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MODBUS
RS485/RS232 Communications Interface
Parameter Address
Data bits or data words exchange information between Master and Slave devices. This data
consists of parameters. All parameters communicated between Master and Slaves have a 16-bit
parameter address.
The MODBUS parameter address range is 0001 to FFFF.
RTU Mode of Transmission
The MODBUS RTU definition of the mode of transmission for a single character is:
A start bit, eight data bits, a parity bit, one or two stop bits
All Parker SSD Drives’ units use one stop bit.
Parity may be configured to be NONE, ODD or EVEN (if NONE, no parity bit is transmitted)
The RTU mode of transmission for a single character is represented as follows:
Start
d7 d6 d5 d4 d3 d2 d1 d0 Parity
Stop
Message Frame Format
A message frame format consists of a number of correctly sequenced characters, as shown
below.
Frame Start
Device Address
Function Code
Data
CRC
EOT
3 bytes
1 byte
1 byte
n bytes
2 bytes
3 bytes
Frame Start
The frame start is a period of inactivity at least 3.5 times the single character transmission time.
For example, at 9600 baud a character with a 1 start, 1 stop and 8 data bits will require 3.5ms
frame start. This period is the implied EOT of a previous transmission.
Device Address
The device address is a single byte (8-bits), unique to each device on the network.
Function Code
Function codes are a single byte instruction to the Slave describing the action to perform.
Data
The Data segment of a message will depend on the function code and the number of bytes will
vary accordingly. Typically, the data segment will contain a parameter address and the number
of parameters to read or write.
CRC
The CRC (Cyclic Redundancy Check) is an error code and is 2 bytes (16-bits) long.
EOT
The EOT (End Of Transmission) segment is a period of inactivity 3.5 times the single character
transmission time. The EOT segment at the end of a message indicates to the listening device
that the next transmission will be a new message and therefore a device address character.
Cyclic Redundancy Check
This is an error check code and is 2 bytes (16-bits) long. After constructing a message (data only
- no start, stop or parity bits), the transmitting device calculates a CRC code and appends this to
the end of the message. The receiving device also calculates a CRC code from the received
message. If this CRC code is not the same as the transmitted CRC there has been a
communication error. Units do not reply if they detect a CRC error in messages sent to them.
Summary of Contents for 6513
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