Section 8. Operation
8.10.3.2.6 Timing
The timeout is a critical parameter of Modbus communication. The response time
of devices is usually not specified by the manufacturer and can range from 100 ms
to more than 5 seconds. When the CR800 is acting as a slave device, it typically
responds very quickly. The default timeout in a master device polling the CR800
will typically not need adjustment from the default. When the CR800 is acting as
a master, the response time of a slave needs particular attention. The best practice
is to monitor the communication between the CR800 and the slave device with the
comms sniffer (terminal mode
(p. 483) W command). The comms sniffer allows you
to see the actual response time of the slave device. The TimeOut parameter of
ModbusMaster() can then be adjusted accordingly.
8.10.3.3 Troubleshooting (Modbus)
Test Modbus functions on the CR800 with third party Modbus software. Further
information is available at the following links:
• www.simplyModbus.ca/FAQ.htm
• www.Modbus.org/tech.php
• www.lammertbies.nl/comm/info/modbus.html
8.10.3.4 Modbus over IP
When the CR800 acts as the Modbus master, a TCPOpen() instruction must
precede the ModbusMaster() instruction. The connection handle returned by
TCPOpen() is used for the ComPort parameter.
In the case of ModbusSlave(), no TCPOpen() instruction is needed. Simply use
502 for the ComPort parameter.
8.10.3.5 Modbus Security
Q: What security options does the CR800 offer for Modbus?
A: The Modbus protocol itself does not include security features, so the CR800
does not offer security on ModbusMaster() or ModbusSlave(). Following are
security issues that come up:
• MAC and IP filtering
• Function code filtering
• Privilege mapping rules by client (by port, IP, etc)
• VPN tunneling
There are some third party Modbus security devices available that can be placed
between the CR800 and the rest of the Modbus network. For example, see
tofinosecurity.com/products.
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