Amtech Axpert-Eazy Series Instruction Manual page 193

Bits per Byte: 1 start bit
8 data bits, least significant bit sent first
1 bit for even/odd parity; no bit for no parity
1 stop bit if parity is used; 2 bits if no parity
Error Check Field: Cyclical Redundancy Check (CRC)
2.1 RTU Framing
In RTU mode, messages start with a silent interval of at least 3.5 character times. This is most easily
implemented as a multiple of character times at the baud rate that is being used on the network
(shown as T1–T2–T3–T4 in the figure below). The first field then transmitted is the device address.
The allowable characters transmitted for all fields are hexadecimal 0–9, A–F. Networked devices
monitor the network bus continuously, including during the 'silent' intervals. When the first field (the
address field) is received, each device decodes it to find out if it is the addressed device.
Following the last transmitted character, a similar interval of at least 3.5 character times marks the
end of the message. A new message can begin after this interval. The entire message frame must be
transmitted as a continuous stream. If a silent interval of more than 1.5 character times occurs before
completion of the frame, the receiving device flushes the incomplete message and assumes that the
next byte will be the address field of a new message.
Similarly, if a new message begins earlier than 3.5 character times following a previous message, the
receiving device will consider it a continuation of the previous message. This will set an error, as the
value in the final CRC field will not be valid for the combined messages. A typical message frame is
shown below.
START ADDRESS
T1-T2-T3-T4 8 BITS
Field Note
Address Valid slave addresses are valid from 0 to 247. 0 is the general call address while 1 to
247 addresses are assigned to individual slave device.
Function Valid function codes are 1 to 255, Which also includes the exception codes. The slave
generates exception codes while it detects any error. In this the 8th bit of function
code is modified to 1 from 0.
Data Data field contains any hexadecimal number, which can have register address as well
as data. All the data addresses in Modbus are referenced to zero. The first
occurrence of data item is addressed as item number zero. For example,
1. The coil known as 'coil 1' is addresses as coil 0000 in the data address field
of Modbus message.
2. Coil 127 decimal is addressed as coil 007E hex (126 decimal).
3. Holding register 40001 is addressed as register 0000 in the data address field
of the message. The function code field already specifies a 'holding register'
operation. Therefore the '4XXXX' reference is implicit.
4. Holding register 40108 is addressed as register 006B hex (107 decimal).
CRC Check The RTU mode uses CRC Checking for error free transmission.
Every byte of the query or response is followed by at start bit and parity or stop bits at the end. With
RTU character framing, the bit sequence is:
With Parity Checking
Start 1 2
Without Parity Checking
Axpert-Eazy AC Variable Frequency Drive
FUNCTION DATA
8 BITS N x 8 BITS
3 4 5
CRC Check END
16 BITS T1-T2-T3-T4
6 7 8 Parity
10
11
Stop
11.5