Industrial Automation Bus System - ASI Bus

ASI（Actuator-Sensor Interface）is a bus network used for bidirectional, multi-site digital communication between controller (master station) and sensor / actuator (slave station). It is composed of master station, slave station and transmission system, and the transmission system is composed of two-core transmission cable, ASI power supply and data decoupling circuit.

1 Transmission cable

The recommended cable model for the ASI bus is CENELEC or DIN VDE 0281 [CENE-90], and it should be marked HO5VV-F2x1.5. It is a two-core flexible power cord with a cross-sectional area of 1.5mm2 and cheap and ubiquitous. The other is the ASI special flat cable with the same electrical characteristics, which is very convenient to install. Because the ASI cable not only transmits the signal but also provides the power supply, two technical indexes must be paid attention to when selecting the cable, the first is the communication spectrum characteristic, and the second is the DC impedance characteristic. When it is considered that there is a large interference, it is necessary to choose to use a shield cable, such as the type (N) YMHCY-02x1.5 cable, but it must also meet the specified spectral characteristics requirements. It should be noted that the shield can only be grounded at the ASI power terminal, but not at the ASI+ and ASI- terminals.

The voltage of the ASI power supply is 29.51-31.5VDC, and the supply voltage provided by each slave station to the sensor / actuator is VDC (+ 10 [%] or-15 [%]). In an ASI bus system, the maximum current that the ASI power supply can provide to 31 slave stations is 2A, so the average current consumed by each slave station is 65mA. If the power of the actuator driven by the slave station is large and the required current is greater than 65mA, an external auxiliary power supply must be connected. The maximum voltage drop on the ASI cable allowed by the whole system is 3V, so the cross-sectional area of the cable can not be less than 1.5mm2, so as to ensure that each slave station in the network can get the specified voltage.

The equivalent circuit model of ASI cable can be divided into two models: two-core cable and shielded two-core cable. The values of resistance (R'), capacitance (C'), inductance (L') and conductance (G') are equivalent parameters of ASI cable. When the transmission rate is 167Kb/s, the range of the total limit parameters of the two-core cable are R’=20-50mΩ /m，L’=200-600nH/m，C’=35-70pF/m，G’=1-3μS/m. At the same transmission rate, the limit parameters of the shielded two-core cable are R’s=10mΩ/m，Ls’=800nH/m，Cs’=300pF/m，Gs’=15μS/m.

The relationship between complex impedance and transmission rate of ASI cable is of great significance to the response characteristics of the system. When the transmission rate is 167Kb/s, the impedance is 80-120Ω, but when it is lower than or higher than 167Kb/s, the impedance will decrease rapidly. So when the transmission rate of 167Kb/s is adopted, the maximum signal amplitude will be obtained.

2 Signal Modulation Process

ASI signal should be modulated before transmission and many factors should be considered in which modulation method is adopted. For example, the transmission signal attached to the power supply voltage must be alternating; the two-way communication between the master station and the slave station requires that both masters can produce a simple, effective and time-saving narrowband transmission signal; there should not be too much interference when using unshielded cables, and so on. ASI signal is modulated by alternating pulse modulation (APM), which is a serial communication mode modulated at fundamental frequency.

The bit sequence of the request signal sent by the master station is first converted into a bit sequence that can perform phase conversion, that is, Manchester II coding, thus generating the corresponding transmission current. When the transmission current passes through the inductor, there will be a voltage sudden change and then result in the request signal voltage. Each increased current produces a negative voltage pulse and each reduced current produces a positive voltage pulse, in this way, the request signal can be easily obtained from the station. Because the signal is superimposed on the power supply, the signal voltage is sometimes greater than the power supply voltage of the slave station. Inductors are not needed in the slave station, which makes the integrated slave circuit with Slave Chip elements on the smart sensor / actuator smaller, simpler and more economical. When the request signal voltage on the cable is received from the slave station and converted into the initial bit sequence, the conversion process of the request signal from the master station to the slave station is completed.

The voltage pulse of the signal transmission is designed as a sine square wave mode, but the reliability can be improved by selecting the appropriate transmission waveform to take into account the influence of low frequency interference. In the prescribed topology of the modulated signal, the interval of every two-bit pulse signal is only 6 μ s.

3 Power and Data Decoupling

ASI power supply connects with the data decoupling circuit and ASI power supply can provide 29.5-31.6VDC voltage for it. ASI power supply and data decoupling connection, ASI power supply can provide 29.5-31.6VDC voltage for the circuit, fully meet the technical requirements of the International Electrotechnical Commission (IEC) for safe isolation of low voltage, maximum output current of 2A, and reliable short-circuit overload protection. The data decoupling circuit consists of two 50 μ H inductors and two 39 Ω resistors in parallel with each other. The current pulse of the transmission signal can be converted into a voltage pulse through the inductor. At the same time, it can prevent the short circuit caused by the data transmission frequency signal passing through the power supply. The two resistors represent the boundary terminals of the network. In order to minimize the signal noise of the circuit, a high symmetry circuit structure must be adopted, the two capacitors CE and two inductors L should be completely equal, the grounding should be reliable, and if the shielded cable is used, the shielding layer should also be connected to the ground. If the 2A current still does not meet the requirements of the slave station, the slave mode with auxiliary power supply or a repeater with additional power supply must be used.

4 Access Methods and Messages

The ASI bus system is a master-slave structure and adopts request-reply access mode. The master station first sends a request signal, which includes the address of the slave station. The slave station that receives the request will reply within the specified time, and only 1 master station and up to 31 slave stations will communicate at any time. There are two general access methods: one is multi-host access with token passing. The other is CSMA/CD, which has priority selection and frame transmission process. The access mode of ASI is relatively simple. In order to reduce the cost of the slave station and improve the flexibility, on the one hand, it includes as many parameters and information as possible without increasing the transmission cycle. On the other hand, the time of the transmission cycle should be automatically adjusted. For example, when there are only 6 slave stations in the system, the transmission period is 1ms. While the transmission cycle of 31 slave stations is about 5ms. If there is a short-term interference on the network, the master station does not receive the response signal from the slave station or receives an error and invalid signal, the master station can resent the information without repeating the whole transmission cycle.

The total transfer rate of the ASI bus is 167Kb/s, which includes all functionally necessary pauses. The allowable network transmission rate is 53.3Kb/s, and its transmission efficiency is 32 [%], which is better than that of other fieldbus systems. However, further measures should be taken to ensure the reliability of data transmission in the environment of electromagnetic interference.

An ASI message consists of four links: master request, master pause, slave reply and slave pause. All the requests of the master station are 14 bits, the response of the slave station is 7 bits, and the time length of each bit is 6 μ s. The suspension of the main station should be at least 3 digits and 10 digits at most. If the slave station is synchronized, the slave station can send a reply signal after the 3-bit pause of the master station. If it is not a synchronous signal, then the slave station must send a reply signal after a 5-bit pause. Because during this period, the slave station will monitor the pause of the master station after receiving a complete and valid request signal to see if there is any other information. However, if the master station does not receive the starting bit of the reply signal of the slave station after 10 pause bits, the master station will think that there is no longer a reply signal and send a request signal for the next address. The pause of the slave station has only 1 or 2 bits of time.

In the ASI message, the master request consists of the following specific information,

ST Start Bit, starts the master request. 0 is valid and 1 is invalid.

SR Control Bit, data / parameter / address bit or command bit, 0 is data / parameter / address bit, 1 is command bit.

A0~A4 Slave Address Bit, the slave address (5 bits) that is accessed.

I0~I4 Information Bit, transmit information (5 bits), request type.

PB Parity Bit, in the master station request information, the sum of the bits whose end bit is 1 must be an even number.

EB End Bit, request ends. 0 is invalid and 1 is valid.

The slave reply in the ASI message consists of the following specific information,

ST Start Bit, starts the slave request. 0 is valid and 1 is invalid.

I0~I3 Information Bit (4 bits), transmit information (4 bits), reply type

PB Parity Bit, in the SLAVE station reply information, the sum of the bits whose end bit is 1 must be an even number.

EB End Bit, reply ends. 0 is invalid and 1 is valid.

5 Master Request and Slave Reply

In the master-slave structure of ASI, the messages sent by the master station play an important role in the system data exchange. There are 9 kinds of request messages in the master station,

(1) data exchange requires the slave station to upload the survey data to the master station, and the master station can issue control instructions to the slave station.

(2) write the parameter setting function of the slave station, such as the measuring range of the sensor, activating the timer, changing the measuring method in the multi-sensor system and so on.

(3) address allocation is valid only when the slave address is 00H. After receiving this request, the slave station answers with 06H, indicating that the correct request from the master station has been received. From then on, the slave station can be called at this new address, and the new address is stored in the EEPROM of the slave station. This process approximately requires 15ms. In this way, the master station can automatically set the original address of the slave station which is damaged and replaced in operation.

(4)  reset to restore the called slave station address to the initial state, and the slave station answers with 06H. The whole process requires 2ms.

(5) delete operation address temporarily change the address of the called slave station to 00H, which is generally used with the "address assignment" message. When the new address is determined, the slave station answers with 06H. If you use the instruction "Reset- ASI - Slave", you can restore the original address.

(6) read the I/ O configuration. Read ID Encoding Slave Station I/ O settings and ID codes have been determined at the factory and cannot be changed.

(7) the purpose of combined use is to determine the identity of the slave station.

(8) State reading reads four data bits in the slave station state buffer to obtain error messages that occur during addressing and reset.

(9) read status and state deletion read out the contents of the slave state buffer, and then delete.

Among the above nine kinds of master station request messages, there are two kinds of data and parameters transmission, two kinds of setting and changing slave station address, and five kinds of slave station identification and query. Table 1 lists the names and contents of 9 kinds of messages from the master station.

6 Transmission Fault Characteristics

If the high-speed ASI transmission communication is carried out on the unshielded cable, then the electromagnetic compatibility (EMC) problem is very important. The emission interference and the field intensity radiation interference should not exceed the limit given by the European standard EN55011. The anti-jamming capability of the ASI system has been described in detail in the IEC801 file. A large number of test data of ASI system show that because the transmission signal uses sine square wave, the transmit interference of ASI system is kept below the specified value of IEC. The resistance of the ASI system to the electromagnetic high-speed instantaneous interference of the static discharge in the 26M-1GHz frequency range can reach level 3. In the worst case, the communication will fail, but the system has the function of detection and can resend the message. Because it is a short message, retransmission will not increase the cycle time, only when a serious error occurs in the message, it will increase the cycle length of the message. When the bit transmission error rate is in 70b/s, the system cycle is about 5ms; if the error rate is higher and the cycle time does not change much. And ASI can still maintain all its functions. Only when the error exceeds the 5000b/s, the normal data transmission is difficult to maintain.

When the ASI cable is cut off (such as incorrect short connection or failure disconnection), the master station will not be able to access the slave station on the other side of the break-point, while the slave station on the other side of the master station can still be called by the master station. The master station can diagnose and send out fault signals through the management service program, but the premise is that the data decoupling circuit and the power supply should be on the same side, otherwise the system will be completely paralyzed. If a repeater is not used in the ASI system, the ASI system will stop working when the power supply fails and no information about the fault will be obtained. However, if a repeater is used, because the repeater can supply power to the network, the impact of power failure will be reduced and part of the function of the system will be maintained.

The transmission system of ASI bus is the path and bridge connecting master station, slave station, power supply, controller, sensor / actuator in the network system. Message signal in the transmission system has to go through many times of transformation and recovery, and to resist a variety of external interference to ensure accurate, fast and reliable information exchange, it is an important part of the ASI bus system.