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General / SerialDataFormat

Serial Data Format

Introduction

Data is in the same format in both the live serial stream and as stored on the compact flash card. However, not all messages are sent in both formats depending on the configuration of the system.

All messages are in the same format:

The header is a single byte, the data length is between 1 and 255 bytes depending on the type of message and the checksum is a single byte which is calculated from the sum off all bytes including the header. In the typical case where the checksum is greater than 255, then only the least significant byte is stored.

Note that since the data stream is binary, there is not possible to have a "unique" header available before each message - as such the data has to be decoded continuously to ensure that correct messages are extracted. For example if we start decoding a stream and we get a value of "9" for the first byte, we can see from the table below "message type 9" is 5 bytes long, so we wait until we get another 4 bytes; and then check the checksum. If the checksum is correct then it is likely (1 in 256) that this was a "type 9 message" and we move onto the next message to decode. If the checksum was incorrect then we use the 2nd byte as the message header and check its checksum and so on. It is unlikely, but possible to get incorrect messages decoded when we first start decoding the data stream however the chances rapidly decrease with just a few messages.

• After decoding 1 message in a row without error there is a 1 in 256 chance of an incorrect message
• After decoding 2 message in a row without error there is a 1 in 65536 chance of an incorrect message
• After decoding 3 messages in a row without error there is a 1 in 16million chance of an incorrect message

The following table gives summary information on the range, resolution and units used for some common serial data channels. For more information refer to specific channel pages.

Serial Physical Layer: For most products the default data rate is 115200 baud, RS232 levels. 1 start bit, 1 stop bit, no parity bit(s), no handshaking.