GE PMCS 6.15 quick start Modbus TCP/IP vs. Modbus RTU, GEMultinet or Modbus TCP/IP/RTU bridge

FAQ

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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