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Modbus TCP/IP vs. Modbus RTU:
Many people assume that Modbus TCP/IP is ‘faster’ than Modbus RTU due to the fact that it runs on
Ethernet or a TCP/IP network. However since Modbus TCP/IP follows the same rules as Modbus RTU
the device update performance is almost identical. The major advantage for using Modbus TCP/IP is
related to ‘ease of transport’. Use of Ethernet is now almost ubiquitous and allows much easier
support vs. a traditional RS-485 network. Both hardware and installation of an Ethernet network is
much cheaper and can be maintained by a facilities networking group. It also allows flexibility in data
access via intra/inter/extranets. Another advantage is that a device with embedded Modbus TCP/IP is
configured in the modbus master as a single device on the same network. As explained in the basic
operation section above a single device on a single network (serial or IP) has no ‘round robin’ time
associated with having multiple devices on the same network. Thus a dedicated Modbus TCP/IP
device gives the appearance of a ‘performance gain’ however this is derived solely from the virtue
of its ‘single device on a network’ status. A serial Modbus RTU device would have similiar update
performance if alone on a serial line.
GEMultinet or Modbus TCP/IP/RTU bridge:
With industry moving toward Modbus TCP/IP devices and Ethernet in general there is a need for
legacy support of Modbus RTU and RS-485 networks. To support these networks and devices a
Modbus TCP/IP/RTU bridge such as the GE Multilin Multinet is required. The Multinet converts the
Modbus RTU protocol to the Modbus TCP/IP protocol. Once converted to Modbus TCP/IP the messages
are then passed on via the available TCP/IP network (Ethernet, fiber, etc.).
The biggest difference between a dedicated Modbus TCP/IP device (such as the UR) and a Multinet is
the fact that the Multinet can have multiple devices daisy chained on its RS-485 port. These multi-
dropped devices follow the same rule as other multiple devices on a ‘same network’. If each device is
polled for data a ‘round robin’ occurs. The update time is then controlled by the number of devices
and how many modbus blocks are needed from each one.
The graphic below illustrates 1 meter and 3 relays. If each device was being polled for registers
requiring four separate modbus blocks than each device would take 1000ms to poll (4 x 250ms/block).
Since there are 4 devices the total update time would be 4000ms (device1+device2_device3+device4 =
4000ms).
Device update times using Multilin Enervista software:
Enervista can communicate to a wide range of GE device ranging from relays, trip/breaker units,
motor controllers and meters. Each device has a unique register map and number of registers.
Whenever a client requests data from the Enervista Modbus server the server processes the request
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