Endress+Hauser EngyVolt RV12, Operating Instructions Manual

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Appendix EngyVolt RV12
48
the timeout period from both the master and the slaves’ specifications. The slave may
define the ‘response time’ as being the period from the receipt of the last bit of the query
to the transmission of the first bit of the response. The master may define the ‘response
time’ as period between transmitting the first bit of the query to the receipt of the last bit
of the response. It can be seen that message transmission time, which is a function of the
baud rate, must be included in the timeout calculation.
Query transmission
time
Slave
processing time
Response transmission
time
Query Response
Start of
query Query received
by slave Start of
response Response received
by master
A0016524-EN
 12 Modbus data transmission
11.4.5 Error checking methods
Standard Modbus protocol serial networks use two error checking processes, the error
check bytes mentioned above check message integrity whilst Parity checking (even or odd)
can be applied to each byte in the message.
Parity checking
If parity checking is enabled – by selecting either Even or Odd Parity - the quantity of “1’s”
will be counted in the data portion of each transmit character. The parity bit will then be
set to a 0 or 1 to result in an Even or Odd total of “1’s”.
Note that parity checking can only detect an error if an odd number of bits are picked up or
dropped in a transmit character during transmission, if for example two 1’s are corrupted
to 0’s the parity check will not find the error. If No Parity checking is specified, no parity
bit is transmitted and no parity check can be made. Also, if No Parity checking is specified
and one stop bit is selected the transmit character is effectively shortened by one bit.
CRC checking
The error check bytes of the Modbus Protocol messages contain a Cyclical Redundancy
Check (CRC) value that is used to check the content of the entire message. The error check
bytes must always be present to comply with the Modbus Protocol, there is no option to
disable it. The error check bytes represent a 16-bit binary value, calculated by the
transmitting device. The receiving device must recalculate the CRC during receipt of the
message and compare the calculated value to the value received in the error check bytes. If
the two values are not equal, the message should be discarded.
The error check calculation is started by first pre-loading a 16-bit register to all 1’s (i.e.
Hex (FFFF)) each successive 8-bit byte of the message is applied to the current contents of
the register.
Only the eight bits of data in each transmit character are used for generating the CRC,
start bits, stop bits and the parity bit, if one is used, are not included in the error check
bytes. During generation of the error check bytes, each 8-bit message byte is exclusive
OR'ed with the lower half of the 16 bit register.
The register is then shifted eight times in the direction of the least significant bit (LSB),
with a zero filled into the most significant bit (MSB) position. After each shift the LSB
prior to the shift is extracted and examined. If the LSB was a 1, the register is then
exclusive OR'ed with a pre-set, fixed value. If the LSB was a 0, no exclusive OR takes place.