PLCs were conceived originally as replacement for timers, relays, and drum sequencers, they were strictly discrete controllers. Analog I/O was added eventually, but only rudimentary operations such as alarming, basic digital conditioning, and set point monitoring were available.
Not until about 15 years ago was true process loop control, in the form of the PID algorithm, added to the PLC. This functionality was only available on high end, high priced PLCs. For industrial OEM applications with process control and discrete needs, it often was cheaper to pair a low end PLC with a single loop controller as opposed to using a high end PLC.
In similar fashion, digital communications beyond RS-232 and RS-422 were not available on low cost PLCs. Industrial OEMs that needed standard communication protocols such as Ethernet, or with a need to perform loop control, were not able to use low end PLCs.
Many industrial OEMs were forced to create their own proprietary control platforms because the market did not provide a low cost controller with analog control, discrete control, and Ethernet communication capabilities.
This state of affairs has changed dramatically in the few past years, and low end and low cost PLCs now are available with analog I/O, Ethernet communication and process control capabilities. These low cost, high performance hybrid PLCs can be extremely attractive options for many industrial machine applications.
BOC Edwards designs and builds gas delivery systems of industry of semiconductor. Its control systems perform discrete control along with controlling and monitoring of gas pressures and flow rates. The control system also interfaces with toxic gas flame detectors and leak detectors.
BOC Edwards was using a custom designed controller based on the Intel 8088 processor, but wanted to deploy hybrid PLC control. It found a low cost PLC from AutomationDirect with all of capabilities. The systems require 32 digital inputs, 16 analog inputs, and 32 digital outputs. It also needs to interface with facility monitoring systems via an OPC interface or via their own proprietary communications protocol using RS-422.