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MPI / DP / PPI are all based on RS485 network, the difference is that the communication protocol used is different, and the application place is also different. MPI / DP for 300 / 400, PPI for 200
MPI is used for short communication distance and small amount of data,
it is generally used for connection programming PG, field OP, master-slave communication, DP is generally used for master-slave communication and field level real-time communication,
the communication rate is high
the difference between DP port and MPI port depends on the CPU used. If the 9-pin DP port represented on the CPU is MPI / DP, then this port can be configured as DP port or MPI port by hardware. If it is only represented as MPI port, then it can only be used as MPI port
and DP port can only be used as DP port
RS485 and RS232 are all serial ports,
RS232: the RS232 interface connector uses a 25 core plug socket of db-25. The RS-232 interface between some equipment and PC does not use the other party's transmission control signal, so only three interface lines are needed, namely "send data", "receive data" and "signal ground". Therefore, DB-9 core plug socket is adopted, and shielded twisted pair is adopted for transmission line
RS485: RS485 has no specific physical shape, and the interface is adopted according to the actual situation of the project
MODBUS is a serial communication protocol, which is a common connection mode between instrial electronic devices. Including ASCII, RTU, TCP and so on, there is no physical layer. The physical interface of Modbus communication can be serial port (including RS232 and RS485) or Ethernet port
its communication follows the following process:
1. The master device sends a request to the slave device
2. The slave device analyzes and processes the request of the master device, and then sends the result to the master device
3. If there is any error, the slave device will return an abnormal function code
Due to the early appearance of RS232 interface standard, it is inevitable to have some shortcomings, mainly including the following four points:1. The signal level value of the interface is high, which is easy to damage the chip of the interface circuit, and it is not compatible with TTL level, so it is necessary to use level conversion circuit to connect with TTL circuit
In asynchronous transmission, the baud rate is 20kbps The interface uses a signal line and a signal return line to form a common ground transmission form, which is easy to proce common mode interference, so the anti noise interference is weak The standard value of the maximum transmission distance is 50 feet, which can only be used for about 50 meters In view of the deficiency of RS232 interface, some new interface standards have appeared continuously, and RS-485 is one of them, which has the following characteristics:1, the electrical characteristics of RS-485: Logic "1" is represented by the voltage difference between two wires as + (2-6) V; Logic "0" is represented by - (2-6) V voltage difference between two lines. The interface signal level is lower than RS-232, so it is not easy to damage the chip of interface circuit, and the level is compatible with TTL level, so it is convenient to connect with TTL circuit
The maximum data transmission rate of RS-485 is 10Mbps The RS-485 interface is a combination of balanced driver and differential receiver, which enhances the ability of anti common mode interference, that is, good anti noise interference The standard maximum transmission distance of RS-485 interface is 4000 feet, which can reach 3000 meters. In addition, only one transceiver can be connected to RS-232 interface on the bus, that is, single station capacity. The RS-485 interface allows up to 128 transceivers to be connected on the bus. That is to say, it has the ability of multi station, so that users can use a single RS-485 interface to establish the equipment network convenientlythe realization of channel between PLC and PC can make the two complement each other in function. PLC is convenient and reliable in control, while PC has strong function in graphic display, data processing, printing report and Chinese display. Therefore, the PLC manufacturers have developed various types of PLC and PC communication interface moles for our company. Fx-232aw interface mole developed by Mitsubishi company is used for communication between FX2 series PLC and computer. There are also interface mole aj71e71 connected with Ethernet, interface mole aj71m51-s1 connected with map network, interface mole aj71m51m1 connected with FAIS map network, etc. Different communication methods have different cost, price and application scope. This paper introces a method of communication between PC and PLC through RS-232 port
the programming interface of FX2 series PLC adopts RS-422 standard, while the serial interface of computer adopts RS-232 standard. Therefore, as the interface circuit of PLC computer communication, RS-422 standard must be converted into RS-232 standard
RS-232 and RS-422 are different in signal transmission, logic and electricity
RS-232 adopts single ended receiver and single ended transmitter, only one signal line is used to transmit information, and the logic "1" (- 3 ~ - 15V) and "0" (+ 3 ~ + 15V) are determined according to the level of the signal line relative to the common signal ground level
RS-422 standard is a balanced transmission standard, that is, two terminal transmission and two terminal reception. The logic state is determined by the potential difference between two transmission lines. RS-422 circuit consists of transmitter, balanced connecting cable, cable terminal load and receiver. It converts the logic level to the potential difference by balancing the transmitter and the differential receiver (+ 2V for "0", - 2V for "1")
max202 of Maxim company is selected to realize the level conversion between RS-232 and TTL. Max202 has voltage multiplication circuit and conversion circuit, which only needs + 5V power supply to work, so it is very convenient to use; MAX490 is selected to realize the conversion between RS-485 and TTL. Each MAX490 has a pair of transmitter / receiver. Because of the full plex communication mode, it needs two max490s and only four capacitors
the RS-422 interface of PLC is equipped with db-25 connector, while PC generally uses DB-9 connector. The hardware circuit diagram is shown in Figure 1
the RS and CS of RS-232 are short circuited, so that the PLC is always ready for the computer to send data. In other words, the computer can send data to PLC at any time. Because Dr and ER are cross connected, the computer must wait until the PLC is ready to receive data<
2 software description of communication device
2.1 communication protocol between FX2 series PLC and computer
the communication between FX2 series PLC and computer adopts RS-232 standard, its transmission rate is fixed at 9600bps, and parity bit adopts even check. The data format is shown in Figure 2. Data is sent and received in frames. A multi character frame consists of five parts as shown in the figure. The sum check value is to add the ASCII codes (hexadecimal numbers) of the characters between the command codes ETX to obtain the lowest two digits of the sum. STX and ETX represent the start and end marks of the character frame respectively
the communication between FX2 series and computer is based on the initial command issued by the host computer and the response of PLC. There are four kinds of commands: 0, 1, 7 and 8. The upper computer can read and write PLC and set it forcibly. Through enq, ACK and NAK, the host computer coordinates the communication response with PLC
2.2 communication process
bland C is used to write the communication program between host and PLC. First of all, the COM1 port must be initialized, the baud rate is 9600bps, the parity bit adopts even check, and seven bits of valid data. The initialization detection process of communication is shown in Figure 4. When the computer receives the response character ack from PLC, it can enter the data communication. The communication sequence diagram is shown in Figure 5
the computer can read, write and force on / off operation to all soft devices in PLC. In addition to the PLC Timer and counter set value constant, as well as the data in the file register, all the switch input, output and software equipment of FX2 series PLC are transparent to the computer. The format of the multi character frame is shown in Figure 3. But different operations are different within the "multiple characters" entry. For example, the computer reads the soft equipment Y20 ~ y37 of PLC, looks up the device address table as 00a2, and reads 2 bytes of data. Its status is shown in Figure 6 (a), the command format issued by the host is shown in Figure 6 (b), and the information returned by PLC is shown in Figure 6 (c)
if there is an error in the transmitted command, the PLC will return the NAK signal. If this operation fails, it will be repeated
in order to ensure the accuracy of the communication between the host and PLC, the upper computer must also carry out the calibration according to the communication protocol. If the received information is wrong, read it again. If it can not be repeated three times, an error message will be displayed
it is easy to realize the above programming with C language
the host computer can also write data to PLC for single point strong proximity setting and reset
for the rapid operation in the control and laboratory environment, this scheme has the advantages of reliable operation, high cost performance, small size, but for the long communication distance and harsh environment, it needs to add photoelectric isolation and other measures.
Instrial Ethernet switch, which is used in the field of instrial control Ethernet switch equipment, e to the use of network standards, it has good openness and wide application; Can adapt to low temperature and high temperature, strong anti electromagnetic interference, anti salt spray, strong earthquake resistance. Using transparent and unified TCP / IP protocol, Ethernet has become the main communication standard in the field of instrial control. It is mainly used in instrial control automation, road traffic control automation, building automatic control system, mine automatic control system, oilfield control automation, hydropower station control automation, power system control automation, computer room monitoring system
Ethernet optical terminals are widely used in CCTV monitoring and security applications from low end to middle end in the instrial security market. It is suitable for 5.5 km (3.5 miles) or shorter distance transmission, such a system can provide
qualitative video performance is quite good, and can always meet the quality requirements of rs-250c long distance transmission. However, am video transmission equipment is only suitable for 850nm. Multimode operating wavelength limits the most
available transmission distance. More significantly, for every 1dB of optical path loss, the linear correlation attenuation of SNR based on am system is 2dB, so the acceptable video transmission quality can only be obtained at relatively short
optical cable distance. Some manufacturers' equipment may need receiver gain adjustment in the initial installation stage, which makes the installation process complicated. Finally, am procts can not meet the technical requirements of rs-250c medium and short distance video transmission that its and high-end instrial security should
meet today.