Dawn CONTROL-1500A Manual page 73

CONTROL-1500A Single Axis Antenna Controller Appendix D The RCI RS422 Interface Spec 66
Figure 2 and 3 show RS-422 Master and Slave connections respectively.
Data Format
The data format supports the industry's standard asynchronous ASCII format with one start bit,
eight data bits (7-bit ASCII with the 8th bit sent as even parity), and one stop bit. The ASCII
control character subset 00-1F (hex) is reserved for message control. The printable ASCII
characters 20-7F (hex) are used for address, command and data characters. The standard
bus data rate via direct connect (up to 1,500 ft.) is 9,600 BAUD; the data rate for devices
connected to a master via modem is 1,200 BAUD, typically.
Message Protocol
Message format and protocol over the bus is a derivative of IBM's binary synchronous
communications protocol (BISYNC). The master station sends a command over the bus to all
remote stations. The station whose address is contained in the second byte of the command
message carries out the requested commands, and then replies with a response message
containing its own address and status information relating to its present condition. A remote
station only sends a response following a command containing its unique address from the
master. This prevents bus contention caused by more than one remote device communicating
over the bus at the same time.
A remote device ignores all commands that contain parity or checksum errors, protocol errors,
a wrong address, or message overrun errors. A remote device replies with a not-
acknowledged (NAK) character, 15 hex, if it receives an invalid command or data.
Message Format
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