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Abstract

A communication interface circuit transfers signals between a TTL microcontroller and a RS232 device while avoiding level translation. The interface circuit includes two switch elements. A first switch element is connected between the TTL receive terminal and the ground supply. The second switch element includes a first node that is an electrical communication with the TTL transmit data terminal, the RS232 transmit data terminal and the RS232 receive data terminal, a second node in electrical communication with the first TTL power supply and a control node in electrical communication with the TTL receive data terminal. The interface circuit is configurable to a first switching state in which electrically connects the first TTL power supply terminal to the RS 232 receive data terminal. In a second switching state, a -12V default voltage signal is conveyed from the RS232 transmit data terminal to the RS232 receive data terminal and a voltage signal is conveyed from the RS232 transmit data terminal to the TTL receive data terminal. [An interface circuit permits communication between an RS232 port and a TTL device without requiring level translation. This is accomplished by interposing a switching transistor between the RS232 port and the TTL device. Selective activation of the switching transistor permits a high voltage signal to be transmitted from the power supply rail of the non-RS232 device to the RXD pin of the RS232 port, where the signal is interpreted as a logical low. This step takes advantage of the fact that the RS232 standard interprets any voltage received at the RXD pin greater than a receiver threshold voltage to be logical zero. Selective deactivation of the switching transistor isolates the RS232 port from the non-RS232 device, permitting a negative voltage signal output by the TXD pin of the idling RS232 port to be conveyed back to the RS232 port at the RXD pin. This negative voltage signal is interpreted by the RS232 port as a logical high signal. This step takes advantage of the fact that the RS232 standard calls for the TXD pin to emit a -12V signal when the RS232 port is otherwise idle.]

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First Claim