Serial Communications Protocol Specifications - Swissvacuum.com

� Read Function: function code 3 (decimal)
� Write Function: function code 16 (decimal)
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Master can use these functions in order to take actions on the slave (chamber controller).
READ FUNCTION (FUNCTION CODE: 3)
The Modbus read operation (function code 3) can be described as follows:
FUNCTION
QUERY LENGTH
ANSWER LENGTH
CODE
( without error check field) ( without error check field)
3 (decimal) 6 bytes 3 bytes + content of the 3 rd
WRITE operation (FUNtION CODE: 16)
The write operation (function code 16) is described as following
FUNCTION
QUERY LENGTH.
CODE
( without error check field )
16 (decimal) 7 + the content of the 7 th
byte
In order to get the total data length, add 2 bytes for CRC.
byte of the answer
LUNGHEZZA RISPOSTA
( without error check field)
6 bytes
Each function is organized in fields with specific meaning. In the following section a short description of the fields is
given, and a detailed explanation of the read and write functions follows.
FIELDS DESCRIPTION
ADDRESS FIELD
This is the first field of the structure and consists of 1 byte. This byte identifies the address of the chamber
controller (slave) to work with.
Each slave must have a unique address and only the addressed slave will respond to a query that contains its
address.
The chamber controller address is always 17 (decimal)
FUNCTION FIELD
Tells the addressed slave what function to perform. The high order bit (High Order HO in the following) in this
field is set by the slave device to indicate that other than a normal response is being transmitted to the Master
device (see the discussion regarding the exceptions in the answer). The bit remains 0 if the message is a query
or a normal response message.
DATA FIELD
Contains information needed by the slave to perform the specific function or it contains data collected by the
slave in response to a query.
ERROR CHECK FIELD
The error check field uses a CRC calculation, which described later in the examples.
Serial communications protocol vers. 2.02 Angelantoni Industrie Spa Massa Martana (PG) Italy