| Citect supports a specific implementation
of this standard. This implementation, using the Citect MODNET
driver, supports the Modicon Quantum Ethernet TCP/IP module. This
KB article outlines the differences between the Modicon
specification (last revision date 3rd of Sept., 1997) and the
MODNET driver operation. This article does not cover the issues of
the 'conformance classes' that are documented in the specification,
as the conformance classes are directly related to the data types
supported by the MODNET driver.
Half-duplex Vs Full-Duplex
The specification states that "Requests are sent in half-duplex
fashion (response MUST be received before the next request may be
sent on a socket)". The MODNET driver operates in a Full-Duplex
fashion, effectively having multiple outstanding requests. This
feature is supported by the Modicon Quantum Ethernet TCP/IP module,
although this feature can be partially disabled in the MODNET
driver by setting [MODNET]MaxOutstanding=1.
Invoke Identifier (also known as simply
"Identifier")
The specification also indicates that the
identifier field is "usually 0". The MODNET driver uses a cycling
module 65536 value.
CRC
The specification does not explicitly highlight whether the Modbus
RTU CRC field is used in the protocol. From experience, we have
found that the CRC field is not used by the Modicon Quantum
Ethernet TCP/IP module, but rather relies on the TCP/IP Checksum.
As a result, the MODNET driver does not use a CRC field but also
relies on the TCP/IP Checksum.
Destination ID (also known as 'Unit Identifier' or 'Slave
address')
The specification indicates that the Destination ID may be used to
take advantage of devices such as gateways and bridges, so that on
IP address can support multiple end units. As the MODNET driver was
designed specifically for the Modicon Quantum Ethernet TCP/IP
module, this Destination ID is permanently hard-coded to 0. Future
revisions of the MODNET driver may allow for the Destination ID to
be user configurable in Citect.
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