Endress+Hauser EngyVolt RV15, Руководство по эксплуатации

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EngyVolt RV15 Appendix
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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
Significa
nt Bit
(LSB)
Most
Significa
nt 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
Significa
nt Bit
(LSB)
Most
Significa
nt 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
Significan
t Bit (LSB)
Most
Significan
t 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 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.