USB to RS485 converters are often used to enable personal computers to connect with RS485-compliant devices on industrial automation networks, such as factory floor automation, video surveillance cameras and door security card readers.
Although the RS485 specification recommends a default topology based on 'master-slave' point-to-point or multi-drop configurations, the use of multiport repeaters allows construction of very large RS485 star topologies. It's into these kinds of networks that a variety of USB to RS485 converters can be deployed to assist with monitoring and control tasks.
The simplest converter allows a single USB interface to connect with a single RS485 interface. Although basic in functionality, these converters support Windows, Mac and Linux device drivers. Plug and play installation means that there are usually few device or resource conflicts. (However, note that some low-end converters do not support Windows 7 or Vista correctly.)
Multiport USB to RS485 converters have two, four or more switch-selectable RS485 / RS422 interfaces and support point-to-point and point-to-multipoint configurations. No external power supply is required; the converter is fully plug-and-play with automatic send data control and auto speed detection. Some multiport converters also come with a choice of RS232, RS422 or (two-wire, half duplex) RS485 interfaces.
Many of these converters have a standard 9-pin D-Sub male connector and some also have an optional screw terminal board that allows easier connection of cable or even individual wires. (Many manufacturers also provide accessories such as gender-changers, DB-9 to DB-25 adapters and DB-9 to RJ-45 adapters.)
As mentioned earlier in this article, one area of concern is with Microsoft's Vista and Windows 7 operating systems and low-cost USB-to-RS485 adapters. In many cases the processor chip hardware simply does not meet the industry standard requirements for serial communications and this weakness has been exposed by the way in which these newer operating systems expect communications to be handled at the hardware level.
Another limitation is the supplied driver software itself - which can interact poorly with the specific combination of Windows 7/Vista, software update packs and peripherals in use on a particular system. The end result is often low-level timing errors that show up as computer freezes and COM-port connection problems. The annoying aspect is that a low-end processor chip may work fine on one computer but not on another computer with an almost identical hardware and software configuration. Troubleshooting the resulting errors can be a tedious and slow process.
The solution to this problem is to use USB to RS485 converters that have been hardware and software-optimized to support Vista and Windows 7 and addition to Microsoft's earlier operating systems. This is especially important when these converters are to be used in less forgiving industrial automation environments.
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