Pick   To:       This article describes how to use COM20022 to implement token bus LAN.
introduction
The bus contention of the bus LAN makes it unsuitable for some real-time control systems that have strict requirements on time, and ARCNET (Attached Resource Computer NET) has high reliability, high communication rate, determinable network performance, and long-distance communication capabilities. Features are an ideal fieldbus technology. It has the following three outstanding advantages: first, it uses the token transfer protocol to ensure the completion of the message transmission within a certain time; second, it supports variable-length data frames (0 ~ 507 bytes ) , and the additional overhead is small , Coupled with its high bus rate ( up to 10Mbps) , it has 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 , the rate can be set in the register. Can support a variety of microprocessors, data bus port width 8/16 bit optional, integrated 2K bytes of dual-port RAM , new programmable DMA channel. Software programmable node ID number, supporting a maximum of 255 nodes. The network automatically rebuilds when adding or reducing nodes, and supports duplicate node ID detection. All nodes on the network form a logical loop in the order of the node ID from small to large. The token is passed between the nodes along the logical ring. The node that obtained the token can send a data frame. If no data is sent, Then the token is passed to the next logical node. Token circulation is passed to provide equal network access services for each node.
Picture 1 Â COM20022 and 8051 interfaces
Picture 2 Â Token bus LAN connected by coaxial cable
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Picture 4 Â Token bus LAN with star bus structure
Interface between COM20022 and microprocessor
In order to adapt to more types of microprocessors, COM20022 does not add extra pins, it has the function of automatically adapting to microprocessors. Not only compatible with INTEL 's microprocessor (see Figure 1) , but also compatible with MOTOROLA 's microprocessor. There are two ways of accessing the bus: composite and non-composite. COM20022 determines the type of its data bus through the state of address line A0 . If A0 is grounded, COM20022 is a composite bus; if A0 is connected to the microprocessor's address line A0 , then 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 * /
                    ...
 }
First write a write operation to the odd address register of COM20022 , and then perform 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, no driver can be added 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. Both Figure 2 and Figure 3 use 75176 , which is a DC-coupled RS-485 driver. Because the RS-485 interface is relatively simple, only 5 types of twisted pairs can be transmitted, so installation and maintenance are more convenient. In order to more easily choose the transmission medium, 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 to transmit a longer distance, you can use fiber optic transceivers, the transmission distance can reach more than one kilometer.
Networking
Using COM20022 can be used as a network card, inserted into the expansion slot of the computer, and the connected computer can be used to form a token bus LAN. The COM20022 can also be used as a data transmission device to achieve point-to-point or multi -point data transmission. 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 with star structure. Active HUB is actually a circuit repeater, which can be interconnected using a variety of connection media. Coverage range is farther than passive HUB , can isolate cable faults and prevent signal reflection, often used in distributed star network. When networking, a terminator is also needed. When using a coaxial cable, any unused passive HUB ports or cables must be terminated with a 93 Ω terminator; when using twisted pair, the terminator is inserted into the network card at the end of the cable Above, the terminator resistance is the value of the characteristic impedance of the twisted pair used. Once the network card, HUB , and terminator are available, you can set up the network. The network formed is the 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 with coaxial cable as 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 -type connector, the top of which is connected to the cable, and the bottom is connected to the network card. Both ends of the cable must be terminated with 93W terminators.
When using twisted pair as the connection medium, the network card suitable for this medium has two ports, one port is connected to this node, and the other port is connected to the next node. In this way, a maximum of 17 nodes can be connected ( using DC coupling the RS-485). Terminators can be connected to the redundant ports on both ends of the network card. Figure 3 shows a network card with two RJ11 connectors. Two unconnected ports are connected with 120W terminators.
Star bus structure
The star bus wiring is centered on the HUB to form 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 twisted pair, you need to use active HUB , HUB is connected to the file server, and the workstation is also directly connected to it. In this way, the distance between the server or workstation and the active HUB can be up to 1220 meters. The unused ports on the network card must be shorted 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 (the bus output rate is 2.5Mbps) when networking .
Conclusion
Using COM20022 to achieve token bus LAN has many advantages. In real-time control and other fields, the media selection is flexible and the networking is convenient. It is an excellent ARCNET controller. At present, it is 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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