ICS ELECTRONICS 9099, Instruction Manual

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3.2.5 Operation with Modbus TCP/IP Protocol
The 9099 appears as a Modbus TCP/IP server (slave) to the Modbus TCP/IP
client (master) and provides a transparent Modbus TCP/IP to Modbus RTU
conversion.
The 9099 will recognize the Modbus TCP/IP packet and extract
parts of its Application Data Unit (ADU) and add a CRC to create the Modbus
RTU packet
The packet is then transmitted serially to the Modbus device. If
the Modbus device returned a response, the information in the response packet
is passed back to the Modbus TCP/IP master. The 9099 does not restrict the
Modbus TCP/IP functions but they are limited by the capabilities of the slave
Modbus RTU device(s) connected to the 9099. The maximum message size
is limited to 255 bytes.
The Modbus TCP/IP operation starts by opening a socket to port 502 on the
9099. The controller has approximately 10 seconds to start communicating
with the 9099. When the 9099 receives a Modbus TCP/IP packet its length
parameter is checked. If a packet is not received within 10 seconds or if the
message length is exceeded, the 9099 closes the socket.
In some cases, Modbus TCP/IP programs have been written for a slave ad-
dress that the slave device cannot be set to. If Substitute Slave Address on the
9099's Configuration page is enabled, the slave address in the TCP/IP packet is
replaced with the 9099's current slave device address when creating the RTU
packet. The 9099's slave device address can be set with a web browser or with
the Modbus “C " command.
Received Modbus RTU responses are checked for length and for a valid CRC.
If the received response is a valid RTU response, a Modbus TCP response
packet is created and the device address is restored if it had been changed. The
packet is then transmitted to the Modbus TCP/IP master. Missing or invalid
responses from the slave device will cause the 9099 to close the socket and set
the appropriate bits in the Modbus and ESR registers.
Packets from multiple Modbus TCP/IP masters (clients) are queued and held
until the current operation is completed. If a controller closes its socket while
its packet is in the queue, it will not be processed. If the socket is closed while
the packet is being processed, any slave response will be discarded.
The Modbus TCP/IP control method has its own Status Reporting Structure
that can be controlled and queried with the SCPI Modbus Status commands
from any other control method. Modbus TCP/IP does not have a way to handle
Service Requests so the user has to query the Status and ESR Registers to
check for errors.