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An Introduction to "Industry Standard" Solid-State Relays and Mounting Racks

Solid state relays and rack
16 position mounting rack, 4 types of modules, and ribbon cable

Interfacing your computer to real-world signals

Solid-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 modules

Solid-state relay modules come in 4 basic types and are color coded for ease in identification:
  • IAC - AC input, coded yellow
  • OAC - AC output, coded black
  • IDC - DC input, coded white
  • ODC - DC output, coded red
Input modules are used to monitor the status of a switch, sensor, or load, and produce a logic level signal that tells the control system the state of the load. Input modules have filtering built in to provide clean signals to the control system, and include transient protection circuitry.

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 racks

Solid-state relay mounting racks are printed circuit boards on which the modules are mounted, and where field wiring connections are made. These racks include:
  • Sockets for a number of modules
  • Screw terminals in barrier strips for external wiring
  • Fuses on the field wiring side of the circuitry
  • LED's to indicate when the individual circuits are activated
  • Circuitry to allows your control system to operate the modules
Racks are available in several sizes, with 4, 8, 16, and 24 position racks being the most common, and most racks contain a connector where you can easily plug in a cable that goes to your control system. Cables of many different lengths are available, both from the companies that manufacture solid-state relays and racks, and from those that manufacture control systems.

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 racks

Many 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.

Opto22      Grayhill      Crydom

Industrologic products using this industry standard

The following Industrologic products utilize a 50 pin connector for easy connection to industry standard I/O racks and modules.
  • SIO-1000 RS-232 Data Acquisition Interface - Digital inputs and outputs, analog inputs and outputs, pulse counter input, pulse generator output, relay
  • SBC-1 Single Board Computer/Controller - analog input, 24 digital inputs, 24 digital outputs, counter inputs

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