Communication interface between a TTL microcontroller and a RS232 Device avoiding level translation

• US 6,738,855 B1
• Filed: 01/14/2000
• Issued: 05/18/2004
• Est. Priority Date: 01/14/2000
• Status: Expired due to Term

• Current Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Original Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Inventors: Goldman, Steven J.
• Primary Examiner(s): Thai, Xuan M.
• Assistant Examiner(s): King, Justin
• Application Number:
  US09/483,628
• Time in Patent Office: 1,586 Days
• Field of Search: 710/305, 710/71, 710/100, 710/315, 710/62, 710/1, 710/65, 326/62, 375/377, 375/257, 375/259, 708/232, 361/58, 327/417, 340/825.52, 370/359, 713/300
• US Class Current: 710/315
• CPC Class Codes: G06F 13/4072 Drivers or receivers G06F13/4086 takes precedence; for multistate logic circuits H03K19/0002
• CPC Class Codes: H03K 19/01806 Interface arrangements
• CPC Class Codes: H04L 25/45 using electronic distributors

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.]

Claims

1. An interface circuit that transfers signals between a TTL microcontroller and a RS232 device,

wherein the TTL microcontroller includes a TTL receive data terminal, a TTL transmit data terminal, a first TTL power supply terminal configured to convey a +5V power supply, and a second TTL power supply terminal configured to convey a ground power supply, and

wherein the RS232 device includes a first RS232 power supply terminal configured to bear a +12V power supply, a second RS232 power supply terminal configured to bear a −

12V power supply, a RS232 receive data terminal configured to convey a received data signal, and an RS232 transmit data terminal configured to convey a −

12V default voltage signal when the RS232 receive data terminal receives a data signal, and wherein the RS232 device interprets the received data signal traversing a +3V receiver threshold value as a low logic state and interprets a received data signal not traversing the +3V receiver threshold value as a high logic state,

the interface circuit comprising;

(a) a first switch element disposed between the TTL receive terminal and the ground power supply; and

(b) a second switch element that includes;

(i) a first node that is in electrical communication with the TTL transmit data terminal, the RS232 transmit data terminal and the RS232 receive data terminal;

(ii) a second node that is in electrical communication with the first TTL power supply terminal; and

(iii) a control node that is in electrical communication with the TTL receive data terminal, and

wherein the interface circuit has a first switching state which electrically connects the first TTL power supply terminal to the RS232 receive data terminal, and a second switching state which the −

12V default voltage signal from the RS232 transmit data terminal to the RS232 receive data terminal, and in which second switching state a voltage signal is conveyed from the RS232 transmit data terminal to the TTL receive data terminal.

2. The interface circuit according to claim 1, and wherein:

the second switch element comprises a PMOS transistor;

the first node comprises a drain of the PMOS transistor;

the second node comprises a source of the PMOS transistor; and

the control node comprises a gate of the PMOS transistor.

3. The interface circuit according to claim 1, and wherein:

the second switch element comprises a PNP bipolar transistor;

the first node comprises an emitter of the PNP bipolar transistor;

the second node comprises a collector of the PNP bipolar transistor; and

the control node comprises a base of the PNP bipolar transistor.

4. The interface circuit according to claim 3, and wherein the TTL microcontroller includes a pin addressable eight-bit parallel port and a software program instructing the TTL microcontroller to synchronize and conduct serial-to-parallel conversion to signals communicated between the RS232 device and the TTL microcontroller.