Dawn CONTROL-1500A Manual page 74

CONTROL-1500A Single Axis Antenna Controller Appendix D The RCI RS422 Interface Spec 67
Command messages (see Figure 4) begin with Start-of-text byte, STX, followed by a remote
address, a command byte and multiple data bytes. The End-of-text byte, ETX, is sent
following the last data byte, and the message is terminated by a checksum character.
Response messages are identical to command messages in format (but not content) with the
exception of the ACK (Acknowledge) or NAK (Not Acknowledge) character at the start of the
message instead of STX. Figure 4 illustrates the format of the command and response
messages. A command or reply message may have a variable length.
Message Delimiters
A command message begins with STX (02 hex), the ASCII Start-of-text control character. A
message-acknowledged reply begins with ACK (06 hex), the ASCII Acknowledge control
character, and a message-not acknowledged reply begins with NAK (15 hex), the ASCII Not
Acknowledge control character. All messages end with the ETX (03 hex), the ASCII End-of-
text control character, followed by the checksum byte.
Address Character
The device address must be a valid ASCII printable character between 31 and 6F in hex; thus,
63 addresses are possible. These are set in the controller in decimal format, or 49-111.
Command Character
The command character (CMD) immediately follows the device address and specifies one of
several possible commands for a particular device. See RCI 'Communication Protocol'
document for a complete description of these commands.
Check Character
The last character of any message is the check character (CHK). This character is simply the
bit-by-bit exclusive OR of all characters in the message starting with the STX character
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