BECKHOFF C1220 - Manual (Page 4)

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Beckhoff Lightbus System Description 4
Beckhoff Lightbus - PC interface card C1220
The telegram frame is required for serial, asynchronous data
communication and consists of 1 start bit, 6 CRC check bits and 2 stop
bits. The telegram frame is generated and checked by the hardware.
Software support is not necessary.
The telegram contents are essentially based on a byte organization.
AD0 - AD7 constitute the so called address field. Up to 254 modules and
terminal rows can be addressed via this address field (the addresses 0x00
and 0x0ff are reserved).
CR0 - CR3 defines the telegram type. The following functions can be
defined in the telegram:
CR3 CR2 CR1 CR0 Function Description
0 0 0 0 READ The addressed module inserts the input
information in the data fields D0 - D3.
0 0 0 1 READ/WRITE The addressed module inserts the input
information in the data fields D0 - D3 and
accepts the output information.
0 0 1 0 ADDRESS
INITIALIZATION The addressed module accepts the contents of
D0 as the module address and sets D0 = 0.
0 0 1 1 RAM A special type of telegram for bus coupler
BK2000
0 1 0 0 ADDRESS
CHECK AND
COUNT
COMMAND Every module that is passed through increments
the contents of D0 by 1. The addressed module
transfers the contents from D0 to D3.
1 0 0 1 LOW INTENSITY
COMMAND The addressed module reduces the send
intensity by 20%.
1 0 1 1 BROADCAST A special type of telegram for bus coupler
BK2000
The bytes D0 - D3 contain the actual user information. Processing of this
user information data is defined by the control field.
The last byte in the telegram contains 2 reserve bits and 6 bits for
generation of a CRC checksum. A Hamming distance of d=3 is achieved
with a length of the contents amounting to 50 bits.