Weidmuller WI-GTWY-9 User Manual page 83

WI-GTWY-9-xxx Wireless Gateway
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.
4.9.3 Endianess
Endianness is the convention that two parties that wish to exchange information will use to send
and receive this information if the information needs to be broken into smaller packets, i.e. data
transmission, radio, etc.
Integers are usually stored as sequences of bytes and the two more common sequences used are
little-endian and big-endian.
Most computer processors agree on bit ordering however this is not always the case.
Below is an analogy of what can happen if the bit orders are different between devices.
Imagine that Device 'A' wants to send a hexidecimal value "ABCD" to another devce'B'.
However device'A' can only do so 2 bits at a time. As device 'A' uses big-endian order, it will
first send "AB" and then "CD".
Device 'B' needs to be using the same convention as Device 'A' when receiving this information
such that when it receives the first part "AB" it knows that this is the beginning of the value, then
when it receives the next part "CD" it knows that it goes after the first part (big-endian).
If Device 'B' is unaware and assumes the inverse (litte-endian), it will end up with the value
around the wrong way, e.g. "CD" and then "AB", eg "CDAB"
Now if you convert these hexidecimal values back into decimal you will see a significant
difference, which can expalin why when connecting different devcices together the values
sometimes do not line up.
"ABCD" = 43981
"CDAB" = 52651
4.9.4 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.
The Fieldbus Interface IN and OUT areas are simply a block of I/O memory, exchanged
according to the configured protocol. For example, with a Profibus slave that supports 244 bytes
of inputs, the fieldbus interface IN area could be addressed either as byte locations 1 to 244 or as
word locations 1 to 122. Note that in either case, the underlying database structure is unchanged,
the difference is limited to the Fieldbus IN/OUT Area address that is displayed by configuration
software.
Certain protocols have an inherent or preferred byte or word structure – for example, Modbus is
a protocol that usually operates on 16-bit (word) registers. Consequently, configuration software
will default to the most common address mode for that protocol. Configuration software may
also apply an offset and/or scaling to the IN/OUT Area addressing to suit the particular protocol.
V1.18
83