Pick  want:      This article describes how to use COM20022 to implement a token bus LAN. introduction The bus contention of the bus local area network makes it unsuitable for some real-time control systems that have strict requirements on time. ARCNET (Attached Resource Computer NET) has high reliability, high communication speed, determinable network performance, and long-distance communication capabilities. Characteristic, it is an ideal fieldbus technology. It has the following three outstanding advantages: First, it uses a token-passing protocol to ensure that the message transmission is completed within a certain time; second, it supports variable-length data frames (0~507 bytes ) with low overhead , Coupled with its high bus rate ( up to 10Mbps) , makes it have a good response to short messages; third, the built-in 16 -bit CRC check and data link layer protocol make it have higher reliability and software easy to use. The ARCNET network is physically a bus network, but logically it is a token ring. COM20022 is SMSC's a high-speed embedded ARCNET controller, use COM20022 easy to achieve 10Mbps token bus LAN. COM20022 COM20022 supports the bus rate up to 10Mbps , and the rate can be set in the register. It can support a variety of microprocessors, the data bus port width is 8/16 bits optional, 2K bytes of dual-port RAM is integrated , and a programmable DMA channel is newly added . Software programmable node ID number, supports up to 255 nodes. The network is automatically reconstructed when nodes are added or reduced, and duplicate node ID detection is supported . All nodes on the network form a logical loop in the order of node ID from small to large. The token is passed between nodes along the logical ring. The node that obtains the token can send a data frame. If there is no data to send, Then the token is passed to the next logical node next to it. The token is passed in a loop to provide equal network access services for all nodes. Figure 1 COM20022 and 8051 interface Picture 2 Token bus local area network connected by coaxial cable   Picture 4 Token bus local area network with star bus structure COM20022 and microprocessor interface In order to adapt to more types of microprocessors, COM20022 has the function of automatically adapting to microprocessors without adding additional pins. It is compatible with both INTEL 's microprocessor ( as shown in Figure 1) and MOTOROLA 's microprocessor. There are two types of connected buses: composite mode and non-composite mode. COM20022 determines the type of its data bus through the state of the address line A0 . If A0 is grounded, COM20022 is a composite bus; if A0 is connected to the address line A0 of the microprocessor , COM20022 is a non-composite bus. The following is the initialization procedure for COM20022 to determine its data bus type: unsigned char Com20022Init(COM20022_CONFIG xdata *cfg) {                     ...    /*inTIalize the bus type of the Com20022*/    TO_ADDR_LO_REG( 0 );      /* Odd address write operation */    temp = FROM_ADDR_LO_REG( );    /* Odd address                                  Read operation */                     ...  } Perform a write operation to the odd address register of COM20022 , and then a read operation. Once the bus type is determined, COM20022 will keep this interface mode unchanged until the next reset. COM20022 network interface driver chip When COM20022 is interconnected, it is not necessary to add a driver when the distance is very close, such as inside the same instrument. When two or more (not more than 255) to be connected ARCNET devices, it is necessary to add the drive. Figure 2 and Figure 3 both use 75176 , which is a DC-coupled RS-485 driver. Because the RS-485 interface is relatively simple, it only needs Category 5 twisted pair to transmit, so installation and maintenance are more convenient. In order to select the transmission medium more conveniently, you can also use SMSC 's HYC9088A , which is a twisted pair and coaxial cable compatible driver, but it only supports a maximum bus rate of 2.5Mbps . If you need a longer transmission distance, you can use fiber optic transceivers, and the transmission distance can reach more than one kilometer. Networking COM20022 can be done by using the card, into the computer's expansion slots, so that the connecting cable composed of computers access token bus LAN, it may also be used for data transmission device COM20022 data transmission or point-multipoint. For better networking, HUB can also be used to expand the network. HUB is divided into two categories, one is passive HUB and the other is active HUB . Passive HUB is cheap. Passive HUB generally has 4 ports, used to connect the coaxial cable transceiver of the star structure. Active HUB is actually a circuit repeater, which can be interconnected using a variety of connection media. The coverage area is far longer than that of passive HUB , which can isolate cable faults and prevent signal reflection, and is often used in distributed star networks. When networking, a terminator is also needed. When using a coaxial cable, any unused passive HUB port or cable must be terminated with a 93 Ω terminator; when using a twisted pair cable, the terminator is inserted into the network card at the end of the cable Above, the resistance of the terminator is the value of the characteristic impedance of the twisted pair used. After the network card, HUB , and terminator are available, the network can be formed, and the network formed is a token bus LAN. There are two main ways of ARCNET token bus LAN wiring: one is bus type, the other is star bus type. Bus structure The bus structure in which the coaxial cable is the connection medium is similar to the Ethernet thin cable method, as shown in Figure 2 . The maximum length of the ARCNET bus is 305 meters. Up to 8 devices can be connected . The connection between the device and the bus is through a T -connector, the top of the connector is connected with the cable, and the bottom is connected with the network card. Both ends of the cable must be terminated with a 93W terminator. When a twisted pair is used as the connection medium, the network card suitable for this medium has two ports, one port is connected to the node, and the other port is connected to the next node. This cascade connection can connect up to 17 nodes ( using DC coupling). the RS-485). The redundant ports of the network cards at both ends can be connected to a terminator. Figure 3 shows a network card with two RJ11 connectors, and the two unconnected ports are connected to a 120W terminator. Star bus structure The star bus wiring is centered on the HUB , forming a multi-star interconnected structure, as shown in Figure 4 . This structure is called a bus because all workstations share a common cable. This structure can use coaxial cable or twisted pair. When using a coaxial cable, the HUB can be active or passive. If an active HUB is used , the bus cable is plugged into one of its ports, and the remaining ports are connected to the workstation. The maximum distance between the workstation and the active HUB is 610 meters. When using a twisted pair cable, an active HUB is required . The HUB is connected to the file server, and the workstation is also directly connected to it. In this way, the maximum distance between the server or workstation and the active HUB can be 1220 meters. The unused ports on the network card must be short-circuited with a 93 Ω terminator. Table 1 shows the maximum allowable length of the network segment and the maximum number of nodes allowed in the single-bus network segment ( bus transmission rate is 2.5Mbps) during networking . Concluding remarks There are many advantages in using COM20022 to implement a token bus LAN. In real-time control and other fields, the choice of media is flexible and the networking is convenient. It is an excellent ARCNET controller. At present, it has been well used in the digital in-vehicle communication system developed by our unit, and has achieved good results in data transmission and real-time voice communication.
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