Serial Communications Protocol Specifications - Swissvacuum.com

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as part of the current data packet. If not, the character will be considered as the first character of a new data
packet.
For example, at 9600 bps the time between the transmission of a character and the next one is approximately
800 microsec. If the next character will be received after more than 800 * 3.5 microseconds,
the character will be the first of a new data packet.
The following picture shows how a received character belongs to the current data packet (frame) or begins a
new frame according to the time elapsed since the previous character was detected.
In this picture the horizontal axis is the time axis and each rectangle identifies a character. A packet consists
of all characters with intercharacter time less than time specified by Modbus (T = 800 * 3.5 microseconds at
9600 bps)
More than
T=2800 microsec.
Fig. 5 Frame identification
Note that this method requires a very precise timing to get data from serial line. Considering that Windows is not a real-time
system (and so it cannot garantee the timing required) it is preferable not to use it under this operating system.
LENGHT SYNCHRONIZATION
Each modbus function has a well defined length, both for queries and answers. Knowing these lengths, it is possible
to identify the end of a packet while receiving.
This way of operation is probably the simplest to use, both for development of a commincation software and for the
debug operation.
In effect you always know how many character you are waiting for according to the query you sent to the slave device.
This is also true when an error occours during the communication: the lenght of an answer with an exception is
however known.
MODBUS FUNCTIONS
Previous frame
Data packet
Time between characters is less
than 800*3.5 microseconds: the
characters belongs to the current
New frame
More than
T=2800 microsec.
All the operations with the chamber controller can be done with only two of the Modbus functions:
Serial communications protocol vers. 2.02 Angelantoni Industrie Spa Massa Martana (PG) Italy