User`s guide

Page 9 of 21
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6. Communications Packet Structure Overview
The serial communication between the HxM device and the paired device operates in a simplex mode. The HxM
device transmits the HxM data packet and does not process any received data.
6.1. Basic Message Format
Figure 8 : Basic Message Format
6.1.1. STX
The STX field (Start of Text) is a standard ASCII control character (0x02) and denotes the start of the message.
Although the protocol does not guarantee this value will not appear again within a message, it gives some
delimiting to the message and therefore a start character to search for when receiving data.
6.1.2. Msg ID
The Message ID uniquely identifies each message type and is in binary format. For the HxM the message ID is
0x26 for the standard HxM data packet.
6.1.3. DLC
The Data Length Code is used to specify the number of bytes within the data payload field of the message. Valid
values range between zero and 128 (inclusive). For the standard HxM data packet the DLC has a value of 0x37.
6.1.4. Data Payload
This field contains the actual data sent between the local and remote units and can contain anywhere between
zero and 128 bytes of data. The number of bytes in this field is dictated by the DLC field.
Calculated over the
Data Payload area
Number of bytes in
the Data Payload
STX
HxM Transmitted Data Packet:
Msg ID DLC CRC ETX
0…128 bytes
Data Payload