Can someone please summarise the difference for me between Modbus Gateways, Modbus Servers and Protocol converters, and examples of applications for each.

 

First warning ... one vendor's "Modbus Gateway" is anothers "Modbus Converter". There are no fixed terms.

I assume you are looking at Ethernet-to-Serial devices.

You find two main classes of product:

1) What I call a 'bridge', which has no ability to CREATE requests. It waits for an external client/master to send a request and obtains an answer. Such 'bridges' can do simple Modbus-to-Rockwell conversion, but with limitations. Most Modbus/TCP to Modbus/RTU serial products will be bridges.

2) What I call a 'gateway' or 'converter', which tend to include a much more complex configuration. Most function as a 'dual-port memory' (or white-board). So one protocol must be configured to pull data and push it into the memory. Then a second protocol (or even third, fourth, etc) is used by other devices to read the MEMORY (not the source device).

Bridges tend to be cheaper than Gateways.

The easiest way to tell the difference is READ THE CONFIG MANUAL. The bridge will have timeouts, but no ability to define 'ranges' of data. The gateway will require multiple data tables to define how the source data is mapped to the memory, and how the other protocols see that data. For example, you might read 100 Modbus 16-bit words and 'serve' that data to Siemens or Rockwell as 50 32-bit floats.

 

I like to think of a Gateway as device which converts one protocol to another protocol. Because in most protocols there is a Master slave architecture, the Gateway must act as a Slave to one devices protocol and then convert the message to the new protocol and act as a master (server) to the second device which is the slave. So in essence, the Gateway acting as a protocol converter.