Series 905
Page 22
Design Guide
RS-485 Protocol
The 905 supports 2400, 4800, 9600, 19200, 38400 and 115200 baud rates (factory default: 9600). The data
format is 8 data bits, no parity, and one stop bit.
Standard Addresses
Valid addresses are 3 digits, 001 to 253 (factory default: 253).
Universal Addresses
The 905 receives and responds to commands sent to address 254. For example, use 254 to communicate
with a device if its address is unknown. The 905 receives and acts upon commands sent to address 255, but
does not respond; use 255 to broadcast messages to multiple devices attached to the same system. For
example, use 255 to change the baud rate for all devices.
Query and Command Syntax
Queries return current parameter settings; commands change the parameter setting according to the value
the user types in the command syntax. Each query or command must begin with the attention character @
and end with the termination string ;FF.
Syntax required for a query is:
@<device address><query>?;FF
Syntax required for a command is:
@<device address><command>!<parameter>;FF
Examples:
Query current baud rate:
@253BR?;FF
Change baud rate to 19200:
@253BR!19200;FF
where:
@
<attention character>
253
<device address>
BR?
<query> (for query syntax)
BR!19200
<command>!<parameter> (for command syntax)
;FF
<terminator>
Response Syntax (ACK/NAK)
The ASCII characters 'ACK' or 'NAK' preface the query or command response string. The ACK sequence
signifies the message was processed successfully. The ‘NAK‘ sequence indicates there was an error.
The response to a query or a successful command is:
@
<device address>
ACK
<data>
;FF
The response to a message with an error is:
@
<device address>
NAK;FF
Examples:
ACK
response:
@253ACK9600;FF
(baud rate changed to 9600)
NAK
response:
@253NAK;FF
