【正文】 eswith no I/O hardware of its own.Most DCSs offer redundant controllers, networks, and I/Os. Most give you builtin redundancy and diagnostic features, with no need for userwritten logic. DCSs allow centralized configuration from the operator or engineering console in the control room. You can change programming offline, and download without restarting the system for the change to be effective.DCSs allow intercontroller munications. You can do data exchange in most DCS systems ad hoc (no need for predefined data point lists). You access data by tag name, regardless of hardware or location. DCSs use multitasking operating systems, so you can download and run applications aside from the realtime control functions and still do fractionalsecond control. DCSs now e in micro systems, to pricepete with PLCsbut with full DCS features and capabilities. The typical DCS has integrated diagnostics and standard display templates that automatically extend/update when your database changes. This database is central to the systemyou don39。t have different databases sitting in the controllers. DCSs have userfriendly configuration tools, including structured English, control block libraries, SFC (sequential function chart), and even RLL (relay ladder logic). Most DCSs allow graphical configuration, provide online diagnostics, and are selfdocumenting. Most provide for userdefined control blocks or customized strategies. The controllers execute control strategies as independent tasks。 thus, making changes to part of the control logic has no impact on the rest. An important difference between DCSs and PLCs is how vendors market them. DCS vendors typically sell a plete, working, integrated, and tested system。 offering full application implementation. They offer many services: training, installation, field service, and integration with your Information Technology (IT) systems. A DCS vendor provides a server with a relational database, a LAN with PCs for office automation, networking support and integration of thirdparty applications and systems. The DCS vendor tries to be your onestop shop. The PLC is more of a doityourself device, which is sometimes simpler to execute. When PLCs were solely replacements for hardwired relays, they had only digital I/O, with no operator interface or munications. Simple operator interfaces appeared, then evolved into increasingly plex interfaces as PLCs worked with increasingly plex automation problems. We went from a panel of buttons and I/Odriven lamps to PLC fullcolor customized graphic displays that run on SCADA software over a network. PLCs now have many DCSlike control functions (., PID algorithms) and analog I/O. They39。ve moved past their birthplace: the digital world (switch and binary sensor inputs and output contacts to run motors and trigger solenoids). PLCs are fast: They run an inputputeoutput cycle in milliseconds. On the other hand, DCSs offer fractional second (1/2 to 1/10) control cycles. However, some DCSs provide interrupt/eventtriggered logic for highspeed applications. PLCs are simple, rugged puters with minimal peripherals and simple OSs. While increasing reliability, PLC simplicity is not conducive to redundancy. Thus, fully redundant (hot, automatic, bumpless) variations of PLCs, with their added hardware and software, sometimes suffer from a reduction in their reliabilitya characteristic PLCs are famous for. Data exchange typically requires you to preassign data registers and hard code their addresses into the logic. If you add registers or need to reassign data, you typically have to deal manually with the Domino Effect. Typical PLC Relay Ladder Logic (RLL) languages include function blocks that can perform plex control and math functions (., PID algorithms). Complex multiloop control functions (., cascade management and loop initialization) are not typical. For functions too messy to implement in RLL, most PLCs provide a function block that calls a userwritten program (usually in BASIC or C). PLCs typically operate as state machines: They read all inputs, execute through the logic, and then drive the outputs. The userwritten logic is typically one big RLL program, which means you may have to take the whole PLC offline to make a change of any size. You also run into database synchronization problems because of the separation of PLCs and the Man Machine Interface (MMI) software packages, as opposed to the central databases of DCSs. A PLC will run in a standalone configuration. A DCS controller normally expects an operator interface and munications, so it can send alarms, messages, trend updates, and display updates. Many PLC installations use interface software from thirdparty vendors for improved graphics and various levels of alarming, trending, and reporting. The PLC and MMI software normally interact by sitting on the network and using the register exchange mechanism to get data from and to the various PLCs. This type of munication presumes you have preassigned data registers and can fetch data on an absolute address basis. This can lead to data processing errors (., from the wrong input) you won39。t encounter with the central database of a DCS. Some PLCs use proprietary networks, and others can use LANs. Either way, the munication functions are the samefetch and put registers. This can result in bottlenecking and timing problems if too many PCs try municating with too many PLCs over a network. A PLC may have a thirdparty package for operator interfaces, LAN interface to PCs and peripherals, PLC data highway or bus, redundant controllers with local and distributed I/O, local MMI and local programming capability. The PLC would have redundant media support, but not the redundant munication hardware or I/O bus hardware you39。d find in a DCS. A PLC would have preprogrammed I/O cards for specific signal types and ranges. In