User manual

905U-G Wireless Gateway User Manual
Page 75 ©September 2004
Fieldbus Mapping Types
Transfer Mode Read Mapping Write Mapping
Single Bit
The 905G reads a block of
consecutive bits from Fieldbus OUT
Area and stores each bit in
consecutive I/O Registers, as hex
FFFF or 0000.
The 905G takes the MSB (most
significant bit) of a block of
consecutive I/O Registers, converting
the 16 bit I/O register values into 0 or
1, and writes to consecutive bits of
Fieldbus IN Area.
Byte (8-bit)
The 905G reads consecutive bytes
(8-bit values) from Fieldbus OUT
Area and stores each byte in the most
significant 8-bits of a consecutive I/O
register.
The 905G takes the most significant
8-bits of consecutive I/O registers
and writes them to consecutive bytes
(8-bit values) of the Fieldbus IN area.
Word (16-bit)
The 905G reads consecutive words
(2x8-bit values) form Fieldbus OUT
Area and stores each word in a
consecutive I/O Register.
The 905G takes consecutive I/O
registers and writes them to
consecutive words (2x8-bit values) of
Fieldbus IN Area.
4.8.2 Transfer Mode
Radio Interface registers are all 16-bit general-purpose input or output registers. That is,
analog inputs or outputs are stored as a 16-bit value. Digital inputs or outputs occupy a whole
16-bit register and are stored as either 0000(hex) or FFFF(hex) for compatibility with the
ELPRO Radio Protocol. However, the Fieldbus Interface may contain (depending on the
protocol) significantly less registers than the Radio Interface (see diagram above). Also,
certain protocols may require a different I/O structure than that used by the Radio Interface
registers. Consequently, depending on the fieldbus mapping transfer mode (see above table),
Radio Interface registers may or may not be compressed.
“Word” transfer mode offers no compression, but rather a direct transfer of 16-bit registers
between Radio Interface and Fieldbus Interface. This mode would suit the transfer of
registers containing pulse counts or analog values with no loss of resolution.
“Byte” transfer mode operates on only the most significant BYTE (the first 8 bits) of Radio
Interface registers, but allows these bytes to be consecutively packed in the Fieldbus
Interface. This mode would suit the transfer of analog values in low-resolution, in cases
where I/O space is at a premium. Byte Address Mode is recommended when using byte
transfer mode (see Address Mode section below).
Bit transfer mode operates on only the most significant BIT of Radio Interface registers, but
allows these bits to be consecutively packed in the Fieldbus Interface. This mode would suit
the transfer of digital I/O in cases where it is not desirable (or possible) to use a whole 16-bit
register just to store a 0 or 1 value.
4.8.3 Address Mode
Configuration software allows the Fieldbus Interface IN and OUT areas to be addressed as an
array of 8-bit bytes (Byte Address Mode) or an array of 16-bit words (Word Address Mode).
The address mode may be required to change depending on the transfer mode, the protocol,
or the particular host device. The Address Mode option is included so that the configuration
software can be setup to use the same I/O addressing method used by the host device. The
actual structure of I/O in this database can only be physically altered via the transfer mode.