USB to RS422 Converters Types And Uses

By Daniel Lorenz

Universal Serial Bus (USB) connectors are well known for the high-speed data transfer and hot swap capabilities they provide to personal computers (PCs). It's also becoming common to see PCs with USB ports used as diagnostics and monitoring devices in industrial automation networks, such as those used by RS422-compliant surveillance and security camera systems.

It's important to remember that the EIA Standard RS422-A specification is effectively the successor of the RS232 specification and it, in turn, has been superseded by the RS485 specification. A key benefit of RS422 over RS232 is in extending the supported communications distance to approximately 4000 feet (1200 meters) and at speeds of approx 115 kbps.

However the 'game changer' for RS432 was in enabling reliable communications over these longer distances in environments where induced electrical noise and earthing/grounding variances were regular annoyances. This was achieved by using a twisted-pair wire for each signal, with these being configured to operate as balanced differential signals in order to minimize the effects of various induced noise voltages.

Another advantage of RS422 over RS232 is that it offers better support for 'multi-drop' networks. Actually, RS232 was specified for point-to-point ('single-ended') connectivity and to use it for multiple devices requires a 'daisy-chain' approach where the devices are looped together along the communication line. The big problem with this is whenever a cable break occurs between slave nodes; all downstream nodes will then be unable to communicate.

The RS422 specification allows up to ten slave receiver nodes to be connected in a multi-drop configuration and, most importantly, these have different transmit and receive paths so that the 'daisy-chain' problems of RS232 will not occur.

For Field Service applications (such as custom data loggers) many engineers use notebook computers with several USB ports and it's here where a range of USB to RS422 converters is to be found.

Models range from single port converters to multi-port converter hubs and many often support both RS422 and RS485. (The multi-port models usually support hot-swap so a PC reboot is not required. For some environments there's also a need to have converters that provide optical isolation and surge protection.

In addition, if your computer is running Microsoft's Vista or Windows 7 operating systems, check that the converter also comes with the relevant software drivers because lower-end models often do not.

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