It is important in serial data transmissions that the receiving device knows when data is
being transferred and when data being transferred is about to stop. This information is
conveyed by the above start and stop bits. The start bit synchronizes the receiving
device so it reads the data properly. The stop bit notifies the receiving device that the
data frame has ended. The RX-400A’s fixed data word frame format does not contain a
parity bit.
7.2.1 Communications Protocols
Data transfers are handled via input and output buffers. The input buffer is handled in
circular fashion allowing simultaneous inputting and processing of data. The input
buffer accepts up to 256 bytes before overflowing. As data in the buffer is being
processed, up to 16 additional characters can be accepted by the unit. Upon receiving
a terminator character, the RX-400A processes any previous messages in the buffer.
The input buffer processing starts on the receipt of a terminator (CR). If the
communications error flag is set, the buffer contents from the end of the last processed
message thru the message terminator are discarded. In the event the buffer is
overrun, its contents are discarded. All other incoming data is buffered and processed
in the order in which it was received. If the radio does not understand a command
sequence or the command is badly formatted, the radio will respond with a ‘Z’ followed
by a carriage return.
The output buffer is handled in circular fashion allowing simultaneous additions and
outputting.
7.3
COMMAND MESSAGE FORMATTING
Command messages for the RX-400A are exclusively ASCII-encoded data, consisting of
command headers and arguments. Command headers consist of a two character
(letter-digit) mnemonics. All queries consist of a question mark (?) followed by a
command header. All set commands consist of an asterisk (*) followed by a command
header.
7.4
TERMINATORS FOR COMMANDS AND QUERIES
Terminators are used to signal the end of a command or string. When a properly
formatted message is ready to be sent, a CR (carriage return) character should be
entered. The CR character instructs the receiver to process the preceding messages.
The RX-400A also transmits a terminator when responding to queries. After the query
response is transmitted the receiver issues a CR (carriage return) indicating the end of
the response.
7-2
PN 74446
