MELSEC A1S Selection Guide
What is MELSEC A1S?
The MELSCEC A1S is a series of programmable logic controllers (PLCs) manufactured by Mitsubishi Electric. This series includes high-performance, low-cost, compact, and micro-modular PLCs. It is a typical general-purpose PLC series capable of solving solutions for almost all types of industrial control applications, from replacing simple relays to networking and system control. Some of the common applications of MELSEC A1S PLC systems include data acquisition, data link control, system monitoring, sequence control, positioning control, analog control, and robot control.
How Do MELSEC A1S Devices Operate?
Similar to most available PLCs, MELSEC A1S-series devices operate by executing a sequence of instructions contained in a user-defined sequential control program. This sequence control program is stored in battery-backed random-access memory (RAM) within the CPU module of the MELSEC A1S PLC. The RAM is battery-backed to prevent program loss in the event of a power outage to the PLC.
A MELSEC A1S PLC will begin operation once the user downloads the control program onto its CPU memory and connects the appropriate I/O modules. The input modules monitor and collect any relevant information from the connected field input devices and send it to the A1S CPU. The CPU processes the received information and stores it as pre-processed data to be used in the following steps. It then executes the instructions from the stored sequence control program and triggers/updates the appropriate outputs based on the pre-processed data, as well as on the execution results from each program section. Thus, the connected field output devices are controlled depending on the received input data and the execution results of the sequential control program.
MELSEC A1S CPUs perform functions in a repeating loop, known as the Program Scan Cycle. During this cycle, the CPU constantly checks the state of the connected inputs while sequentially executing the control program and updating the conditions of the inputs and outputs. The timing of the updates depends upon the processing mode to which the A1S CPU module is set (via the dip switch setting), which can either be Direct or Refresh Mode. With the Direct Mode, the outputs and inputs are updated when and as they are being used within the sequential control program, whereas with the Refresh Mode, the outputs and inputs are updated at the end of the program scan cycle.
MELSEC A1S System Selection
This section outlines the steps to follow when selecting a general-purpose MELSEC A1S PLC system.
1. Select the Input Modules
The input modules provide an interface between the MELSEC A1S CPU and the external input devices used in various applications, such as push buttons, proximity switches, auxiliary contacts, limit switches, etc. There are many types of input modules available for use with the A1S series, and each of these modules has a differing input voltage specification and/or input points count.
Thus, to select appropriate input modules for a MELSEC A1S PLC system, simply specify the number of the different types of inputs to be used in your application. Then select the compatible input modules. Compatibility is to be determined in terms of the type of input module: AC or DC (Sink or Source), TTL (Transistor-Transistor Logic), High-Speed, etc. Compatibility is also determined in terms of the number of input points supported by each module and the input voltage specifications.
2. Select the Output Modules
The output modules interface the MELSEC A1S CPU with external/field output devices such as lamps, digital displays, interposing relays, contactors, and so on. Like with input modules, there are many different types of output modules for use with the A1S series, each of them having a differing number of output points and switching output voltage specifications.
Therefore, to select suitable output modules for a MELSEC A1S PLC system, you’ll need to specify the number and the different types of outputs to be used in your application. Then, select the correct output modules based on the type of output, the count of output points per module, and the switching output voltage specification.
3. Select the Special Function Modules
The special function modules in a MELSEC A1S PLC system are intelligent Input/Output modules that provide dedicated control solutions for specialized applications, such as Networking (Master/Local or Remote I/O), Computer Linking, Position Control (Voltage or Pulse train), Analog to Digital (A/D) Conversion, Digital to Analog (D/A) Conversion, High-Speed Counting, High-Speed Interrupts, Pulse Catch, Analog Timers, Temperature Sensing, etc. They convert or interpret information between an A1S CPU and application-specific field devices, thus enabling complete integration of the MELSEC A1S PLC system.
Since each special function module has its own function, select the A1S special function modules that will accomplish the various specific control functions entailed in your application.
4. Select the CPU Module
The CPU module is the main component of the MELSEC A1S PLC system, and it can control up to 512 Input/Output points. It consists of integrated battery-backed random-access memory (RAM), a programming port, real-time clock function, key switch control for the two different operation modes (Refresh and Direct Modes), and operation mode LEDs.
Each MELSEC A1S PLC system requires one A1S CPU module, which is mounted in the CPU slot on the CPU base unit. Therefore, for you to have an operating MELSEC A1S PLC system, you must select an A1S CPU module that can support the total number of I/O points you selected previously for the input, output, and special function modules. Other key features to consider when selecting an A1S CPU include:
- I/O Control Method: Switch selectable Direct Mode or Refresh Mode
- Processing Speed: Consider the processing speed for both Direct and Refresh modes
- Number of Instructions: Basic, Application, and Sequence instructions
- Programming Language: It should be dedicated to sequence control, with the combined use of logic symbolic type and relay symbol type.
- Memory and Program Capacity
5. Select Power Supply Modules
Power supply modules are used to power the MELSEC A1S PLC system. A typical A1S power supply module should provide the 5 VDC required by the selected A1S CPU and I/O modules to function properly. The A1S power supply modules are mounted in the power supply module slots on the CPU base units, and they output the 5 VDC they produce to a power rail also on the CPU base unit. The input supply voltage to an A1S power supply module depends on the selected model number.
Available A1S power supply modules include:
- Model No. A1S61P– Input supply voltage: 110/230 VAC. Output Voltage: 5 VDC, 5A
- Model No. A1S62P– Input supply voltage: 110/230 VAC. Output Voltage: 5 VDC, 3A, and 24 VDC, 0.6A (Can be used to power relay output modules, input devices, etc.)
- Model No. A1S63P– Input supply voltage: 24 VDC. Output Voltage: 5 VDC, 5A
Select an A1S power supply module whose output voltage is sufficient to power the A1S CPU and Input/Output modules included in your MELSEC A1S PLC system. Also, be sure to check the current consumption of all the A1S modules to be powered by the power supply module that you intend to use.
For example, if you select the A1S62P power supply module, then ensure that the current consumption of all the A1S modules being powered by this power supply does not exceed 3A. If the current consumption of the selected A1S modules is more than 3A, then the A1S61P power supply unit should be selected for 5 VDC output voltage, but if 24 VDC output voltage is required with a current consumption above 3A, then an external power supply module should be used.
6. Select the CPU Base Units
The A1S CPU base units are used for mounting one (1) power supply module, the selected A1S CPU module, and various A1S I/O modules. The number of A1S Input/Output modules that can be mounted on a given A1S CPU base unit depends on the model number of the selected base unit. Therefore, select a CPU base unit model that has enough I/O slots to mount the total number of A1S I/O modules you intend to use.
The available A1S CPU base unit models include:
- Model No. A1S32B: Includes spare I/O slots for two (2) A1S I/O modules.
- Model No. A1S33B: Includes spare I/O slots for three (3) A1S I/O modules.
- Model No. A1S35B: Includes spare I/O slots for five (5) A1S I/O modules.
- Model No. A1S38B: Includes spare I/O slots for eight (8) A1S I/O modules.
Note: Like with the A1S CPU modules, each MELSEC A1S PLC system requires one A1S CPU base unit.
7. Select the Expansion Base Units
Expansion base units in a MELSEC A1S PLC system are used to mount an extra power supply module and/or additional A1S I/O modules if required. If, for example, the number of A1S I/O modules you intend to use on your A1S PLC system exceeds the number of I/O slots on the selected A1S CPU base unit, then you’ll need an expansion base unit. An expansion cable is used for connecting the expansion base unit with the A1S CPU base unit.
The expansion base unit model number determines the number of I/O slots on the unit, and whether the unit has a power supply module slot or not. Therefore, select an extension base unit whose number of I/O slots can mount the extra I/O modules, and if your MELSEC A1S system requires an extra power supply module be sure to select an expansion base unit with a power supply module slot.
The following are the available A1S expansion base units:
- Model No. A1S52B: Includes spare slots for two (2) I/O modules.
- Model No. A1S55B: Includes spare slots for five (5) I/O modules.
- Model No. A1S58B: Includes spare slots for eight (8) I/O modules.
- Model No. A1S65B: Includes spare slots for five (5) I/O modules. This extension base unit also has a power supply module slot.
- Model No. A1S68B: Includes spare slots for eight (8) I/O modules. It also includes a power supply module slot.
Note: If you select either the A1S68B or A1S65B expansion base unit, then an additional power supply module will be required.
8. Select the Necessary Accessories
Select the essential accessories required for the proper functioning of your MELSEC A1S PLC system. These may include:
A) Memory Modules: These modules allow for the use of EPROMs and EEPROMs for program storage, in addition to the random-access memory in the selected A1S CPU module. Depending on the memory requirements of your MELSEC A1S PLC system, you can select any of the following A1S memory modules:
- Model No. A1SMCA-2KE: This is an Electrically Erasable Programmable Read Only Memory (EEPROM) memory module with a capacity of 2K steps, which is equivalent to 8K bytes.
- Model No. A1SMCA-8KE: This is also an EEPROM memory module but with a capacity of 8K steps (32K bytes).
- Model No. A1SMCA-8KP: This is an Erasable Programmable Read Only Memory (EPROM) memory module whose memory capacity is 2K steps (8K bytes).
B) Expansion Cables: As previously stated, extension cables are used to connect the expansion base units to the A1S CPU base units. Different expansion cable lengths are available as per the model number. Therefore, select the expansion cable model number with the length that will be most appropriate for your application. Available A1S expansion cables include:
- Model No. A1SC01B: This is a 5.5-centimeter-long cable for expansion base to CPU base connection.
- Model No. A1SC03B: This is a 33-centimeter-long cable for expansion base to CPU base connection.
- Model No. A1SC12B: This is a 120-centimeter-long cable for expansion base to CPU base connection.
- Model No. A1SC30B: This is a 300-centimeter-long cable for expansion base to CPU base connection.
- Model No. A1SC05NB: This is a 45-centimeter-long cable for AnN expansion base to A1S CPU base connection.
C) Terminal Block Units: Select the compatible remote terminal block units and terminal block cables for use with connector wiring type A1S I/O modules. There are a variety of remote terminal block units that can be used with connector wiring type input and output modules within the MELSEC A1S Series. These terminal blocks differ in the number of terminal points they support and/or their compatibility with different A1S I/O modules.
On the other hand, there are two types of terminal block cables available for connecting the remote terminal block units to compatible A1S I/O modules. One type is for use with the sink-type A1S I/O modules, while the other type is for use with the source-type A1S I/O modules. Therefore, terminal block cables should be selected depending on whether the A1S I/O module to be connected to the selected remote terminal block unit is of sink or source-type.
Selecting the correct MELSEC A1S PLC system for your application may seem like a daunting task when you look at the so many different types of A1S modules and accessories to choose from. However, the selection process can be made easier if you follow the selection steps outlined in the above section because the reason for all the different types of modules available with the A1S Series is so that the MELSEC A1S controller platform can be used for any type of control application. Follow those steps carefully and you’ll be able to select the right MELSEC A1S PLC system that matches the requirements of your control application(s).