DCS, PLC, PAC and HMI

David L. Neal, P.E. – Advanced Industrial Systems, Inc.

9/17/07

DCS - Distributed Control System.

Generally a vendor specific solution for a larger facility. Usually includes hardware, software, systems engineering, start-up, and training services. Often includes an advanced language for extended programming. May include installation. May include hardware and software from any or all of the HMI, PAC or PLC vendors.

DCS vendors often supply a complete system from proposal to start-up. Long term support is generally a characteristic of a DCS vendor.

DCS accept a wide variety of signals including analog, digital, serial and network information. The DCS can usually be tied to other plant computer systems.

Proprietary hardware and protocols have been common in the past. Many DCS vendors now incorporate more open standards in their systems. Some open standards are not as open as they appear.

PLC – Programmable Logic Controller. traditionally a hardware solution for I/O and logic control. Most large PLC’s accept analog inputs, some small PLC’s accept analog but many do not.

In general the PLC hardware and software is totally specific to the supplier. Even though many suppliers claim openness, it may come at the cost of performance, reliability or high cost.

Soft PLC programs have been around for many years. They use a variety of I/O hardware and can often be used in a multi-vendor environment.

Most PLC vendors offer some form of HMI or HMI compatibility.

Large PLC’s have extensive communications capability. Most small PLC’s have very limited communications capability (or none at all).

The microPLC has developed at a very low cost with small point count. MicroPLC’s lack many of the features of their big brothers, but are often cost effective for specific applications.

Programming, engineering, training, and start-up are normally provided by the end user or a third party. PLC vendors almost never do installation.

PAC – Programmable Automation Controller.

A hardware/software solution that may or may not include multi-vendor capability. This group of products is generally designed around PC hardware with connectivity to a variety of I/O and software. The PAC supplier provides connectivity and drivers for their hardware, and vendor recommendations for compatible programs. Most are designed to accept user programming in C or Basic as well. Some have better documentation than others.

VME, and STD were early forms of PAC systems.

Many PAC’s are built around the Modbus protocol. Some include HMI’s, however most rely on third party HMI’s.

These systems, by their nature are VERY open.

Programming, engineering, training, and start-up are normally provided by the end user or a third party. PAC vendors almost never do installation.

The microPAC has developed at a low cost with small point count. microPAC’s lack some of the features of their big brothers, but are often cost effective for specific applications. Most microPAC’s have limited interface capability.

Generally a software program with capability to talk to a wide variety of hardware. HMI’s in their most basic form display information about the control system, while control is performed in other places. Modern HMI’s provide control feedback, recipes, alarm handling, data logging, trending and calculation capability. Some HMI products provide the capability of distributing information locally and across the internet. Advanced HMI’s provide interconnectivity among many different devices with highly distributed nodes.

HMI systems are usually very open, however it may be impossible to add extended programming. Some HMI systems have the capability to exchange data with other programs giving those systems an advanced degree of openness.