The RapidHA serial protocol has a frame-based architecture. All communication between the Host and Module is facilitated by the exchange of data frames.

Serial Interface Parameters

The serial UART interface uses the following parameters:

Table 1: RapidHA Serial Parameters

Parameter	Value
Baud Rate	115200
Parity	None
Stop Bits	1
Data Bits	8

Frame Structure

Every RapidHA frame has the structure illustrated below:

Figure 1: RapidHA Frame Structure

Header

The frame header is a fixed 5 bytes in length. Each byte is described in Table 2.

Table 2: Frame Header Description

Byte	Field Name	Description
0	Start of Frame	Fixed value of 0xF1; will be referred to as START_OF_FRAME throughout this document.
1	Primary Header	Defines the primary frame type.
2	Secondary Header	Identifies the command subset of the given primary frame type.
3	Frame Sequence Number	0 – 127: Host initiated transactions 128 – 255: Module initiated transactions The Frame Sequence Number enumerates a serial message exchanged between the Host and the Module, and vice versa. Use of a Frame Sequence Number for each such message is recommended. For best practice, we also recommend the Frame Sequence Number be incremented by 1 for each newly initiated message. Any transaction, whether initiated by the Host or the Module, may be acknowledged (i.e., ACK'ed) by the recipient with a response using the same Frame Sequence Number. The Frame Sequence Number is used for tracking purposes but is not a mandatory feature. If its use is not desired, it should be assigned a value of 0x00.
4	Payload Length	Defines the length of the payload of a received or transmitted frame as 0-n bytes; at the time of this writing, the maximum acceptable values for n are: Transmitted by Module – 0x7A or 122 bytes Received by Module – 0x7A or 122 bytes

Payload

Unlike the header, the payload is of variable size, ranging from 0 to n bytes; the size is based on the Payload Length field in the header. All fields contained in the payload are ordered with the least significant byte first.

The payload contains the command’s parameters. Command parameters are detailed in Frame Payload Definitions.

Checksum

The Checksum is a fixed two bytes in length, with the MSB indicating the End of Frame (EOF). It is calculated by summing the values of byes starting from the Primary Header to the end of the Payload. Any overflow above two bytes is discarded.

The checksum calculation (CS) is given here:

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