Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 114)

Publication AG-UM008C-EN-P - February 2005
3-46 Configuring MicroLogix 1100/1200/1500 Controllers
Modbus RTU Protocol This section provides the configuration parameters for Modbus RTU
(Remote Terminal Unit transmission mode) protocol. For more
information about the Modbus RTU protocol, see the Modbus Protocol
Specification (available from http://www.modbus.org).
The driver can be configured as Modbus RTU Master or Modbus RTU
Slave. The Modbus RTU Slave driver maps the four Modbus data
types; coils, contacts, input registers, and holding registers, into four
binary and/or integer data table files created by the user.
Modbus RTU Master
Message instructions are used to transfer information between the data
files in the Modbus RTU Master and the Modbus RTU Slaves.
Modbus addressing is limited to 16 bits per memory group, each with
a range of 1 to 65,536. There are four memory groups, one for each
function:
•
coils (generally addressed as 0xxxx)
•
contacts (1xxxx)
•
input registers (3xxxx)
•
holding registers (4xxxx)
Coils and contacts are addressed at the bit level. Coils are like outputs
and can be read and written to. Contacts are like inputs and are
read-only. Input registers and holding registers are addressed at the
word level. Input registers are generally used for internally storing
input values. They are read-only. Holding registers are general
purpose and can be both read and written to.
The most significant digit of the address is considered a prefix, and
does not get entered into the Modbus Data Address field when
configuring the message instruction.
When the message is sent, the address is decremented by 1 and
converted into a 4-character hex number to be transmitted via the
network (with a range of 0-FFFFh); the slave increments the address
TIP
Modbus RTU Master driver can be used with the
following controllers:
•
All MicroLogix 1100
•
MicroLogix 1200, FRN 8 and higher
•
MicroLogix 1500, FRN 9 and higher