![]() TOP Server effectively supports all Modbus addresses because Modbus uses a type of addressing referred to as offset addressing. The TOP Server Modbus Suite does, in fact, support 4001, 40001 and even 400001. So how can TOP Server claim to have such wide support for Modbus devices given this data? Modbus Holding Register Addressing With holding registers, for example, in a device like this, an address of 4001 comes nowhere close to 400001 as a starting address. The listed address ranges supported by TOP Server are as follows:įor many devices, the supported address ranges are much smaller than this. One of the most common questions asked when considering if the TOP Server Modbus Suite is compatible with a device is whether a particular address range is supported by TOP Server. That being said, with the birth of the Modbus protocol occurring when memory sizes were still relatively modest, the total amount of memory on a device in the late 70's was small compared to the large quantities of memory available today. Offset Addressing: Is There a Difference between 4001?Īs I've already mentioned, the Modbus protocol has been around for decades and is arguably the most widely used protocol for device communications in the world. You may also see registers referred to as analogs or coils referred to as discrete or digital addresses due to the type of data they represent. Since some device documentation does not provide a detailed listing of Modbus addresses containing references to the type of address such as 4xxxxx or Holding Register, it is useful to think of Modbus addresses in terms of the type of data a particular address will contain and whether or not it can be read from and written to or only read from. Output Coils – 1-bit (Boolean) addresses with read/write access (0xxxxx).Input Coils – 1-bit (Boolean) addresses with read only access (1xxxxx).Internal Registers – 16-bit addresses with read only access (3xxxxx).Holding Registers – 16-bit addresses with read/write access (4xxxxx).When we discuss the Modbus protocol and how to address particular locations of data, there are generally four different types of Modbus addresses: So the intention of this post is to provide a somewhat standard explanation for how Modbus addressing works to make it easier to understand how to access the data in your Modbus devices using TOP Server. Given the large number of Modbus protocol implementations in the world by almost as many different device manufacturers, device documentation can sometimes be unclear about what numerical address should be requested for a particular piece of data. ![]() In this blog post, I will step through the basics of Modbus addressing including how offsets come into play, and how they affect what addresses to use in TOP Server for AVEVA applications. And with those different Modbus interpretations, much confusion about the different terminology for accessing data. Given how long Modbus has been around and in use as an "open" protocol (starting in 1979), there have been many different interpretations of Modbus throughout the years. ![]() Which is both its blessing and its curse, to some extent. It's one of the most widely used protocols (if not the most widely used) in the world. I'm sure you've all at least had some interaction with Modbus, working in the automation industry.
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