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SLC 500 Processor Selection Guide

I. Introduction
Central Processing Units (CPU) or processors are considered the most important and expensive component of a Programmable Logic Controller (PLC) assembly. Processors not only execute the downloaded user program but also manages communication between devices. It may also be operated as a stand-alone device for the purpose of testing, familiarization and learning.

II. SLC 500 Processors
Before selecting an SLC 500 Processor, familiarization with the platform and the hardware styles must be identified.

Generally, the SLC 500 product family must were released in Two (2) designs – Fixed Style and Modular Style hardware.

                1. Fixed Style Hardware


                                The fixed style hardware was released in the early ‘90s. This design comes with a built-in  power supply, processor and I/O channels – all installed in one compact assembly. Fixed style hardware belongs to the Compact type PLCs due to the built-in components. This SLC 500 hardware style permits limited expansion as this design is especially made for micro and small automation requirements.   Fixed hardware style hardware generally comes with Three (3) models, as listed below:

  • 1747-L20: 20 total I/O (12 inputs and 8 outputs)
  • 1747-L30: 30 total I/O (18 inputs and 12 outputs)
  • 1747-L40: 40 total I/O (24 inputs and 16 outputs

                2. Modular Style Hardware

   Modular style SLC 500 are designed for use in medium to large automation requirements. This type of SLC 500, are highly modular and expandable. Components   are purchased individually according to the function needed. Modular Style processors are generally categorized into Five (5) models, listed below:

  • SLC 5/01
  • SLC 5/02
  • SLC 5/03
  • SLC 5/04
  • SLC 5/05

Processors in this SLC 500 product family have significantly higher computing capability    and designed to implement complex tasks.        

III. SLC 500 Processor Selection
Now that we have identified the hardware styles of the SLC 500 family, we can now discuss the considerations needed to select the right processor for your control requirement.
Basically, the considerations to follow are:

                1. Selecting whether fixed or modular style
                                A. Application size         
                                B. Application criticality 
                2. Selecting the appropriate SLC 500 modular processor           
                                A. I/O points
                                B. Processor performance
                                C. Advanced Programming Instruction support
                                D. Communication

                1. Selecting whether fixed or modular style
                                A. Application size
                                Application size can be considered as small, medium and large applications. It may also                                   refer to as simple and complex application. We have identified that Fixed style SLC 500                                   hardware are ideally used for micro and small automation applications which are expected to be of simple applications. With Modular style SLC 500 hardware, we have   determined that this product version is designed for medium to large applications.  
                                How to identify the type of application?
                                We can identify this following the simple sizing table below:

No. Of Input and Output (I/O) devices Application size Ideal Processors
< / = 256 I/O Small Fixed Style
> 256 I/O and < / = 1024 I/O Medium Modular Style
> 1024 I/O Large Modular Style

                                Table 1: Identifying application size

                                Referencing the table above (table 1), the application size can be determined by                                                identifying the number of Input and Output (I/O) devices, regardless of the type of I/O                                    (Discrete and Analog). This easily eliminates the confusion on which hardware style to                                     use.

                                B. Application criticality
                                The criticality of an application requires a processor with increased uptime. Such a requirement is met by configuring a Simplex and Hot-Backup configuration. With simplex configuration, an SLC 500 system has One (1) SLC 500 processor. Once the processor has faulted, the entire system shuts down. With hot-backup configuration, an SLC 500 is installed with Two (2) SLC 500 processor working synchronously, linked by a pair of 1747-BSN or the Backup Scanner module installed in the primary and secondary chassis. When a processor is faulted, control is transferred to the partner processor within a very minimum time.

                                Hot back-up configuration may only be implemented with the modular style SLC 500 hardware.                          

Table 2: SLC 500 processor hot backup support

Hot back-up configuration Supported
Fixed Style Hardware No
Modular Style Hardware Yes
SLC 500 Modular Processor No. of I/O support
SLC 5/01 up to 3040 I/O
SLC 5/02, 5/03, 5/04, and 5/05 up to 4096 I/O

                2. Selecting the appropriate SLC 500 modular processor
The following considerations apply to SLC 500 modular processor only. As previously mentioned,  the modular hardware style covers Five (5) processor models:

                                –              SLC 5/01
                                –              SLC 5/02
                                –              SLC 5/03
                                –              SLC 5/04
                                –              SLC 5/05

                The next set of items help identify the actual modular SLC 500 processor for an intended application:

A. I/O Points
An SLC 5/01 processor is capable of handling up to 3940 I/O while SLC 5/02, 5/03, 5/04,  and 5/05 supports up to 4096 I/O.   

                                Table 3: SLC Processor I/O support

This information easily confirms that an SLC 500 modular processor is ideally used for Medium to Large application.

                                B. I/O architecture
As modular SLC 500 processors are designed to handle a large number of I/O, it may be     configured to control extended number of chassis with the use of:
  – Chassis interconnect cables and;
  – I/O scanner modules
  Chassis interconnect modules allow extension of the primary chassis (where the processor is installed) using chassis interconnect cables. Up to Thirty (30) I/O chassis extension may be installed and interconnected however, each chassis permits Four (4) meters distance each.

Figure 4: Chassis Interconnection architecture
Figure 5: Distributed I/O architecture using Remote I/O (RIO) protocol

Another method of maximizing the supported I/O count is with the implementation of   distributed I/O system. A distributed I/O system uses I/O scanner modules and may be configured with more than 30 racks, depending on the requirement. Additionally, distance between each chassis is not limited to Four (4) meters as this is dictated by the                                 type of communication protocol used.

Distributed I/O is supported by SLC 5/02, SLC 5/03, SLC 5/04 and SLC 5/05 processors                                           only.

                                Table 4: I/O Expansion support

Extended I/O support SLC 500 modular processor
Chassis Interconnection All SLC 500 Modular processors
Distributed I/O SLC 5/02, 5/03, 5/04, and 5/05 only

                                C. Advanced Programming Instruction Support
                                Program capabilities of SLC modular processors is dictated by the type of internal instructions it support. By differentiating the support for advanced instructions, the processors capability of handling complex control requirements may be identified. Refer to the table below for the instructions supported by each processor:      

Table 5: Support for Advanced Instructions

Advanced Program Instructions SLC 5/01 SLC 5/02 SLC 5/03 SLC 5/04 SLC 5/05
Bit Logic Yes Yes Yes Yes Yes
Timer and Controller Yes Yes Yes Yes Yes
Comparison Yes Yes Yes Yes Yes
Basic Arithmetic Yes Yes Yes Yes Yes
Move, Copy, and Bit Shift Yes Yes Yes Yes Yes
Sequencer Yes Yes Yes Yes Yes
Jump and Subroutine Yes Yes Yes Yes Yes
Messaging   Yes Yes Yes Yes
STI   Yes Yes Yes Yes
FIFO/LIFO   Yes Yes Yes Yes
PID   Yes Yes Yes Yes
Advanced Math and Trig     Yes Yes Yes
Indirect Addressing     Yes Yes Yes
Floating Point Math     Yes Yes Yes
ASCII     Yes Yes Yes
No. of instructions 7 11 15 15 15

Some notable advanced instructions commonly used are:

  •   Messaging: Used for implementing controller to controller communication. Data is sent over the supported communication protocol .
  • Proportional + Integral + Derivative (PID):
    Used to implement automatic control of analog control loops
  • Advanced Math and Trig: Used for implementing complex mathematical formulae
  • Floating point Math: 32-bit, data format that supports decimal notation.
  • It is critical that the advanced functionality of the system be identified prior to selection of an SLC 500 modular processor.

                              D. Communication
On-board communication interfaces of SLC processors enable these devices to communicate with a variety of automation devices. These devices could be Supervisory Control and Data Acquisitions (SCADA), Human Machine Interface (HMI), Enterprise  productivity and reporting tools, databases, gateways, protocol converters and other devices.   

Communication capabilities of each SLC 500 modular processor are different from each other except SLC 5/01 and SLC 5/02.

 Table 6: Built-in communication protocol supports of SLC Processors

Communication Protocol SLC 5/01 SLC 5/02 SLC 5/03 SLC 5/04 SLC 5/05
DH-485 Supported Supported Supported    
RS-232     Supported Supported Supported
DH+       Supported  
Ethernet         Supported

                                SLC 5/01 and SLC 5/02 has One communication channel (channel 0) only supporting DH- 485 communication protocol.      SLC 5/03, SLC 5/04 and SLC 5/05 has One (1) RS232 physical interface (channel 0) supporting DH-485, DF1 full-duplex, DF1 half-duplex master/slave, ASCII, DF1 radio modem, and Modbus RTU Master protocols.
Additionally, SLC 5/03, SLC 5/04 and SLC 5/05 has a secondary channel that supports    DH-485, Data Highway Plus (DH+) and Ethernet I/P communication protocol, respectively.
In terms of popularity, Ethernet is the most prominent protocol nowadays. This is due to the fact that Ethernet I/P is compatible for use with non-OEM devices specifically, with commercially available networking components and cables unlike other protocols which  uses proprietary connectors and communication devices. 

Of course, a modular SLC 500 processor supports installation of separate communication modules however, selecting a processor with an appropriate on-board  communication capability contributes to the cost-effectiveness of the design, simplified configuration and maintenance. Consider installing optional communication modules if   the application requires a dedicated communication interface, existing communication  interface has failed and if, due to migration, partner device(s) no longer support the existing communication capability of the processor.        

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This entry was posted on March 26th, 2021 and is filed under Allen-Bradley, Hardware Comparison, PLC, Uncategorized. Both comments and pings are currently closed.

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