Forms of Signal in Programmable Logic Control

Task 1
Evaluate the different forms of signal used in programmable logic control. Your evaluation will need to include:

Analogue

Digital (include Discrete)

Task 2

Describe the resolution and relationship between analogue inputs and outputs and word length. Your description will need to include:

10-bit

12-bit

Task 3

Create a table to describe and compare the typical protocols used in signal communication.

Task 4

Evaluate networking methods and networking standards. Your evaluation will need to include:

Master to Slave control

Peer to Peer networks

7 layer ISO model

IEEE 802

MAP Manufacturing Automation Protocol

Analogue

Analogue signals are continuously varying signals, between the two predefined limits, positive peak and negative peak. These signals usually have multitude of values that limit in between the temperature and pressure level, etc. The Analogue signals are able to detect the signals at various levels. Let us consider a micro-oven, which works heating the food at various temperatures run for various timeframes. The temperature needed is set by the user and for example, the maximum temperature that can be set is 200 degree centigrade, then the scale of analogue unit is to be divided into 200 units. The user set temperature is set through the PLC to produce the corresponding temperature. The temperature is set with 8 bits, which can produce 256 combinations and the maximum input and output required here is 0 to 200 and so, 8 bits are changed continuously to produce the required output needed for the circuit and the output is given to the actual processing unit or microcontroller. So, analogue signals can be used to provide input to the PLC in the form of continuous signals for pressure, position, level, temperature, etc. These bits are stored in the memory of the PLC and can be given to motor to manipulate the speeds, valves and other devices.

Digital, including Discrete

Digital signals means, signal on or signal off, or simply 1 or 0, two possible states. With the digital signals, in simple terms, a switch can be on the motor runs and when switched off, motor stops. One discrete digital signal is considered as a bit. The bits can be turned on or off to generate the pulses. When the bit is given to a counter, the number of times the bit is turned on and off is counted by the PLC. For example, if one revolution of the wheel of the motor moves the vehicle or crane to one foot and if the crane has to move further ten feet, then the count in PLC must be ten. The bits are also given to the timer. If eight bits are given to the temperature unit, it is going to give 256 combinations from 0 to 255, where 0 to 200 combinations would be enough for setting the temperature required for the micro-oven. . These signals are used for operating motors and opening and closing of the valves, etc.

Resolution and Relationship between Analog Inputs and Outputs and Word Length

The word length is the size of the word, which is represented as number of bits that are considered as a single set, given at one instant of input. Input output units stand as an interface between the external world and PLC controller. Analog input can be given and analog output is obtained, to and from the PLCs. When the PLC controller is given analog input, then it is converted into digital, using analog to digital converter. A suitable analog input card is given to the rack. The card is connected to multiple analog inputs and the inputs are multiplexed. Cards usually have the inputs of size, 4,8 and 16. In simple terms, the analog input is given in varied voltages that will be converted into the digital signals, using analog to digital converter and will be passed to the PLC controller. when the output is produced from the controller, the output is in the form of digital and it will be converted into analog, using digital to analog converter.

9 Bit

When 9-bit is considered, 1 analog input is divided into the word length of 9, with series of 9 bits in a digital form.

10 bit

When 10-bit is considered, 1 analog input is divided into the word length of 10, with series of 10 bits in a digital form.

12-bit

When 12-bit is considered, 1 analog input is divided into the word length of 12, with series of 12 bits in a digital form. The word length is varied according to the requirement.

The PLC communication is established by five protocols, called RS-232, IEEE 488, RS-422, USB and Ethernet. Each of the protocol has its advantages and disadvantages and so are used, based on the usage and application.

S.NO.	PROTOCOL	MAXIMUM SPEED	MAXIMUM SPEED	DEVICES	BEST IN
1	RS-232	Up to 50 m	700 Hz	1
2	RS-422	Up to 1000m	700 Hz	31	Maximum distance covered; more number of devices
3	IEEE-488	UP to 2m	1 MHz	14	Maximum speed
4	Ethernet	185 m	36000 Hz	29
5	USB	5m	80 Hz	8

Master to slave control

PLCs are usually connected with many intelligent devices, like computers. The capabilities of the PLC are maximized, when the PLC and controller are combined. PLC gets the data acquisition and control and computer for operator interface, documentation and data processing. The communication in a PLC or computer network is started by the hosting computer, which is connected in a daisy wheel chain pattern with PLCs. Each of the PLC is communicated for each specific information and the respective PLC passes the information for storage in the computer and also further use. Usually, one PLC has centralized control and is treated as a master PLC, which provides the controlling instructions to the rest of the PLCs. The other PLCs that receive the signals from the master PLC are called slave PLCs. It is done with tremendous speed that hundreds of cycles are completed in a second. Host computers help PLCs for powerful programming, using documentation software. The programs are loaded in the PLC, after writing them on computer, using relay ladder logic. This way, PLC programs can be created, modified, debug and monitored.

Protocols Used in Signal Communication

Peer to peer networks

Peer-to-peer networks decentralize the control functions, along with coordinated control. Here, many PLCs are connected each other in a daisy-chain fashion with each memory table duplicated. When data is written in one PLC, it is duplicated in all other in the network and this data is used in respective operating systems. Each PLC is programmed to control its control site, reducing the complexity in programming and debugging. Communication will eventually be transparent with reduced programming effort.

7 layer ISO model

International Standards Organization has devised a model called ISO/OSI model for using as a standard for Open System Interconnection or OSI, for facilitating the communication among multiple devise to reduce the interconnecting problems. Various digital equipment items are connected with a communication link among them, by defining in terms of protocol, electrical, physical and user standards. 

Figure: Layers in ISO

IEEE 802

IEEE 802 is considered as a IEEE standards family that deals with LANs and MANs, restricting the networks to carry packets of variable sizes. Data transmission is done with the units of cells that are sized uniform and short. The protocols and services map to Data and Physical layers of the OSI model. It splits data link layer into logical link control and media access control layers, LLC and MAC layers.

MAP

Manufacturing Automation Protocol was a standard for computer networking for devices interconnection, manufactured by many different manufacturers. The protocol was developed for combating the incompatible communication standards proliferations used by automation products suppliers, like programmable controllers.

MAP is revised many times and MAP 3.0 was released in 1987 with many improved technology. The network standard is used and promoted by many manufacturers, like Boeing, General Motors and many others, however, it was not adopted widely, because of contemporary Ethernet standard. The drawbacks of the protocol were, changing specifications of protocols, MAP interface links expenses and token-passing network speed penalty.

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