9.15 Modbus Slave
This is the ‘client’ of the master unit. Each slave is given a unique identifier in the range 1 to 247. When
in this mode, the slave listens on the appropriate port until a query from a master unit is heard. When the query is
received, the slave address is decoded and if it is not the same as the instruments, the packet is ignored. In the
standard protocol, an address of 0 indicates that all slaves should reply. This is not yet implemented in the
InSpec.
9.16 Modbus Master
The Modbus master initiates the reading and writing of information from all the slave units attached to
the communication link. On Modbus serial there is only one master per communication link and the master
cannot be changed during operation. In ModbusTCP however a device can be a slave and master
simultaneously and no master is in overall control of the link.
9.16.1 General
The master initiates the queries from a list of queries which specify what is required to be read
or written. The master works its way through the list of queries, sending each one in turn. If a query is not
responded to in time or not correctly, then the query will be retried a configurable number of times before
the next query is processed.
9.16.2 Queries Blocks
The instrument has 10 Modbus query blocks these can be used by 10 ModbusTCP masters to
simultaneously connect to 10 IP address to poll for data. However, block 2 and 3 are also used by serial
ports 2 and 3 are not available if Modbus master is used by these serial ports.
Modbus master queries are created and edited in the menu. A maximum of 20 queries can be
enabled for each query block, and these can be any combination of reading and writing of Modbus
addresses.
To configure a query, you must select the slave’s node number that is to be accessed, whether
you wish to read or write to that slave and what the associated Modbus address is within the slave.
Different nodes can be selected within each block if required.
You must then select which register in the InSpec the data is read from or written to.
Finally, you must select the number of consecutive InSpec registers that are to be used for this
query. If the number of registers is more than one, then consecutive Modbus registers are read or written
so handling the extra InSpec registers. For ModbusTCP master you will also need to provide the
associated block URL or IP address as 4 bytes separated by dots (e.g., 0.0.0.0).
9.16.3 Modbus poll delay (serial only)
The Modbus poll delay is the delay between receiving a reply to a message and sending a new
query. Some systems especially RS485 based ones will require a delay small delay so that that systems
can tell the difference between the end of one packet and the start of the next.
9.16.4 Modbus retries
This is the number of times a Modbus master will try and resend a request to slave if there is an
error in the reply. A value of zero means if any error is encountered the master will just move onto the
next query.
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