Signal functions in the EIA232 standard can be subdivided into six categories. These categories are summarized below, after which each signal described.
a) Signal ground and shield.
b) Primary communications channel. This is used for data interchange, and includes flow control signals.
c) Secondary communications channel. When implemented, this is used for control of the remote modem, requests for re transmission when errors occur, and governance over the setup of the primary channel.
d) Modem status and control signals. These signals indicate modem status and provide intermediate checkpoints as the telephone voice channel is established.
e) Transmitter and receiver timing signals. If a synchronous protocol is used, these signals provide timing information for the transmitter and receiver, which may operate at different baud rates.
f) Channel test signals. Before data is exchanged, the channel may be tested for its integrity, and the baud rate automatically adjusted to the maximum rate that the channel can support.
a) Pin 14 - Secondary Transmitted Data
b) Pin 16 - Secondary Received Data
c) Pin 19 - Secondary Request to Send
d) Pin 13 - Secondary Clear to Send These signals are equivalent to the corresponding signals in the primary communications channel. The baud rate, however, is typically much slower in the secondary channel for increased reliability.
System Operation
Symptom Troubleshooting and Corrective Actions for PLC system
All LEDs are OFF
1. Check the power supply wiring.
2. Check If the power supplied to the PLC control units is in the range of the rating.
3. Be sure to check the fluctuation in the power supply.
4. Disconnect the power supply wiring to the other devices if the powersupplied to the PLC control unit is shared with them.If the LEDs on the PLC control unit turn ON at this moment, the
capacity of the power supply is not enough to control other devices
as well. Prepare another power supply for other devices or increase
the capacity of the power supply.
5. If the POWER LED still does not light up when the power is on after
the above corrective actions, the PLC should be sent back to the
dealer or the distributor whom you purchased the product from
ERROR LED is
flashing
1. If the ERROR LED is flashing, the problem may be an invalid
commands, communication error, invalid operation, or missing
instructions, error indication is given by self-checking function and
corresponding error code and error step are stored in special
registers. The corresponding error codes can be read from the
WPLSoft or HPP. Error codes and error steps are stored in the
following special registers.
Error code: D1004
Error step: D1137
2. If the connections between the PLC are failed and the LED will flash
rapidly, this indicates the DC24V power supply is down and please
check for possible DC24V overload.
3. The LED will be steady if the program loop execution time is over thepreset time (set in D1000), check the programs or the WDT (Watch Dog Timer). If the LED remains steady, download user program again and then power up to see if the LED will be OFF. If not, please check if there is any noise interference or any foreign object in the PLC.
Diagnosing Input
Malfunction
When input indicator LEDs are OFF,
1. Check the wiring of the input devices.
2. Check that the power is properly supplied to the input terminals.
3. If the power is properly supplied to the input terminal, there is
probably an abnormality in the PLC’s input circuit. Please contact
your dealer.
4. If the power is not properly supplied to the input terminal, there is
probably an abnormality in the input device or input power supply.
Check the input device and input power supply.
When input indicator LEDs are ON,
5. Monitor the input condition using a programming tool. If the input
monitored is OFF, there is probably an abnormality in the
PLC’s input circuit. Please contact your dealer.
6. If the input monitored is ON, check the program again. Also, check
the leakage current at the input devices (e.g., two-wire sensor) and
check for the duplicated use of output or the program flow when a
control instruction such as MC or CJ is used.
7. Check the settings of the I/O allocation.
Diagnosing Output
Malfunction
PLC Fault Finding & Troubleshooting Procedures Corrective Actions for PLC system
When output indicator LEDs are ON,
1. Check the wiring of the loads.
2. Check if the power is properly supplied to the loads.
3. If the power is properly supplied to the load, there is probably an
abnormality in the load. Check the load again.
4. If the power is not supplied to the load, there is probably an
abnormality in the PLC’s output circuit. Pleas contact your dealer.
When output indicator LEDs are OFF,
5. Monitor the output condition using a programming tool. If the output monitored is turned ON, there is probably a duplicated output error.
6. Forcing ON the output using a programming tool. If the output
indicator LED is turned ON, go to input condition check. If the output
LED remains OFF, there is probably an abnormality in the PLC’s
output circuit. Please contact your dealer.
1)Extension modules:-
PLC I/O number can be increased through certain additional modules by system extension through extension lines. Each module can contain extension both of input and output lines. Extension modules can have inputs and outputs of a different nature from those on the PLC controller. When there are many I/O located considerable distances away from the PLC an economic solution is to use I/O modules and use cables to connect these, over the long distances, to the PLC.
2)Remote I/O connections:-
When there are many I/O located considerable distances away from the PLC an economic solution is to use I/O modules and use cables to connect these, over the long distances, to the PLC.
3)Remote PLCs:-
In some situations a number of PLCs may be linked together with a master PLC unit sending and receiving I/O data from the other units.
4)Cables:-
Twisted-pair cabling, often routed through steel conduit. Coaxial cable enables higher data rates to be transmitted and does not require the shielding of steel conduit. Fiber-optic cabling has the advantage of resistance to noise, small size and flexibility.
5)Parallel communication:-
Parallel communication is when all the constituent bits of a word are simultaneously transmitted along parallel cables. This allows data to be transmitted over short distances at high speeds. Might be used when connecting laboratory instruments to the system.
6)Parallel standards:-
The standard interface most commonly used for parallel communication is IEEE-488, and now termed as General Purpose Instrument Bus (GPIB).
Parallel data communications can take place between listeners , talkers , and controllers. There are 24 lines: 8 data (bidirectional), 5status & control, 3 handshaking, and 8 ground lines.
7)Serial communication:-
Serial communication is when data is transmitted one bit at a time. A data word has to be separated into its constituent bits for transmission and then reassembled into the word when received. Serial communication is used for transmitting data over long distances. Might be used for the connection between a computer and a PLC.
8)Serial standards:-
RS-232 communications is the most popular method of plc to external device communications. RS 232 is a communication interface included under SCADA applications. Other standards such as RS422 and RS423
are similar to RS232 although they permit higher transmission rates and longer cable distances.
9)There are 2 types of RS-232 devices:-
DTE – Data Terminal Equipment and a common example is a computer.
DCE – Data Communications Equipment and a common example is a modem.
PLC may be either a DTE or DCE device.
10)ASCII:-
ASCII is a human-readable to computer-readable translation code
(each letter/number is translated to 1’s and 0’s). It’s a 7-bit code, so we can translate 128 characters (2^7 is 128).
11)Protocols:-
It is necessary to exercise control of the flow of data between two devices so what constitutes the message, and how the communication is to be initiated and terminated, is defined. This is termed the protocol.One device needs to indicate to the other to start or stop sending data.
Interconnecting several devices can present problems because of compatibility problems.
In order to facilitate communications between different devices the International Standard Organization (ISO) in 1979 devised a model to be used for standardization for Open System Interconnection (OSI).
 Pico : Non modular small PLCs
 Micrologix 1000, 1200 and 1500 Series
 SLC : SLC 5/01, 5/02, 5/03 ….
 Control Logix Flex Logic and Soft PLC
Data Highway : The proprietary data network used by Allen Bradley PLCs to communicate information to and from other PLCs on the network or to and from host computers attached to the network.
ControlNet : A real-time, control-layer network providing high-speed transport of both time-critical IO data and messaging data, including upload/download of programming and configuration data and peer-to-peer messaging, on a single physical media link
DeviceNet : A low-cost communication link that connects industrial devices to a network. It is based on broadcast-oriented communication protocol - the Controller Area Network (CAN).
Ethernet : The standard for local communications networks developed jointly by Digital Equipment Corp., Xerox, and Intel. Ethernet baseband coaxial cable transmits data at speeds up to 10 megabits per second. Ethernet is used as the underlying transport vehicle by several upper-level protocols, including TCP/IP.
The protocols are distinguished with following specifications,
No. of nodes supported, total network length , Speed of communication .
The delay time between the end of one communication and the start of another. During this time, the processes associated with the communication are hung up and cannot continue. The latency has to be minimum.
Typical component on Schneider Redundant PLCs
The backplane used is either 4 slot o6 6 slot with
- Power Supply
- Controller with built-in Modbus Plus and Modbus ports
- Optional dual cable Modbus Plus
- Optional fiber optic Modbus Plus
- CHS Hot Standby module
- Dual cable Remote I/O Head
The master and Standby configuration must be identical
Sinking Source type modules gives out the current to the field digital devices while sink type modules draw current when the device is in high state
Sinking – When active the output allows the current to flow to a common ground.
Sourcing – When active, current flows from a supply, through the output device and to ground.
