Endress+Hauser EngyVolt RV12, Operating Instructions Manual

Page: 47 / 64

EngyVolt RV12 Appendix
47
11.4.3 Modbus protocol message timing
A Modbus protocol message has defined beginning and ending points. The receiving
device recognizes the start of the message, reads the “Slave Address” to determine if they
are being addressed and knowing when the message is completed they can use the Error
Check bytes and parity bits to confirm the integrity of the message. If the Error Check or
parity fails then the message is discarded.
In RTU mode, messages start with a silent interval of at least 3.5 character times. The first
byte of a message is then transmitted, the device address. Master and slave devices
monitor the network continuously, including during the ‘silent’ intervals. When the first
byte (the address byte) is received, each device checks it to find out if it is the addressed
device. If the device determines that it is the one being addressed it records the whole
message and acts accordingly, if it is not being addressed it continues monitoring for the
next message.
Following the last transmitted byte, a silent interval of at least 3.5 character times marks
the end of the message. A new message can begin after this interval.
The entire message must be transmitted as a continuous stream. If a silent interval of
more than 2.5 character times occurs before completion of the message, the receiving
device flushes the incomplete message and assumes that the next byte will be the address
byte of a new message. Similarly, if a new message begins earlier than 3.5 character times
following a previous message, the receiving device may consider it a continuation of the
previous message. This will result in an error, as the value in the final CRC field will not be
valid for the combined messages.
11.4.4 How characters are transmitted serially
When messages are transmitted on standard Modbus protocol serial networks each byte is
sent in this order (left to right):
Transmit Character = Start Bit + Data Byte + Parity Bit + 1 Stop Bit (11 bits total):
Least
Signific
ant Bit
(LSB)
Most
Signific
ant Bit
(MSB)
Start 1 2 3 4 5 6 7 8 Parity Stop
Transmit Character = Start Bit + Data Byte + 2 Stop Bits (11 bits total):
Least
Signific
ant Bit
(LSB)
Most
Signific
ant Bit
(MSB)
Start 1 2 3 4 5 6 7 8 Stop Stop
Transmit Character = Start Bit + Data Byte + 1 Stop Bit (10 bits total):
Least
Significa
nt Bit
(LSB)
Most
Significa
nt Bit
(MSB)
Start 1 2 3 4 5 6 7 8 Stop
The master is configured by the user to wait for a predetermined timeout interval. The
master will wait for this period of time before deciding that the slave is not going to
respond and that the transaction should be aborted. Care must be taken when determining