NAPOPC Server 001

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 Q: The digital input channel shows "-1" which displays within our OPC client software when using NAPOPC

     DA server and the channel  with "ON" condition. This reading changes from 0 to -1 (OFF & ON) instead of 

     0 to 1. Other OPC server software displays these numerals correctly.

 Ans: 

 Please refer to the "Automation Programmer's Reference/Microsoft Press" book, page 164 to 165.

 ======= Cut from "Automation Programmer's Reference" =======

 VT_I2
A 2-byte integer value is stored in iVal.

 VT_BOOL
A Boolean (True/False) value was specified. A value of 0xFFFF (all bits 1) indicates True; a value of 0 (all bits 0) indicates False. No other values are valid.

 ================ End of cut ==================

 In C/C++ computer language, a value of 0 indicates False; other values (For examples: 0xFFFF, -1, 1 or....) indicates True.

 But in OLE/COM/DCOM/OPC, a value of 0xFFFF (-1) indicates True; a value of 0 indicates False. No other values are valid. Microsoft defines this in the OLE specification.

 In the NAPOPC DA server, the digital bit input uses the VT_BOOL data type. Thus, it always has to returns 0xFFFF (equal to -1) or 0. And the client program can explain the value of -1 to "TRUE", "ON", "High" or...

 If other OPC servers return 0 or 1 to indicates False or True, they should use the VT_I2 data type to return a integer value. Then the client programs have to check the returned value if it is between 0 and 1 (It may return an invalid value).

Written by Gary 

Date:2004/9/7