2. The principle of the access controller is that the access controller works in two modes. One is the inspection mode and the other is the recognition mode.
In patrol mode, the controller constantly sends query codes to the card reader and receives reply commands from the card reader. This mode will remain until the card reader detects the card. When the card reader senses the card, the card reader gives different replies to the patrol command of the controller. In this response command, the card reader sends the code data in the proximity card to the access controller, so that the access controller enters the identification mode.
In the identification mode of the access controller, the access controller analyzes the internal code of the induction card and compares it with the card data stored in the device to implement the follow-up actions. After receiving the data, the access controller will send a command to reply to the card reader, so that the card reader can be restored, and at the same time, the access controller will return to the inspection mode.
Extended data:
fundamental function
1, data buffering: Because the speed of I/O devices is low but the speed of CPU and memory is high, a buffer must be set in the controller. When outputting, this buffer is used to temporarily store the data transmitted by the host at high speed, and then the data in the buffer is transmitted to the I/O device at the rate owned by the I/O device; At the time of input, the buffer is used to temporarily store the data sent by I/O devices, and then transmit the data in the buffer to the host at high speed after receiving a batch of data.
2. Error control: The device controller is also responsible for error detection of data transmitted by I/O devices. If an error is found in transmission, an error detection code is usually set and reported to the CPU, so the CPU invalidates the data transmitted this time and retransmits it. This can ensure the correctness of data input.
3. Data exchange: This refers to the realization of data exchange between CPU and controller, and between controller and equipment. For the former, the CPU writes data to or reads data from the controller in parallel through the data bus; For the latter, it is a device that inputs data into the controller or transmits data from the controller to the device. Therefore, a data register must be set in the controller.
4. State description: The controller that identifies and reports the state of the device should write down the state of the device so that the CPU can know it. For example, only when the device is in the transmission ready state can the CPU start the controller to read data from the device. Therefore, a state register should be set in the controller, and each bit in the register reflects a certain state of the device. When the CPU reads the contents of this register, it can know the status of the device.
5. Receiving and identifying commands: The CPU can send various commands to the controller, and the device controller should be able to receive and identify these commands. Therefore, there should be corresponding control registers in the controller to store the received commands and parameters and decode the received commands. For example, the disk controller can receive 15 different commands from the CPU, such as reading, writing and formatting, and some commands also have parameters; Therefore, there are many registers and command decoders in the disk controller.
6. Address identification: Just as every unit in the memory has an address, so does every device in the system. The device controller must be able to identify the address of every device it controls. In addition, in order for CPU to write (or read) data into registers, these registers should have unique locations.
References:
Baidu Encyclopedia-Controller