GE 90-30 PLC, User Manual

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4-18 Series 90™-30 PLC LONWORKS® Bus Interface Module User's Manual – June 1997 GFK-1322A
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Menu Commands
There are four command menus available in the Reference Configuration screen. They
are File, Tools, View, and Options. The File menu includes the commands Open New
File, Open Existing File, Save Current File, Save current file under a different name,
and Exit. The Tools menu contains the commands to edit transceiver type, form the .XIF
file, download the .XIF file to the module, and rebuilding the valid bit table. The View
menu allows the user to choose either the reference configuration or the valid bit table.
The Options menu allows the user to edit screen colors.
Defining Custom Variables
The user has the ability to create custom or user-defined variable types to be used with
the LBIM. These variable types are limited to 31 Bytes or less. LONWORKS messages
are processed in 8-bit words. The PLC registers are configured for 16-bit words. In
order to form the 16-bit words from the 8Bit data, the interface needs instructions on how
to place the data into the registers in the desired order. This is accomplished using the
bytemap information.
Bytemapping
To control mapping of data, a Bytemap is required for each network variable. The
bytemap is a 32-bit (8 hex digits) value. The first 31 bits (bit0-bit30) represent each
of the 31 possible bytes in a network variable. The most significant bit (bit 31)
determines whether all the bytes in the network variable are packed into the PLC
register locations.
Network variables can be organized as nearly any combinations of bits, 8-bit bytes,
and 16-bit words up to 31 bytes total. The PLC %AI and %AQ registers are 16-bits
wide. If an 8-bit network variable is mapped to a %AI register, it would normally be
placed into the lower 8-bits with the upper 8 bits left empty. The next reference
would be stored in the low byte of the next register. To conserve memory space or to
affect logical association of byte-wide data, the data can be “packed” such that a byte
can be stored in the lower byte of the register and the next byte value stored in the
upper byte of the same register. The bytemap configuration value controls this
process.
To pack data, the most significant bit of the bytemap is set to 0. With the remaining
31 bits associated with the possible 31 bytes of the network variable, a 0 means the
byte at that location is to be handled as an individual byte. A 1 means the byte is part
of a larger word. Since a 1 signifies that the byte is combined with the next higher
byte, a 0 in that next location would be meaningless since that byte cannot be an
individual (it is joined with the next lower byte). In this case, a 0 means that this
byte completes the data word, while a 1 means that these two bytes form the lower
word of a double-word value. In this case, the remaining two bits for the double word
bytes should be set to 0 (see example below).