At present, the mainstream CAN protocol controller generally uses an I/O bus (SJA1000, etc.) or an SPI interface (MCP2515, etc.) to communicate with the MCU. Because this design uses PC / 104 bus expansion card to expand multiple RS232 and RS485 interfaces, no redundant I / O chip selection line is available, so the final use of 9200 SPI interface and MCP2515 for multi-channel CAN bus interface expansion. The MCP2515 is a standalone CAN controller with SPI interface from Microchip. It fully supports the CAN V2.0B specification, with a communication rate of up to 1 Mbps, including 3 transmit buffers, 2 receive buffers, 6 29-bit acceptance filter registers, and 2 29-bit acceptance mask registers; The SPI interface clock frequency can be up to 10 MHz, which can meet the needs of an SPI host interface to extend the multi-channel CAN bus interface.
The figure is a block diagram of the peripheral CAN bus interface of the MCP2515. The interface parts of the MCP2515 and 9200 are omitted. Since the equipment needs to be installed along the railway, it must have the ability to resist lightning strikes. Therefore, the MCP2515 and the CAN bus transceiver use a high-speed optocoupler for complete electrical isolation, and the power supply of the circuit at both ends of the optocoupler must also be isolated by the power isolation module, so as to truly isolate. Two 30 pF capacitors can be connected between the CANH and CANL pins of the TJA1050 and ground to filter high frequency interference on the CAN bus; two diodes can protect against transient voltage disturbances on the bus voltage. When the optocoupler works normally, the input current is about 10 mA, and the forward voltage drop of the internal LED is about 1.7 V. Therefore, pay special attention to the resistance selection of the series resistor at the input end.
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