An Introduction to "Industry Standard" Solid-State Relays and Mounting Racks
16 position mounting rack, 4 types of modules, and ribbon cable
Interfacing your computer to real-world signalsSolid-state relays provide a way of interfacing microprocessor and computer based control systems to external devices and loads, while providing the necessary electrical isolation needed to protect the sensitive circuitry of the control system.
These solid-state relay modules (often called "bricks" because of their shape) optically isolate the control voltage from the external input or load inside the module so that you can concentrate on wiring your system (your "field wiring") rather than wondering if you are properly interfacing these signals.
"Industry Standard"The modules are based on several standard sizes and pin arrangements, and plug into mounting racks that contain screw terminals for your field wiring and a connector that goes to the control system.
Modules and racks are manufactured by a number of companies, and most are compatible and interchangable with each other within one of the standard module sizes.
(In the early 1980's Opto22 began using personal computers for industrial automation applications and developed a number of solid stated relay and mounting rack products. Over the years, other companies began making systems that were compatible, and this system has now become a defacto "standard".)
The modulesSolid-state relay modules come in 4 basic types and are color coded for ease in identification:
Output modules are used to switch loads such as motors, lamps, and solenoids by driving them with logic level signals from your control system. Many AC output modules have "zero-crossing" turn-on circuitry built in to minimize generated noise, and include transient protection circuitry.
Mounting racksSolid-state relay mounting racks are printed circuit boards on which the modules are mounted, and where field wiring connections are made. These racks include:
These connectors and cables have standardized pin-outs, and like the modules and mounting racks themselves, are compatible with digital input and output boards from many computer control system manufacturers.
Most mounting racks use the 50 pin connector standard which allows you to use mounting racks with up to 24 modules. The cables are usually made from "ribbon cable", with the odd number pins used for signals and the even number connected to ground to reduce noise.
Since +5 volts is required to power the circuitry inside the modules, most control system manufacturers provide enough current on the +5 signal coming from their interface to power several mounting racks, but even if they did not, a screw terminal connector is provided on most mounting racks to connect to an external source of +5 volts.
Sources of solid-state relays and mounting racksMany companies produce control systems and sold-state relay components that use this "industry standard" method of connecting control systems and I/O components. Listed below are links to several companies that provide these products.
Industrologic products using this industry standardThe following Industrologic products utilize a 50 pin connector for easy connection to industry standard I/O racks and modules.