Owners manual

Modbus Protocol User Guide 8
2: Configuring Modbus
Network Protocols
The IAP Device Server uses TCP/IP protocols for network communication. The
supported standards are ARP, UDP, TCP, ICMP, Telnet, TFTP, DHCP, and SNMP.
For transparent connections, TCP/IP (binary stream) or Telnet protocols are used.
Firmware upgrades can be made with the TFTP protocol.
The IP protocol defines addressing, routing, and data block handling over the
network. The TCP (transmission control protocol) assures that no data is lost or
duplicated, and that everything sent into the connection on one side arrives at the
target exactly as it was sent.
For typical datagram applications where devices interact with others without
maintaining a point-to-point connection, UDP datagram is used.
Packing Algorithm
Traditional Modbus/RTU requires a “character timeout” to signal the end of a
Modbus/RTU packet. This stretches out the overall response cycle. Fortunately, the
IAP Device Server uses an intelligent length-predictive algorithm to detect the end of
standard Modbus messages. This allows better performance, and the IAP Device
Server falls back to using a user definable “character time-out” to manage non-
standard or user-defined Modbus functions.
IP Address
Every device connected to the TCP/IP network including the IAP Device Server must
have a unique IP address. When multiple Modbus devices share a single IP, then
Modbus/TCP includes an additional address called the Unit ID. See the User Guide
for your specific IAP Device Server for a complete description of IP Addressing.
When the IAP Device Server is receiving Modbus/TCP messages from remote
masters, the Unit ID is converted to use in the Modbus/RTU message as the slave
address.
When the IAP Device Server is receiving Modbus/RTU messages from local serial
masters, a user-defined lookup table is used to match the 8-bit Modbus slave
address to a remote IP address. The Modbus slave address received is used as the
Unit ID.