Plc Slot Numbering
What Everybody Ought to Know about PLC Numbering Systems. Once we know the Data Types that are in the PLC, we can look at the Memory Types. This is the Memory that the PLC utilizes within the program. This includes where to look for the actual input and output points, timers, counters, data registers, etc. The slot is the slot number the I/O module is plugged into. Slots are numbered with slot 0 being the first slot on the left side of the rack and continues counting up by 1 going to the right. The Type specifies one for four types of data: I for inputs; O for outputs; C for configuration; S for status.
Introduction
Understanding the Hardware is the first step of working with Programmable Logic Controllers (PLCs). In this tutorial, we will be going over the entire ControlLogix platform which is the flagship product from Rockwell Automation.
ControlLogix is classified as a large control system which is typically used in plant-wide applications. This platform allows PLC Programmers to have full control over various local and remote systems. ControlLogix hardware is critical in many facilities and provides a level of flexibility, unlike any other PLCs. Furthermore, it has the most memory and fastest speed available when compared to other systems from Allen Bradley.
In this article, we will talk about the basics you need to know: Chassis, Power Supply & 1756 Control Logix Family.
ControlLogix Chassis Hardware Components
ControlLogix is characterized by the 1756 prefix. You will find this number in front of every component which works with this system. This includes PLCs, I/O Modules, Chassis, Communication Modules and more. This is important to know as Rockwell has secondary identifiers for their devices which often confuses new Automation Engineers. The part number is different from the revision number; the part number for ControlLogix will always include “1756” as the prefix.
ControlLogix Chassis Selection
The Chassis is what will physically hold the modules which allow the system to interface field devices: EtherNet, ControlNet, DeviceNet, Analog Inputs, Digital Outputs, etc. The chassis can be selected based on the constraints of the system as well as the mechanical layout of the panel. However, based on my experience, it’s always best to purchase the largest chassis possible in order to leave room for future expansion. The cost difference in the different models is fairly low, so going with the 17 slot chassis is always the best option in my opinion.
The chassis for 1756 ControlLogix modules can be of following sizes:
- 4 slot (1756-A4)
- 7 slot (1756-A7)
- 10 slot (1756-A10)
- 13 slot (1756-A13)
- 17 slot (1756-A17)
ControlLogix Power Supply Selection
A power supply will provide power to the chassis as well as the modules connected to it. ControlLogix power supplies will snap into place on the left side of the chassis and become an integral part of the PLC rack.
Here’s a breakdown of standard power supplies which should be your number one option in most cases. Do note that you need to select what’s best for your system depending on where it’s going to be used. In my experience, going with the 24VDC option is always the safe bet as factories can always provide this voltage level regardless of their location and thus you don’t need to be concerned with input voltage levels.
If you’re going to deploy a control system within an environment with extreme temperatures, it would be wise to invest in a properly rated power supply. Here are your best options for such environments:
Lastly, if the system which will be attached to this chassis requires redundancy, the following selection would be best. These systems require an uptime which is highly critical and could result in damage to the equipment or personnel in case of failure. Investing in a redundant power supply would be recommended.
ControlLogix Hardware Important Terms
Chassis Backplane
As mentioned above, the chassis will provide a connection between all the modules inserted into it. The primary goal is to send data between the PLC and other cards such as I/O, Communication modules and Specialty Modules.
The backplane, in particular, refers to the portion of the chassis which send the signals between the modules. This term is often used within RSLogix 5000 & Studio 5000 documentation as well as the software. It’s important to understand that this term will be referenced in order to identify which modules are positioned in the chassis.
Chassis Slot
As we discussed above, the chassis for the 1756 ControlLogix family will have 4, 7, 10, 13 or 17 slots. This number refers to the available spaces within the chassis. A single slot will be able to accommodate a single 1756 card. The slot numbers on the chassis will be labeled starting from 0. The same numbers will be used within the RSLogix & Studio 5000 environments as you configure which cards are positioned in which slot.
There’s no right or wrong way to position your modules within the chassis, but I would try to be consistent and group similar modules. My preferred method has been as follows:
- PLC cards
- Comm cards (EtherNet, DeviceNet, ControlNet, Sercos)
- DC Inputs
- DC Outputs
- Analog Inputs
- Analog Outputs
- AC Inputs
- AC Outputs
- Relay Outputs
- Specialty Modules
Conclusion
The ControlLogix family offers an excellent control system for a large application. It’s highly scalable and supports a wide variety of modules for all purposes. Some examples include high-performance PLCs, Inputs, Outputs, Analog I/O, Relay Cards, Specialty Cards (RTD, HART, High-Speed Counter, etc.), Communication (EtherNet, DeviceNet, etc.)
Understanding the hardware is important as an engineer, PLC programmer and technician. Remember that a chassis is what’s used to host your cards, the backplane is the medium over which the cards communicate and the slot refers to their position within the chassis. These terms are frequently used within the documentation as well as program configuration through RSLogix & Studio 5000.
Video Tutorial
Programmable Logic Controller (PLC) also known as Industrial Computer is the major component in the industrial automation sector. Due to its robust construction, exceptional functional features like PID controllers, sequential control, timers and counters, ease of programming, reliable controlling capabilities and ease of hardware usage – this PLC is more than a special-purpose digital computer in industries as well as in other control-system areas. Different types of PLCs from vast number of manufacturers are available in today’s market. Therefore, in the subsequent paragraphs, let us study about PLCs and their types.
What is a PLC System?
PLC is invented to replace traditional control panels whose operations depend on the electromagnetic logic relays that are based on timers in industrial control systems. PLCs are capable of monitoring the inputs continuously from sensors and producing the output decisions to operate the actuators based on the program. Every PLC system needs at least these three modules:
- CPU Module
- Power Supply Module
- One or more I/O Module
CPU Module
CPU module consists of a central processor and its memory. The Processor is responsible for doing all the necessary computations and data processing by accepting the inputs and producing appropriate outputs. Memory includes both ROM and RAM memories. The ROM memory contains the operating system, driver and application programs, whereas the RAM stores user-written programs and working data. These PLCs use retentive memory to save user programs and data when the power supply breaks or fails and to resume the execution of a user program ones the power is restored. Thus, these PLCs do not need any use of a keyboard or monitor for reprograming the processor each time. The retentive memory can be implemented with the use of long-life batteries, EEPROM modules and flash memory methods.
Plc Slot Numbering Tool
BUS or Rack
In some modular PLCs bus or rack is provided in the backplane of the circuit into which all the modules like CPU and other I/O modules are plugged to the corresponding slots. This bus enables the communication between CPU and I/O modules to send or receive the data. This communication is established by addressing the I/O modules according to the location from CPU module along the bus. Suppose, if the input module is located in the second slot, then the address must be I2:1.0 (second slot first channel only as an example). Some buses provide necessary power to I/O module circuitry, but they do not provide any power to sensors and actuators connected to I/O modules.
Power Supply Module
These modules supply the necessary power required for the whole system by converting the available AC power to DC power required for CPU and I/O modules. The output 5V DC drives the computer circuitry, and in some PLCs 24DC on the bus rack drives few sensors and actuators.
I/O Modules
Input and output modules of the PLC allow to connect the sensors and actuators to the system to sense or control the real-time variables such as temperature, pressure flow, etc. These I/O modules vary in type, range, and capabilities and some of these include the following:
Digital I/O module: These are used to connect the sensors and actuator that are of digital in nature, i.e., only for switch ON and OFF purpose. These modules are available on both AC and DC voltages and currents with variable number of digital inputs and outputs.
Analog I/O modules: These are used to connect the sensors and actuators that provide the analog electric signals. Inside these modules, analog to digital converter is used to convert the analog to processor understandable data, i.e., digital data. This module’s number of channel’s availability is also can be varied depending on the application,
Communication Interface Modules: These are intelligent I/O modules that exchange the information between a CPU and communication network. These are used for communicating with other PLCs and computers that are placed at a remote or far away distance.
Types of PLCs
Programmable Logic Controllers (PLCs) are integrated as either single or modular units.
An integrated or Compact PLC is built by several modules within a single case. Therefore, the I/O capabilities are decided by the manufacturer, but not by the user. Some of the integrated PLCs allow to connect additional I/Os to make them somewhat modular.
A modular PLC is built with several components that are plugged into a common rack or bus with extendable I/O capabilities. It contains power supply module, CPU and other I/O modules that are plugged together in the same rack, which are from same manufacturers or from other manufacturers. These modular PLCs come in different sizes with variable power supply, computing capabilities, I/O connectivity, etc.
Plc Slot Numbering Strategy
Modular PLCs are further divided into small, medium and large PLCs based on the program memory size and the number of I/O features.
Small PLC is a mini-sized PLC that is designed as compact and robust unit mounted or placed beside the equipment to be controlled. This type of PLC is used for replacing hard-wired relay logics, counters, timers, etc. This PLC I/O module expandability is limited for one or two modules and it uses logic instruction list or relay ladder language as programming language.
Medium-sized PLC is mostly used PLC in industries which allows many plug-in modules that are mounted on backplane of the system. Some hundreds of input/ output points are provided by adding additional I/O cards – and, in addition to these – communication module facilities are provided by this PLC.
Large PLCs are used wherein complex process control functions are required. These PLCs’ capacities are quite higher than the medium PLCs in terms of memory, programming languages, I/O points, and communication modules, and so on. Mostly, these PLCs are used in supervisory control and data acquisition (SCADA) systems, larger plants, distributed control systems, etc.
Some of the manufacturers or types of PLCs are given below:
- Allen Bradley PLCs (AB)
- ABB PLCs (Asea Brown Boveri)
- Siemens PLCs
- Omron PLCs
- Mitsubishi PLCs
- Hitachi PLCs
- Delta PLCs
- General Electric (GE) PLCs
- Honeywell PLCs
- Modicon PLCs
- Schneider Electric PLCs
- Bosch PLCs
Applications of PLC
The below figure shows the operation of a PLC for a simple process control application wherein the conveyor belt operation, the number of boxes’ measurement and other control operations are performed by the PLC. Here, the position sensor and other sensor outputs are connected to the input module of the PLC, and from the output modules – a motor is controlled. When the sensors are activated, then the CPU of the PLC reads the inputs, and correspondingly processes them according to the program and produces the outputs to operate the motor so that the conveyor is controlled.
PLC and SCADA combination of control structure is mostly used in industrial automation sector and also in electrical utility systems like power transmission and distribution systems. Programmable sequential switching operation is another major application area of the PLC.
Therefore, the selection of PLCs for certain application needs several considerations over different types of PLCs. Thus we believe that the information here about the topic gives you a better understanding well supported by some appropriate and effective images. Do write to us if you have any technical doubts on this topic, and also on PLC based projects for students as well as for industries.
Photo Credits:
Plc Slot Numbering Machines
- Programmable Logic Controller (PLC) by blogspot
- CPU Module of PLC by aotewell
- ABB PLC Power Supply by tlauk
- PLC I/O Modules by thomasnet
- Integrated or Compact PLCs by bse-tech
- A modular Types of PLC by delta
- Small, Medium and Large sized Types of PLCs by ecogate
- Manufacturers or Types of PLCs by amci
- Applications of PLC by ytimg