User Manual Chapter 8 - Modbus RTU Protocol 125
Chapter 8 - Modbus RTU Protocol for DVS drives
8.1 Introduction
In the chapter the Drive parameters are referred to as 16-bit Modbus registers; a 32-bit Drive parameter covers therefore
two Modbus registers.
See chapter 7 for the following correspondences: parameter index and Modbus register.
8.2 The MODBUS Protocol
The MODBUS protocol defines the format and the communication modes between a system controlling master and one
or more slaves aimed at answering to the master requests. The protocol states how the master and the slaves start and
stop their communication, how the messages can be exchanged and how the errors can be detected. A common line can
host one master and 247 slaves; this is a protocol logic limit, the device number can be further limited by the physical
interface; the present implementation foresees a maximum number of 64 slaves to be line-connected.
A transaction can be started exclusively by the master. A transaction can have a direct demand/response format or a
broadcast format. The former is addressed to a single slave, the latter to all the line slaves, which, on their turn, give no
response. A transaction can have a single demand/single response frame or a single broadcast message/no response
frame.
Some protocol features have not been defined. They are: interface standard, baud rate, parity, stop bit number. The
protocol allows also to choose between two communication modes: ASCII and RTU (Remote Terminal Unit). The RTU
mode, which is the most efficient, is implemented in the Drives.
The JBUS protocol is similar to the MODBUS protocol; the only difference is given by the address numbering
system: in MODBUS the numbering system starts from zero (0000 = 1
st
address) while in JBUS it starts from one
(0001 = 1
st
address); this variance is maintained throughout the whole system. The following descriptions, if not
otherwise stated, refer to both protocols.
8.3 Message format
In order to communicate between the two devices, the message has to be contained into a casing. The casing leaves the
transmitter via a port and it is brought along the line to a similar port on the receiver. MODBUS states the format of the
casing, which, both for the master and for the slave, contains:
The slave address for the master stated transaction (the address 0 corresponds to a broadcast message sent to all
the slaves).
The code of the function (already performed or to be performed).
The data to be exchanged.
The error control according to the CRC16 algorithm.
If a slave detects an error in the received message (a format, parity or CRC16 error), the message is invalid and therefore
rejected; when a slave detects an error in the message, it does not perform the required action and does not answer to the
demand as if the address does not correspond to an on-line slave.
8.3.1 The address
As stated above, the MODBUS transactions always involve the master (which controls the line) and one slave at the time
(with the exception of broadcast messages). In order to detect the message receiver, the first sent character is a byte
containing the numeric address of the selected slave. Each slave owns therefore a different address number for its
identification. The legal addresses go from 1 to 247, while a master message starting with the address 0 means that this
is a broadcast message simultaneously addressed to all the slaves (the address 0 can not be allocated to a slave).
Broadcast messages are those messages which do not need a response to perform their function, i.e. the allocations.
8.3.2 The function code
The second character of the message states the function to be performed by the master message; the slave response
contains the same code, thus stating that the function has been performed.
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