Burkert Type 8056 User Manual
Page 6
- 6 -
RS232_485_ETP_MODBUS_BU_REV02.doc
reception phase, the reverse operation is executed by the receiver integrated circuit: the eight data bits are extracted
and the others are eliminated. These operations are executed entirely on a hardware level.
The 8 data bits must be serialised staring from bit 0 (the least significant one).
3.2.3.
DATA BLOCK FORMAT
Communication takes place through data blocks or packs of variable length which do not exceed 256 bytes overall. A
data block or pack is composed thus::
•
ADDRESS
TO, Containing the address of the device the block is sent to;
•
ADDRESS
FROM,
Containing the address of the device which sent the block;
•
COMMAND, Containing the command code the block refers to;
• LENGTH, Containing the length of the data block in bytes
• DATA BYTES, The block of data of variable length from zero to 250
•
CRC CHECKSUM BYTE, Calculated in the following way:
1. CRC initialised to zero
2. CRC rotated to the left by one bit
3. CRC = CRC + block bytes
The process is repeated starting from point 2. for all the block bytes, excluding CRC..
NOTE:
The CRC must be subjected to an operation of
rotation to the left
and not just a simple
shift
operation.
The difference is shown in the following example:
byte value:
10100010
shifting :
1 <= 0100010 <= 0
rotating: 1 1 <= 0100010 <= 1
A block can contain a maximum of 90 data bytes. A block which does not contain data bytes is formed solely by
ADDRESS TO + ADDRESS FROM + COMMAND + LENGTH + CRC CHECKSUM, with LENGTH value equal to
zero.
3.2.4.
COMMUNICATION SPEED
The millennium series instruments have 4 communication speeds::
•
4800
bps
•
9600
bps
•
19200
bps
•
38400
bps
3.2.5.
SERIAL PORT SETTINGS FOR RS 232 AND RS 485
Serial port settings:
• Data bits: 8
• Parity: none (no parity)
• Stop bits: 1
• Flow control: none (no control lines no xon/xoff characters used)