7bit Modbus Proxy

Overview

7Bit ModBus Proxy is a caching gateway from ModBus TCP protocol to Modbus RTU. Unlike typical gateways that only retransmit packets from one network to another and back, ModBus Proxy operates as a stand-alone data server that periodically reads necessary registers from the devices on the RS-485 bus and exposes them on ModBus TCP. Thus, a conventional gateway only provides a connection of devices with a serial interface to the control system using Ethernet, while ModBus Proxy allows to speed up the process of data acquisition in systems with a large number of registers and devices using a low-speed fieldbus connection.

Why this is necessary?

Most data acquisition and control systems are built according to the following principle: first step – the data (registers) are read, then the necessary calculations are made, then follows control actions (writing values to registers) and the cycle repeats again. Thus, until the system get all needed data, there will be no any control actions. One single duty cycle is called “scan”, and the time it takes to complete these 3 steps is called “scan time”. If the scan time will be great, all system will slow down.

How long does it take to read data via RS-485?

In real projects, with a large number of devices and stretched networks, it is rather difficult to achieve stable communication at baud rates above 9600 kbps (1200 bytes per second). Modbus is a simple but cumbersome protocol that allows reading only individual registers or groups of consecutive registers. Reading of a single register looks as this: 8 bytes data request, remote device processing time (several ms), 7 bytes data response, pause 3 bytes. So, at best, it takes all about 10ms, or approximately 100 registers per second, for one-by-one reading,  provided there are no errors. If there are 2500 registers in 25 different devices in your project with a polling rate of once per 30 seconds, then, in the best case, the scan time will be near 25 seconds, but in reality, it may take even a minute! It doesn’t look as real-time at all! The speed of data transfer over Ethernet with Modbus TCP can be about 100 times faster!

How does it work?

ModBus Proxy works totally transparently. After receiving the data request on the TCP port, it starts reading the needed register from devices via RS-485 to internal memory (cache) and transmitting them in response. After that, the gateway continues reading and updating these registers in memory for some time, responding to requests with data from the cache. Thus, the data access time via Modbus TCP does not depend on “scan time” from the RS-485 side.

More advantages

• The built-in Wi-Fi interface allows you to connect to the data collection system in the device where there is no way to lay communications.
• Since, from a functional point of view, the gateway must be located somewhere “in the field”, for ease of integration, it is powered by the same cable as the Ethernet network (universal power supply with a voltage of 9-48 V or PoE)
• The standard compact form factor of the distribution circuit breaker is attached directly to the DIN profile in the cabinet.
• Convenient configuration of the module using the built-in web page
• The module has all the necessary timing settings for reliable polling of devices – timeouts, stabilization pauses, provides diagnostic data in its own registers