MPI Ultrasonic generator Manual page 25

Bus description
 bus address of the device (1-255) – selecting via LCD panel
 transmission speed – selecting via LCD panel
 no parity check
 the number of Data-bits is RTU 8 Data bits
 the number of STOP-bits is 2 STO-bit
important remarks for operation in the Master/Slave system :
The bus address must be differently adjusted for each device !!
Transmission mode, baud rate and parity must be identical
All data in a MODBUS-slave are allocated to addresses. Data access (read or write) is made by the
corresponding control command and the indication of the corresponding data address. In general, a
MODBUS telegram starts with the address of the slave, followed by a control command (e.g. read
register) and the data. By means of the checksum at the telegram end, the bus participants can
recognize transmission errors.
CRC Generation
A procedure for generating a CRC is :
1. Load a 16-bit register with FFFF hex (all 1's). Call this the CRC register.
2. Exclusive OR the first 8-bit byte of the message with the low-order byte of the 16-bit CRC register,
putting the result in the CRC register.
3. shift the CRC register one bit to the right (toward the LSB), zero-filling the MSB. Extract and
examine the LSB.
4. (if the LSB was 0) : repeat step 3 (another shift). (if the LSB was 1) : exclusive OR the CRC register
with the polynomial value A001 hex (1010 0000 0000 0001).
5. Repeat steps 3 and 4 until 8 shifts have been performed. When this is done, a complete 8-bit byte
will have been processed. When the 16-bit CRC (two 8-bit bytes) is transmitted in the message,
the low-order byte will be transmitted first, followed by the high-order byte.