Interfacing an AT Keyboard with RS232 Communication

A complete fetch and decoding of AT keyboard scan patterns are being shown in this project along with the RS232 transmission of ASCII characters to the RS232 client.

The device has a unidirectional communication between the microcontroller and keyboard and also between microcontroller application and remote RS232 client. It can support all keyboard characters typed with shift button active and inactive. The Caps Lock is implemented as well as the Num Lock being always active. A terminal program such as the Windows Hyperterminal is used to visualize the ASCII data sent by the microcontroller application.

The keyboard can send data to the host microcontroller and the host can send commands to the keyboard, implementing a bidirectional protocol on the data transfer. The host can send a command to the keyboard anytime since it has the ultimate priority over the direction. The keyboard generates the serial clock but can be used by the host as a clear-to-send line. A so-called release-flag is used as a key for release handling when the keyboard sends slightly different scan patterns for both key hit and key release. This executes the scan pattern decoding routine twice.

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