I/O Terminal /
MANUAL CONFIGURATION
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6.3
Cyclic Data Exchange
This section explains and provides data on Cyclic Data Exchange.
6.3.1 Process Data and Image
The Bus Coupler includes different memory areas, each having a capacity of 256 words.
Telegrams passing over the Lightbus can specifically access any desired memory cell. The
control and status bytes in the Lightbus telegrams can be used to distinguish between two
relevant regions of the memory and to address them separately. In order to initiate a Bus
Coupler update, the value in the control and status bytes must be 0x10, while the data byte
must contain the constant 80hex. It is possible to access the Bus Coupler data after this. For
this purpose the control and status byte contains the value 0x30. Two bytes can be written
and two bytes can be read simultaneously with one access. The process is described in
detail in the following sections.
After being switched on, the Bus Coupler determines the configuration of the inserted
input/output terminals. The assignment of the physical slots for the input/output channels and
the addresses in the process image is carried out automatically by the Bus Coupler.
The Bus Coupler creates an internal assignment list, in which the input/output channels have
a specific position in the process image of the Bus Coupler. A distinction is made here
according to inputs and outputs, and according to bit-oriented (digital) and byte-oriented
(analog or complex) signal processing.
Two groups are created, one for inputs and the other for outputs. Each group has the byte-
oriented channels in ascending sequence, starting from the lowest address, and these are
followed by the bit-oriented channels.
Digital Signals
The digital signals are bit-oriented. This means that one bit in the process image is assigned
to each channel. The Bus Coupler creates a memory area containing the current input bits,
and ensures that the bits in a second (output) memory area dedicated to the output channels
are written out immediately, following the update command.
The details of the assignment of the input and output channels to the controller's process
image is explained fully with the aid of an example in the appendix.
Analog Signals
The processing of analog signals is always byte-oriented. Analog input and output values are
represented in memory by two bytes each. Values are represented in SIGNED INTEGER
format. The number 0 stands for the input/output value 0 V, 0 mA or 4 mA. The maximum
value of an output or input value is represented, according to the standard settings, by
0x7FFF. The intermediate values are correspondingly proportional. A range with a resolution
of 15 bits is not achieved for all inputs and outputs. If the actual resolution is 12 bits, the last
three bits have no effect in outputs, while as inputs they are read as 0. Each channel also has
a control and status byte. The control and status byte is the most significant byte in the most
significant word. An analog channel is represented by 4 bytes in the process image, of which
3 bytes are used.
