Modbus Plus (Available On The Tpu2000R Only); Modbus Plus Theory Of Operation - ABB TPU2000 Technical Manual

If delay between transmissions is < 3.5 Character Times, the message is received.
If delay < 3.5 character times, receiving device appends characters to last message.
If delay is sensed > 1.5 message times, receiving device flushes the buffer. Next character is new
message.
The Modbus RTU emulation senses timeouts quicker than the Modbus ASCII emulation. The Modbus RTU
emulation also uses a CRC –16 checksum in contrast to the Modbus ASCII using a LRC (Longitudinal
Redundancy Check). The CRC –16 is a much more robust checksum. With parity, internal protocol message
length field checks and the CRC-16, the error detection is exceptional.
IMPLEMENTATION TIP- When commissioning a Modbus system, it is always advisable to connect a
communication analyzer in-line with the host. It is always uncertain whether the host is sending the command
correctly. Within the TPU2000 and TPU2000R, an incorrect address request will always generate an exception
response from the relay. If an exception response is generated, many host devices will not display the Modbus
exception response generated by the unit. A communication analyzer allows for rapid troubleshooting of a
malfunctioning network connection.

Modbus Plus (Available on the TPU2000R Only)

Modbus capabilities were expanded in a significant way during the late 1980's. The base command set was not
changed from the Modbus protocol, however, the protocol access method was modified. The limitations of
Modbus exposed themselves in a few areas:
The throughput was dependent upon the physical interface (RS232 and 485) which limited the speed of
data transfer.
The Modbus protocol did not efficiently manage its bandwidth. Exorbitant amounts of time could be
spent waiting for the slave device to respond with data or timeout.
The Modbus protocol only allowed connection of a single host (or multiple hosts with the addition of
hardware multiplexers) to up to 247 IEDs.
The originator of the protocol Modicon AEG, had devised a way to use the Modbus protocol and present it to the
attached nodes to eliminate the deficiencies found in large Modbus installations.
Modbus Plus was developed using a proprietary physical interface allowing communication over a twisted
shielded pair medium. The baud rate of the network was fixed at 1 megabaud. If this had been the only change
from Modbus to Modbus Plus, the network's introduction would not have been significant. The Modbus
repackaging into a Modbus Plus format afforded the following significant benefits:
Up to 34 simultaneous conversations may occur on a network.
Each device on a Modbus network is capable of being a host.
Each device may broadcast a data, which is received by all other nodes on the network.
Node to node network throughput time may be deterministically calculated.
The Modbus Plus interface was afforded though the manufacturer entering into a "MODCONNECT
AGREEMENT" allowing sharing of technology between the IED implementers and Modicon AEG. The IED
implementers received Modbus Plus chipsets and technology allowing network implementation.
implementation was completed, a certification process ensued and upon the IED's successful test of the
implementation, certification was bestowed upon the IED.

Modbus Plus Theory of Operation

Modbus Plus is a token passing network based upon an HDLC like protocol implementation. The frame structure
of the protocol is illustrated in Figure 5-3. As illustrated, the Modbus command structure is imbedded in the
Modbus Plus structure. Thus, all Modbus commands are used for Modbus Plus. The manufacturer of the
protocol supplies drivers allowing DOS, Windows [3.1, 95 or 98] to communicate with the Modbus Plus hardware.
The implicit understanding of Modbus Plus protocol frames is not needed by the operator. This discussion is
meant to inform the reader of the commonality between Modbus and Modbus Plus.
TPU2000/2000R Modbus/Modbus Plus Automation Guide
Once the
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