The present invention relates to circuitry for isolating a detachaole external device from electronic equipment containing the signal processing circuitry for the external device.

Current consumer electronic equipment often is expandable by the consumer. For example, current receivers for direct satel¬ lite television systems (DSS) include plugs nto which infrared (IR) remote control receivers and transmitters may be plugged. The external IR remote control receiver connection permits a user to place a television m one room and the satellite receiver in another room, and still allow the user to control the satellite receiver using an IR remote control unit. The external IR remote control transmitter permits the satellite receiver to control an external device, such as a VCR, which is responsive to IR remote control signals, all in a known manner.

However, whenever an external unit is connected by a wire to electronic equipment, it is possible to create ground loops, which results in distortion and possible malfunction of the equipment. It is further possible that an electrostatic charge present in the external device will discharge through the elec- tromc equipment when the external device is plugged in, thus, damaging the electronic equipment. It is also possible for the wire connecting the external device to the electronic equipment to emit electromagnetic interference (EMI) , which can affect external equipment. In addition, in consumer electronic equip- ment it is possible that the external device plugged into the plug will be the wrong device, or that it will be configured incorrectly. For all of these reasons, it is desirable to

provide circuitry n electronic equipment which includes a plug for external devices to electrically isolate the external device from the electronic equipment, to isolate potential electrostatic discharge (ESD) damage from the electronic equipment, and _t o minimize EMI emissions.

In accordance with principles of the present invention, an electronic system, including a connector for an external device, includes a signal processor, responsive to the external device and an electrical isolation device, coupled between the external device connector and the signal processor. Such a system can further include electromagnetic interference suppression circuit¬ ry coupled between the external device connector and the electri¬ cal isolation device. Such a system can also further include electrostatic discharge suppression circuitry coupled between the external device connector and the electrical isolation device.

In the drawing:

Fig. 1 is a schematic diagram of an isolation circuit according to principles of the present invention.

In Fig. 1, electronic equipment is illustrated generally as box 10. This electronic equipment 10 is controllable in response to user commands, either from a control panel on the enclosure in which the electronic equipment 10 is placed, or via a remote control device connected by wires to the electronic equipment 10, or via a infrared (IR) remote control which may be connected to the electronic equipment via a plug. In the illustrated embodi¬ ment, the electronic equipment 10 also includes a channel for controlling external equipment via an IR link, although this is not necessary, and is not a part of the present invention.

In Fig. 1, a IR receiver 12 s configured to receive IR signals (indicated by the lightning-like symbol) frcm an IR remote control device (not shown), in a known manner. Respective first and second output terminals of the IR receiver 12 are coupled to a first plug Pl. The first plug Pl is connectable to a first jack Jl on the electronic equipment 10. A first terminal of the first ack Jl is coupled to a first electrode of a first capacitor Cl, and, through a first passage in a common ferrite bead 14, to a first electrode of a first resistor Rl . A second electrode of the first capacitor Cl is coupled to a source of reference potential (ground) . A second electrode of the first resistor Rl is coupled to an anode of a first light emitting diode LED1. A cathode of the first light emitting diode LED1 is coupled to a first electrode of a second resistor R2. A second electrode of the second resistor R2 is coupled through a second passage through the common ferrite bead 14 to a first electrode of a second capacitor C2 and a second terminal of the first jack Jl . A second electrode of the second capacitor C2 is coupled to ground.

The first light emitting diode LED1 supplies a light signal to a photo transistor PT. The combination of the first light emitting diode LED1 and the photo transistor PT form an opto- isolator 18. An emitter electrode of the photo transistor PT is coupled to ground. A collector electrode of the photo transistor PT is coupled to respective first electrodes of a third resistor R3, a third capacitor C3 and a fourth capacitor C4, and to an IR- received remote control input terminal of a processor 20. Respective second electrodes of the third resistor R3 and the third capacitor C3 are coupled to a source of operating voltage V- _: . A second electrode of the fourth capacitor C4 is coupled to

ground. A remote control unit 22 is coupled oy wires to a „:red remote control input terminal of the processor 20.

The processor 20 also includes an IR-transmit (IR XMIT remote control output terminal coupled to a first electrode of a fourth resistor R4. A second electrode of the fourth resistor R4 is coupled to a gate electrode of a first NPN transistor T . Ar emitter electrode of the first transistor Tl is coupled to ground. A collector electrode of the first NPN transistor Tl is coupled to a first electrode of a fifth resistor R5. A second electrode of the fifth resistor R5 is coupled through a first passage in a second common ferrite bead 26 to a first electrode of a fifth capacitor C5 and a first terminal of a second ack J2. The second ack J2 is coupleable to a second plug P2. When inserted, a first terminal of the second plug P2 is coupled to the first terminal of the second ack J2. The first terminal of the second plug P2 is coupled to a cathode of a second light emitting diode LED2. The second light emitting diode ED2 emits an IR light signal (indicated by the lightning like symbol^ which may be used to control external IR remote control controllable electronic equipment (not shown) such as VCRs and/or television receivers. An anode of the second light emitting diode LED2 is coupled to a second terminal of the second plug P2. The second terminal of the second plug is coupled to a second terminal of the second ack J2. The second terminal of the second jack J2 is coupled to a first input terminal of a sixth capacitor C6 and, through a second passage in the second common ferrite bead 26, to a first electrode of a sixth resistor R6. A second electrode of the sixth resistor R6 is coupled to the source of operating voltage V-~. Although the first and second common ferrite beads, 14 and 26, are described as being separate, in a preferred

emoodiment, they are formed as a single ferrite bead with fo_.r respective passages, as illustrated in phantom m Fig. 1.

In operation, the IR receiver 12 produces an electrical signal which represents the IR signal received from tne external IR emitting device (not shown) . The IR representative signal from the IR receiver 12 is applied to the first light emitting diode LED1, which produces a signal corresponding to the IR light signal being received. The first and second resistors, Rl and R2 operate as current limiters for the first light emitting diode LED1. They also provide some protection against electrostatic discharge. The common ferrite bead 14 minimizes common mode EMI radiation, and the combination of the common ferrite bead 14, and the first and second capacitors, Cl and C2, respectively, operate to minimize electromagnetic interference signals from adversely affecting operation of external electronic equipment, and the internal electronic equipment 10. The opto-isolator 18 provides electrical isolation between the external circuitry (e.g. the IR receiver 12) plugged into the first ack Jl and the electronic equipment 10. This prevents spurious signals, which are so e- times caused by ground loops when separate pieces of electronic equipment are connected together, from adversely affecting operation of the electronic equipment 10, and further prevents damage to the electronic equipment 10 from electrostatic dis¬ charge. The opto-isolator 18 also provides protection against possible short circuits in the external circuitry, and prevents damage to the electronic equipment 10 from a user plugging the wrong device into the first ack Jl.

In a preferred embodiment, the IR signal from the remote control device (not shown) represents a command in the form of a

series of code pulses. The code pulses are pulse modulated cn a carrier signal of around 56 kHz. The first light emitting diode LED1 produces light pulses which correspond to the IR carrier light pulses received by the IR receiver 12. That s, a series of 56 kHz light pulses occurs during a code pulse, and no light pulses between code pulses. The photo transistor PT operates to switch on and off in response to those light pulses. The third resistor R3 provides a load for the photo transistor PT. The third and fourth capacitors, C3 and C4, respectively, operate to filter out the 56 kHz carrier component, leaving only the code pulse envelope. This code pulse envelope is supplied to the IR- received input terminal of the processor 20. The processor 20 processes the signal at the IR-received input terminal to decode the code pulses and determine which command has been sent by the IR remote control unit (not shown) . The processor 20 then controls other elements (not shown) of the electronic equipment 10 to execute the received command, in a known manner.

The processor 20 also may control an external device. In a preferred embodiment, such an external device is controllable through an IR link. The processor 20 produces a command for the external device at its IR transmit (IR XMIT) output terminal. The second light emitting diode LED2 is an IR light emitting diode, and is coupled in the collector path of the first NPN transistor Tl. The load for the first NPN transistor Tl, coupled to the source of operating voltage V _cc consists of the serial connection of the sixth resistor R6, the second light emitting diode LED2 and the fifth resistor R5. The signal at the IR transmit output terminal of the processor 20 controls the first NPN transistor Tl to turn on and off in a manner appropriate to send a command to the external device. When the first NPN

transistor Tl is on, the second light emitting diode ED2 emits light, and when the first NPN transistor Tl is off, tne second light emitting diode ED2 is dark.

In a preferred embodiment, the processor 20 produces a signal at its IR transmit output terminal which represents a series of command code pulses modulated on a carrier, as de¬ scribed above, in such a manner that the external device w ll respond appropriately. The specific required command codes and carrier frequencies vary depending upon the type of external device being controlled, and the manufacturer of that device. Generally, the carrier frequency for commands sent to such external devices can vary from 30 to 60 kHz. This series of modulated command pulses are converted into IR light pulses by the second light emitting diode LED2. The external device responds appropriately to the receipt of this command. In a similar manner as described above, the combination of the second common ferrite bead 26, and the fifth and sixth capacitors, C5 and Cβ, respectively, operate to minimize the adverse affects of electromagnetic interference on external electronic equipment, and the internal electronic equipment 10.

Although the illustrated embodiment of the invention was of electronic equipment including a source of external control plugged into a jack, one skilled in the art will understand that the isolation circuit of the present invention is applicable to any piece of electronic equipment which includes a external jack nto which a user may plug an external device. In such a system, the present invention will provide protection against the adverse affects of EMI, ESD and ground loops on the electronic equipment, and against the possibility of a short circuit in the external

equipment or of a user plugging the wrong device into tne exter¬ nal ack or plugging a device in incorrectly.

Table I, below, discloses preferred values for the compo¬ nents illustrated in Fig. 1.

Table I

Component Values