I have been asked this question from time to time. Since both Serial Device Servers and Modbus TCP Gateway are able convert the serial data to Ethernet TCP/IP, can the Serial Device Server be used on a Modbus application?
It has been over a decade since the Serial to Ethernet Devices Server and the Modbus TCP Gateway were introduced to the industrial world. During these years, these products were getting more mature, reliable and cost effective. It became one of the most popular devices in the industrial networking. The price of serial device servers are from $75 to $150 (single serial port) and the Modbus serial (RTU/ASCII) to Ethernet TCP gateways are from $200 to $500. When it comes the Modbus application, does the Modbus TCP Gateway have to be used? Or Can the serial device server be used in the Modbus applications?
Serial Device Servers are very simple units. It does nothing but convert RS-232, RS-422 or RS-485 to the Ethernet. The most common way to use this unit is via the virtual serial com port on a computer in a point to point communication. The device servers packetize the serial data and send it over the Ethernet, and the virtual serial com software unpacketize the Ethernet packet and reveal the serial data. Since the device server doesn’t perform any protocol conversion, it spits out whatever it has.
Modbus applications are usually more complicated. In most cases, there are multiple Modbus ASCII/RTU devices perform within a serial network, and each one of them has a unit ID number to identify itself. Moreover, it perform different “roles” such as master or slave in the applications. A “master” is the controller which may send commands or requests to the slaves which are the end devices. Vice versa, the slave may receive the commands or requests from multiple masters. In these types of applications, device servers are unable properly taking a modbus frame and input it into its own IP packet. Sometimes Serial Device Server encapsulate multiple Modbus frames or parts of Modbus frames in a single IP packet. This will make the communication inefficient and can cause problems. This where the timing problems can occur. In this case, Modbus TCP/IP Gateway is able to offer multi-master capabilities, which requires buffering Modbus requests until responses are received from the slaves.
To make the long story short. The serial device server can be used in the Modbus applications ONLY if is a simple point to point communication. For other applications such as multiple masters/slaves, the Modbus TCP Gateway is still the best choice.