Modbus I/O

Modbus master

FIXME This page not yet written, it just contains the relevant sections from demo/basic_modbus/matplc.conf for now.

[PLC]
# Uncomment the modbus protocol version you wish to run...
module modbus_m "../../io/modbus/modbus_m_rtu"
#module modbus_m "../../io/modbus/modbus_m_asc"
#module modbus_m "../../io/modbus/modbus_m_tcp"


# Configure the modbus module, that will copy the
#  matplc points over the modbus network.


[modbus_m]
# Network configuration table
#
# network <network_name> <protocol> <parameters>
#
#  <network_name>   : the name of the network. Used later in the node address table.
#  <protocol>       : {rtu | ascii | tcp}
#
#      For rtu and ascii networks...
#  <parameters>     : [device <filename>]
#                     [baudrate <x>]
#                     [parity {even|odd|none}]
#                     [data_bits <x>]
#                     [stop_bits <x>]
#                     [ignore_echo {true|false}]
#                     [timeout <x>]
#                     [send_retries <x>]
#
#      For tcp networks...
#  <parameters>     : [host <x>]
#                     [port <x>]
#                     [timeout <x>]
#                     [send_retries <x>]
#                     [TCP_close {true | false}]
#  host <x>         : the IP address or DNS name of the server
#  port <x>         : x is either the port number on which the server is listening,
#                     or the service name, which will be mapped onto the port number
#                     according the configuration in the /etc/services file
#  TCP_close {true | false}
#                   : The openmodbus specification _sugests_ that all TCP connections
#                     should be closed if they are going to stay idle for longer than 1
#                     second. Setting this option to true will close all open TCP
#                     connections at the end of the scan. Note that connections will
#                     be re-established automatically for the next scan.
#                     Default is true, i.e. to close the connections at the end of the
#                     scan. This is due to the fact that some slaves may support a
#                     limited number of simultaneous TCP connections (i.e. limited number
#                     of simultaneous masters), and if we do not close the connections, we
#                     may be blocking those masters out of the slave!
#                     NOTE: true is the safest option, but with slightly longer delays.


# Sample network configurations...
network rtu_net         rtu   device /dev/ttyS1 baudrate 9600 parity none data_bits 8 stop_bits 1 ignore_echo false timeout 1.5 send_retries 2
network asc_net         ascii device /dev/ttyS1 baudrate 9600 parity none data_bits 8 stop_bits 1 ignore_echo false timeout 1.5 send_retries 2
network tcp_net         tcp   host   localhost  port 502  timeout .1 send_retries 1 TCP_close true


# node address table
#
# node <slave_name> <network_name> <slave_addr>
#
#  <slave_name>   : the name of the slave node
#  <network_name> : the name of the network
#  <slave_addr>   : 0..255
#


# Sample slave configurations...
node speed_drive    tcp_net 34
node digital_IO     rtu_net 0
node analog_IO      asc_net 23


#  I/O accessible over modbus network
#
# map [inv | invert] {in | out} <io_addr> <matplc point>
#
#  inv    -> invert the value being read from/writen to physical IO
#  invert -> invert the value being read from/writen to physical IO
#
#  in  -> copy from the input process image to the matplc point
#  out -> copy the state of the matplc point to the outout process image
#
#
#  <io_addr>  : modbus address
#               <slave>.<reg_type>.<reg_addr>
#
#  <slave>      : the name of the slave node, as it appears in the node address table.
#  <reg_type>   : in_bit | out_bit | in_word | out_word
#  <reg_addr>   : 1..10000

Note that, as per the standard, <reg_addr> is 1-based. If your documentation lists Modbus addresses starting with 0, you will have to add 1 to each of them.


# Sample IO table
map out digital_IO.out_bit.1 L1
map out digital_IO.out_bit.2 L2
map out digital_IO.out_bit.3 L3
map out digital_IO.out_bit.4 L4

Modbus slave

The modbus slave module is not yet implemented.

Useful links

Modbusfw - Modbus/TCP Filtering on Linux Firewalls: A set of kernel patches for Linux Netfilter to allow firewall policy decisions (DROP, DENY, ALLOW, etc.) to be made based on Modbus/TCP header values including modbus function code, providing better access control than simply blocking port 502. However, as of Sep 2003 this is an initial prototype release and they've done only limited testing so any feedback, new feature requests, and/or help with development is appreciated. See http://modbusfw.sourceforge.net for details.

$Date: 2003/09/20 08:14:00 $