Reach System Communication Protocol
2 OVERVIEW
Reach System is equipped with sophisticated electronics enabling it to perform a variety of functions without
the need for external logic or feedback. Communication with the device is made via a full duplex RS232
connection using the Reach Communication Protocol laid out below. The protocol is designed to be
expandable for multi axis configurations and robust in dealing with bad connections.
3 Packet Structure
The Packet structure is as follows
Table 1 Packet Structure
OVERHEAD
BYTE
DATA
PACKET_ID
DEVICE_ID
LENGTH
8BIT_CRC
0x00
1
1:LENGTH-4
LENGTH-3
LENGTH-2
LENGTH-1
LENGTH
COBS Byte
DATA
FOOTER
TERMINATOR
OVERHEAD BYTE: Based on an implementation of COBS
(\https://en.wikipedia.org/wiki/Consistent_Overhead_Byte_Stuffing)
DATA: This is the data contained within the packet. Different packets expect different data types. In all cases
they are sent as individual bytes and it is up to the encoding and decoding software to parse them accordingly.
Packets cannot be larger than 254 bytes including the footer.
PACKET_ID: This is a unique identifier to tell the parsing software what the DATA refers to and how it should
be interpreted. The packet ID is a single byte ranging from 0x00 to 0xFF.
DEVICE_ID: This identifies to which device the packet is intended. Within one Reach System product there may
be any number of actuators or peripherals. Certain device IDs are used to send commands to multiple devices
where as others ensure only a single device will receive the command. The device ID is a single byte between
0x00 and 0xFF. Refer to Table 6 for Device ID assignments
LENGTH: The length of data and the 4 Byte footer in bytes. One is not required to use this in parsing the data
as the footer length remains constant. It is useful in checking for incomplete packets and in cases where
communication is unreliable. After byte stuffing the packet is one byte larger than LENGTH.
8BIT_CRC: This is an 8 bit polynomial (x^8+x^6+x^3+x^2+1 ; 0x14D or 0xA6 in Koopman notation) CRC
conducted over the full packet excluding the terminator byte. This implementation is reflected, processing the
least-significant bit first, the initial CRC value is 0xFF and the final value is exclusive-or’d with 0xFF. Refer to
Section 4 CRC.
0x00: The terminating byte signifies the end of the packet stream. Byte stuffing ensures no other 0x00 appear
in the data stream.
4 CRC
CRC implementation in Reach Serial Protocol is as follows.
• Polynomial used for CRC table generation is 0x4D.
• Initial CRC value is 0x00.
• Final XOR value is 0xFF.
• Input data is reflected/reversed.
• Result is reflected/reversed (before final XOR step).
An example implementation is shown in Figure 1 below.
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