Slave Address: 8-bit value representing the address of slave that is responding.
Function Code: 8 bit value which is the function code in the query OR'ed with 80 hex,
indicating that the slave either does not recognise the query or could not carry out the action
requested.
Error Code: 8-
bit value indicating the nature of the exception detected. (See “Table Of
Exception Codes“ later).
Error Check (Lo): The bottom (least significant) eight bits of a 16-bit number representing the
error check value.
Error Check (Hi): The top (most significant) eight bits of a 16-bit number representing the error
check value.
3.2 Serial Transmission Modes
There are two MODBUS Protocol serial transmission modes, ASCII and RTU. Eastron Digital
meters do not support the ASCII mode.
In RTU (Remote Terminal Unit) mode, each 8-bit byte is used in the full binary range and is not
limited to ASCII characters as in ASCII Mode. The greater data density allows better data
throughput for the same baud rate, however each message must be transmitted in a
continuous stream. This is very unlikely to be a problem for modern communications
equipment.
Coding System: Full 8-bit binary per byte. In this document, the value of each byte will be
shown as two hexadecimal characters each in the range 0-9 or A-F.
Line Protocol: 1 start bit, followed by the 8 data bits. The 8 data bits are sent with least
significant bit first.
User Option Of Parity No Parity and 2 Stop Bits
And Stop Bits: No Parity and 1 Stop Bit
Even Parity and 1 Stop Bit
Odd Parity and 1 Stop Bit.
User Option of Baud 2400; 4800 ; 9600 ; 19200 ; 38400
The baud rate, parity and stop bits must be selected to match the master’s settings.
3.3 MODBUS Protocol Message Timing (RTU Mode)
A MODBUS Protocol message has defined beginning and ending points. The receiving
devices recognizes
the start of the message, reads the “Slave Address” to determine if they
are being addressed and knowing when the message is completed they can use the Error
Check bytes and parity bits to confirm the integrity of the message. If the Error Check or parity
fails then the message is discarded.
In RTU mode, messages starts with a silent interval of at least 3.5 character times.
The first byte of a message is then transmitted, the device address.
Master a
nd slave devices monitor the network 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
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