BECKHOFF BK8000 - Manual (Page 18)

RS 485 Coupler BK8000
18 BK8000
Data communication
Data transfer protocol Data communication with the bus coupler is realised by way of a simple
data transfer protocol. During process data exchange with the BK8000, the
complete process image is always transferred, i.e. when requested by the
master the bus coupler receives the complete output data and then sends
the current process input data in the response to the master. Data
communication between the bus coupler and the individual bus terminals is
realised via the "K bus". In the bus coupler's default setting, this access to
the I/O signals of the terminals is asynchronous (the bus coupler is in the
"free run" mode). Using the Beckhoff KS2000 configuration software, it is
very easy to set this mode to synchronous. In the synchronous mode, the
bus coupler accesses the bus terminals synchronously with access by the
master to the bus coupler.
The data packets are transferred in a fixed format as a binary string. The
data frame is set permanently to 8 data bits, even parity and one stop bit (8
E 1). The baud rate is set by default to 38400 baud. As already mentioned,
the station addresses are set by means of the two rotary switches on the
bus coupler. When the address 0 is set, the BK8000 operates as the
master and the slave bus coupler must be assigned the address 1.
Request In the request, the master sends the process output data to be transferred
to the slave. In the response, the slave sends its status and its process
data inputs.
Description of request Byte Value range
Start identifier 0 ’P‘ (0x50)
Number of process data output words 1 0 – 255
Message ident 2 0 – 255
Multipoint address 3 0 – 99
Process data output LOW Byte
( optional ) 4 + 2 x n
(n = 0, 1, 2,..., 125) 0 – 255
Process data output HIGH Byte
( optional ) 5 + 2 x n 0 – 255
Checksum 6+2 x n + 1 0 – 255
The "start identifier" consists of one byte and identifies the start of a data
packet. The "number of process data output words" specifies the size of
the output process image of the addressed bus coupler in words. If the
number of bytes of the process image is odd, it must be rounded up. A 0
must be entered if only the process input data of the bus coupler is to be
read.
The "message ident" is any value that is returned by the recipient in the
response string to enable the sender to assign received strings to the
strings that have been sent.
The "multipoint address" specifies the recipient. The address must have a
value unequal to 0 because 0 is the master address.
The "process data outputs" are entered as data words in INTEL format.
The "checksum" is generated by adding up the contents of the individual
bytes (complete request string without checksum byte). Any possible
overflow is ignored.