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Packet Format - Honeywell hmr3000 TruePoint User Manual

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HMR3500 User's Guide
3.0 UART COMMUNICATION INTERFACE
The 5-pin power supply and UART interface connector on the HMR3500, mates with a Harwin latching connector (Part #
M80-8980505) per the pin configuration table earlier in this document. The interface hardware protocol is RS-232 without
any further handshaking or hardware control lines.

3.1 PACKET FORMAT

The Serial Data Interface operates both ways with messages structured as packets. Each packet starts with a header
followed by zero or more bytes of data, and ends with a checksum trailer.
Message Packet
Number of bytes:
Header Field
Number of bytes:
The first three bytes of the header field are fixed and are, in sequence, 0x0D, 0x0A, and 0x7E. These are ASCII codes for
carriage return, line feed, and the tilde (~) and are intended to permit using a terminal emulator for diagnostic purposes.
Using three fixed bytes also improves re-synchronizing if a transmission error upsets the determination of packet
boundaries.
The fourth byte is a packet type identification, different for each message type, as defined later. With one exception
(RSET), there is a corresponding message from the module for each message from the host, either to acknowledge a
command or provide the requested data. Although different mnemonics are assigned, the actual codes for these
corresponding types are the same. Almost all packet identification codes are printable ASCII, but are not themselves
mnemonic.
The count byte contains, as a binary number, the number of bytes in the data section of the message.
Data Field
The size of the data field depends upon the message. Many messages have zero bytes of data. At this time, the longest
data field is sent from the module with the DPOWER message, whose data is actually a readable ASCII string. Up to 80
bytes are currently allocated for it.
Trailer Field
The trailer field consists of one check byte, which is the sum of all other bytes in the message, that is, all five header bytes
and the data bytes, modulo 256.
3.2 NUMERIC DATA FORMAT
Each packet type description below includes a detailed description of the function and the data passed in the message.
Numeric data can be represented using specific data types described in Table 1 below.
6
Header
5
Start of Packet
3
Data
Variable
Packet ID
1
Trailer
1
Count Byte
1
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