Optimizing Serial Port Networks

Let's first look at a scenario where you have an application that needs to gather data from four PLCs. The PLCs in this case only support RS-232. Using your standard PC, you would at most be able to access two of these PLCs.

There are a couple of solutions to this problem, first you could add more serial ports by using an off the shelf PCI card.  This solution can be time consuming and sometimes difficult.  Depending on the PC, there is typically only one spare interrupt available.  To address the issues of adding a PCI card, companies like Edgeport offer a solution using USB-to-Serial converters.  The converters provide a simple plug-and-play solution.  With only one USB port you can expand your serial ports by 1, 2, 3, 4, and 8 depending on your needs. 

The benefits of USB-to-Serial converters can also be realized in applications where you have a serial network of RS-485 devices. In the application above, the PLC supports only RS-232 communications. In order to communicate with the four PLCs required by the application, a multi-port solution is a necessity. If the PLC supported multi-dropping via RS-485, a single serial port would have been all that was needed. What isn't directly apparent in the above application is the four fold increase in acquisition speed that comes with using multiple serial ports.

If we look at a typical RS-485 application in most cases you would connect all of the devices to a single PC serial port.

In this case, the single serial port can be used to gather all of the data required by the application. Though, by doing so, the aquisiton must be done in round robin fashion where the acquisition cycle time is the total of all time required to gather information from each PLC in succession.

In KEPServerEX, each serial port you configure in the server is essentially its own thread of execution. If we apply a multi-port USB-to-Serial converter to this same application we will see an almost four fold improvement of the acquisition cycle time for the application. By using four serial ports, KEPServerEX can dedicate a single thread of execution to the task of gathering data from only a single device instead of round robin from four devices.

If your application requires the best performance available, try using a multi-port USB-to-Serial Converter to fine tune your application.  Check out Kepware's available Hardware for other serial communications converters.

The Edgeport 1, 1i, and 4s USB-to-Serial converters from Edgeport have been tested with Kepware's KepserverEx communications products. If you are looking to buy a multi-port converter for your application, we can give you direct support on Edgeport products. There is no additional cost in KEPServerEX to use additional serial ports. Up to 100 ports are supported by KEPServerEX.

While most applications can benefit from the methods discussed here, each application is unique and may not see the full performance gains described in this document.