B&B Electronics RS-485 Digital I/O Module 485SDD16 - Manual (Page 27)

485SDD16-1005 Manual Appendix D D-1
B&B Electronics -- 707 Dayton Road -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 433-5104
APPENDIX D
Adding Data Field Comfirmation
D-2 Appendix D 485SDD16-1005 Manual
B&B Electronics -- 707 Dayton Road -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 433-5104
With serial communications in a laboratory environment, the
possibility of a communication error occurring is minimal. However,
in a harsh or an industrial environment the possibility increases. A
communication error occurs when a bit transmitted as a “1” is
received as a “0” or vice versa. If the 485SDD16 receives a error in
one or more of the first four command characters (“!0xx”), the unit
will not execute the command. However, if the 485SDD16 receives
an communication error on a data byte (I/O byte for Read Digital
command or state byte for Set Output State command), the
command will be executed since the unit has no way of knowing that
there was an error.
To provide the 485SDD16 with a way of detecting errors in the
data fields, an additional set of commands can be used. This set of
commands begins with the “#” (23h) character, instead of the “!”
(21h) character. Refer to Table D-1. With these commands every
data byte that is transmitted or received is followed by its
complement. For example: To read I/O lines:
Command syntax:
#{addr}RD
Response syntax:
{I/O msb}{~ I/O msb}{I/O lsb} {~ I/O lsb}
Where “~” is used to indicate the “complement of.” If I/O has a
reading of 1, the following would be received:
{00}{FF}{01}{FE}
Where FFh is the complement of 0 and FEh is the complement of
1. The complement of number “x” can be calculated in QuickBasic
as follows:
comp = (NOT x) AND &HFF