Appendix A: Adding Data Field Confirmation

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 485SDA10 receives an error in one or more of the first four command characters (“!0xx”), the unit will not execute the command. However, if the 485SDA10 receives a communication error on a data byte (channel byte for Read Analog 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 485SDA10 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 A-1. With these commands every data byte that is transmitted or received is followed by its complement.

Example A.1 - To read A/D channel zero: Command syntax:

#{addr}RA{00}{FF}

Response syntax:

{ch0 msb}{~ ch0 msb}{ch0 lsb} {~ ch0 lsb} Where “~” is used to indicate the “complement of.”

If A/D channel 0 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

485SDA103798 Manual

Appendix A

A-1

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B&B Electronics RS-485 manual #addrRA00FF, 00FF01FE, Comp = not x and &HFF
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RS-485 specifications

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