RESEARCH CONCEPTS RC2000C Manual page 87

RC2000C Az/El Tracking Antenna Controller Appendix E Communications Protocol 79
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 through the ETX
character. This forms a Longitudinal Redundancy parity check over the entire message.
Message Timing
The NAK or ACK reply does not signify that a function has actually taken place, but only that the
message was received and understood. The user should query the controller later to see if the
command was actually carried out, or is still in progress. Figure 5 shows the controller state diagram.
Command Restrictions
All slaves will respond to a command "0", 30 (hex), with 6 data bytes of ASCII characters in the
following form:
ACK ADDR 30 type D5 D6 ETX CHSUM
Where type is RC2K for an RC2000A, RC1K for an RC1000A, 2KCA for an RC2000C for El-over-Az
mounts, 2KCP for an RC2000C for polar mounts, 2KCE for Az-over-El mounts, 2500 for the RC2500A,
25CA for an RC2500B for El-over-Az mounts, 25CP for an RC2500B for polar mounts, and 25CE for
Az-over-El mounts. For RCI products, D5 and D6 denote the software version number.
Slave State Diagram: Introduction
General Description: The slave State diagram (see Figure 5) presents the required protocol
implementation at the slave device that guarantees the proper transfer and processing of
communication messages sent by a Master controller.
State Diagram Notation. Each state that a slave can assume is represented graphically as a circle. A
single-digit number is used within the circle to identify the state.
All permissible transitions between states are represented graphically by arrows between them. Each
transition is qualified by a condition that must be true in order for the transition to occur. The device will
remain in its current state if conditions, which qualify transitions leading to other states, are false, or
conditions that qualify pseudo-transitions are true. A pseudo-transition is a transition that occurs within
the same state and is represented graphically by arrows leaving from and arriving at the same state.
Table 1 describes mnemonics used to identify transitions in the state diagram.
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