACK | 06 | Indicates the positive acknowledgment of an information block | |
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| or start sequence. | |
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NAK | 15 | Indicates the negative acknowledgment of an information | |
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| block or start sequence. | |
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DLE < | 10, 3C | Indicates the receiver interrupting to ask for sending rights. | |
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DLE : | 10, 7C | Indicates the receiver interrupting to ask the sender to stop | |
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| transmission and to release the data link. | |
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DLE ? | 10, 3F | Used as the negative acknowledgment of block and indicates | |
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| that the receiver temporarily cannot receive data from the | |
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| sender. | |
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Transmission Protocol
Before a message can be sent, a start sequence (also called a selecting sequence) must be sent to urge the receiver to receive the data. Every transmission sequence will begin with the sender sending a start sequence.
The start sequence is a three byte sequence:
0 1 2
‘1’
‘!’ 05H
SA UA ENQ
Figure 2-1 Start Sequence
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