Abstract:
A circuit for converting data of a signal line in the form of logical pulses of current to RS232 level voltages between an air-conditioner and a computer is described. The conversion circuit is able to troubleshoot the communication signals between the indoor unit and outdoor unit of an air-conditioner.

Description:
FIELD 
     A conversion circuit of the communication signals from an air-conditioner to RS 232 level voltages for inputting to a computer. 
     BACKGROUND 
     Microcontrollers control and monitor the overall operation of the air-conditioning system. By using a serial communication circuit to relay vital information to one another, microcontrollers are able to make the necessary decisions concerning the process being controlled. When a glitch occurs in the system, the communication signals exchange between the indoor unit and outdoor unit can be intercepted for diagnosis. By means of a data logger, the intercepted data can be recorded and viewed at a later time. Computer based data logging systems can be used for this purpose. It is common for a serial port or RS232 port of the computer to be used for receiving data. For the purpose of relaying the data transmitted by the air-conditioning system to the computer, an RS 232 converter circuit is required. 
     Generally, an RS232 transceiver integrated circuit such as the MAX232 and an opto-coupler can be used to perform the RS232 conversions as shown in  FIG. 1 . The opto-coupler first converts the logical pulses of current of the signal line to Transistor-Transistor Logic (TL) signals. The RS 232 transceiver integrated Circuit (IC) in turn, converts the TL voltage levels to RS 232 level voltages, which are then fed into the serial port of the computer. The transceiver IC uses charge pump circuits to step up the voltage in order to generate the RS 232 level signals from the TTL signals. These charge pump circuits however, require external capacitors to multiply the voltage. Usually, four capacitors with values between 0.1 pF to 1 pF need to be added externally to the transceiver IC in order for it to operate properly. The use of external capacitors will take up space on the printed circuit board. In addition to that, the transceiver IC also requires a stable external power supply typically in the region +5V for operation. 
     The Electronic Industries Alliance (EIA) standard RS 232 requires a receiver to distinguish between voltages as low as +3V and −3V, where positive level voltages are defined as logic ‘0’ whereas negative level voltages are defined as logic ‘1’. Typically, the positive level voltages can be in the range from +5V to +12V while the negative level voltage ranges from −5V to −12V, depending on the hardware. The large voltage swing between positive and negative is intended to make the transmission of data signals less susceptible to electrical noise. 
     It is an object of the circuits described herein to convert the data signal line from the air-conditioner, which is in the form of logical pulses of current to the RS232 voltage signals on the personal computer. 
     It is another object of the circuits described herein to troubleshoot the communication signals between the indoor unit and outdoor unit of an air-conditioner via the conversion circuit. 
     SUMMARY 
     A circuit for converting data of a signal line in the form of logical pulses of current to RS 232 level voltages between an air-conditioner and a computer comprising:
         a plurality of opto-couplers each having a phototransistor and a photodiode for inverting and electrically isolating the signals received from the signal line of the air conditioner,   one or more resistors,   a diode for protecting the photodiode against an accidental of reverse voltage,   a serial port for receiving an output voltage from the circuit to be connected to the computer, and   a light emitting diode for indicating the presence of the signals in the circuit.       

     A method of configuring a conversion circuit for converting data of a signal line in the form of logical pulses of current to RS 232 level voltages between an air-conditioner and a computer by means of software comprising:
         enabling pin  4  [Data Terminal Ready (DTR)] of the serial port as an output positive level voltage,   enabling pin  7  [Request To Send (RTS)] of the serial port as an output positive level voltage, and   idling pin  3  [Transmitted Data (TXD)] of the serial port as an output negative level voltage.       

     A method of generating logical ‘0’ by a conversion circuit into positive RS 232 voltage comprising:
         turning on the photodiodes in a first opto-coupler and a second opto-coupler wherein the phototransistor in the first opto-coupler will pull a first end of a first resistor to be low,   turning off the photodiode in a third opto-coupler wherein the phototransistor in the third opto-coupler will disconnect the negative level voltage at pin  3  from pin  2  of the serial port, and   turning on the photodiode in the first opto-coupler and the second opto-coupler wherein the phototransistor in the second opto-coupler will channel the positive level voltage at pin  4  to pin  2  of the serial port.       

     A method of generating logical ‘1’ by a conversion circuit into negative RS 232 voltage comprising:
         turning off the photodiodes in a first opto-coupler and a second opto-coupler wherein the phototransistor in the second opto-coupler will disconnect the positive level voltage at pin  4  from pin  2  of the serial port,   turning off of the photodiodes in the first opto-coupler and the second opto-coupler wherein the phototransistor in the first opto-coupler will turn off, and   turning on the photodiode in a third opto-coupler wherein the phototransistor in the third opto-coupler will connect the negative level voltage at pin  3  to pin  2  of the serial port.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a prior art circuit for linking a signal line to an RS 232 port of a computer. 
         FIG. 2  illustrates a schematic diagram of a converter circuit. 
         FIG. 3  illustrates a waveform of a signal line after its conversion by a converter circuit. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  illustrates a schematic diagram of a converter circuit. The converter circuit includes three opto-couplers, namely IC 1 , IC 2  and IC 3 , two resistors (R 1  and R 2 ), a light emitting diode (LED 1 ), a diode (D 1 ) and RS 232 serial port of a computer. The anode of photodiode of the first opto-coupler IC 1  is connected to the indoor signal line while the cathode of the photodiode of the second opto-coupler IC 2  is connected to the outdoor signal line. The cathode of the photodiode of the first opto-coupler is in turn, connected to the anode of the photodiode of the second opto-coupler, forming a series connection with a signal line. 
     The collector of the phototransistor of the first opto-coupler IC 1  is configured as an inverter and is connected to the anode of the photodiode of the third opto-coupler IC 3  to invert the signal entering IC 3 . The phototransistor of the second opto-coupler IC 2  is configured as a switch, linking pin  4  to pin  2  of the RS232 serial port. The phototransistor in the third opto-coupler IC 3  is configured as a switch, linking pin  3  to pin  2  of the RS232 serial port. The first resistor R 1  is connected between the collector of the phototransistor in the first opto-coupler and pin  7  of the RS232 serial port as part of the inverting circuit. The emitter of the phototransistor of the first opto-coupler and the cathode of the photodiode of the third opto-coupler are connected to pin  5  of the RS232 serial port. The second resistor R 2  and the light emitting diode LED 1  are connected in series between pin  2  and pin  3  of the RS232 serial port to function as an indicator for presence of signals. Further, the cathode of diode D 1  is connected to the indoor side of the signal line, while its anode is connected to the outdoor side of the signal line. 
     Two pins of the RS 232 port (DB9) namely  7  and  4  provide positive voltage (+10 V) while pin  3  provides negative voltage (−10V). Pin  7  [Request To Send (RTS)] and pin  4  [Data Terminal Ready (DTR)] are enabled by software to obtain positive voltage (+10V) while pin  3  [Transmitted Data (TXD)] is left idle to obtain negative voltage (−10V). Pin  2  [Receive Data (RXD)] is the receive pin of the RS232 port. Pin  5  is the common ground of the RS 232 serial port. 
     For use, when there is low or no current flow through the signal line, IC 3  is turned on linking pin  2  to pin  3 . This puts pin  2  at the same potential as pin  3  (−10 V). When a positive pulse (high) is sent through the signal line, IC 1  and IC 2  are turned on. When IC 1  is on, the voltage at point A is pulled low, towards common ground, causing IC 3  to turn off. With IC 3  off, pin  2  is disconnected from pin  3  (−10 V). At the same time, the turning on of IC 2  connects pin  2  to pin  4  (+10 V). In this way, the pulses of current flowing through the signal line are converted RS232 voltages. Low pulse is converted to negative voltage, −10 V (logic ‘1’) while high pulse is converted to positive voltage, +10 V (logic ‘0’) as shown in  FIG. 3 . 
     D 1  is a general-purpose diode connected across the input to protect the photodiodes of IC 1  and IC 2  against accidental application of reverse voltage. Whenever the signal line is at logic high, LED 1  lights up. When data is transmitted though the signal line, LED 1  is turned on and off intermittently (blinks). 
     The converter circuit further uses a passive approach in the RS 232 conversion process compared to the RS 232 transceiver integrated circuit (IC). The converter circuit is formed predominantly of opto-coupler circuits, which interfaces directly with the computer serial port (RS 232). The ±10V to ±5V range of voltages available at the serial port of computers are manipulated by the opto-couplers based on the logical state of the incoming signal line and are channeled back to the receiving pin of the serial port. These opto-couplers also electrically isolate the RS 232 port of the computer from the signal line and allow data to be intercepted without obstructing the current flow of the signal line. 
     One of the advantages of the converter circuit described herein is that the circuit does not require an external regulated power supply typically +5V, vital in most digital and logic circuits. Further, the converter circuit described herein is able to support and connect to various computer hardware, which may have different RS232 level of voltages. This circuit has the inherent property of electrical isolation between the signal line and the serial port of the computer due to the use of opto-couplers, which will protect RS232 port of the computer from electrical surges. 
     Finally, while the above discussion and figures describe an embodiment of an interface circuit between an RS 232 device and a dissimilar peripheral, the circuits described herein are not limited to an RS 232 device. 
     Given the above description and the variety of embodiments described therein, it is intended that the following claims define the scope of the present invention, and that the devices and processes within the scope of these claims and their equivalents be covered hereby.