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Futaba servo signal interface
3 Understanding Circuit Operation
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NOTE: During this section it may be of help to refer to Appendix B containing the schematic diagram of the interface circuit and the component layout of the PCB.
The operation of the circuit is fairly simple. At the heart of the unit is a PIC16F877 Microcontroller.
The logic voltage is taken from the relay coil supply voltage and regulated with an LM7805 +5v regulator, it is then filtered with a 3300?F capacitor to prevent brownouts during the start cycle of the relay coils inductive load. Without this capacitor the logic voltage would dip so low that it would send the Microcontroller into its reset mode, causing stuttering and errors in the circuit operation. This setup is an advantage because it only requires one power source for both the logic and relay driver voltage reducing external hardware and possible failure of a common ground link.
The microcontroller measures the width of the servo pulses from the Futaba FM servo receiver. Four input channels are available on pins 0 - 4 of port D. They are JP1 - JP4 on the component layout. These pins are separated from the signal connector with a 220 ? Resistor to prevent a direct short that could potentially and permanently damage both the receiver and the PIC16F877 if the signal lines were connected wrong.
The pin corresponding to the ground pin on the Futaba receiver cannels on the interface board is connected to the circuit's common ground. This provides a common ground between the receiver and the interface board. The center pin, normally used to supply operating voltage to a servo connected to the receiver is left unused on the interface as it is not needed. This setup provides ease of installation by only requiring a two wire direct link between each servo channel and its corresponding input on the interface.
The relay driver section of the circuit occupies the entire bus of port B. This provides a maximum of two relay drivers per servo channel. This section of the circuit, once again is separated from the PIC16F877 by a 220 ? resistor to prevent accidental short circuiting. Each pin on port B is pulled to a low state by a 10K ? pull down resistor to prevent the pin from floating, causing the relay to fail to be disengaged. The opposite end of the 220? resistor is then connected to the base of a TIP122 Darlington transistor. Due to the architecture of Darlington transistors no driver transistor is required to drive the TIP122 into its conductive state. This aids in simplicity of both the circuit and PCB. The emitter of the TIP122 is connected directly to ground to properly bias the transistor. I failed to realize this as I mentioned earlier on my first attempt of the circuit design. Between the collector and the unregulated positive supply is where the load for the driver will be placed. A Two pin SIP connector was placed here for connection to a relay. Paralleled with this connector, a 2.2v LED in series with a 1K? resistor was added to aid in visual testing. These LEDs were added to allow for testing code without actually having the device attached to relays and motors. When a pin on port B is driven High, it triggers the base of the TIP122 and sends it into its conductive stage. Acting like a switch it allows current to pass through a load connected to its collector and out of its emitter to ground.
The port A input section is once again separated from the PIC16F877 with 220? resistors to prevent short circuiting. The section was added for use with external sensors and limit switches to send feedback and hardware states to the Microcontroller. Port A was chosen for this particularly because it can also be used as an Analogue to Digital Converter. However it was never used in the Skills Canada Robot design. Each pin is pulled High by a 10K? pull up resistor. The six pins of port A (0 - 5) are connected to a 12 pin male ICD connector. Each pin adjacent to the input pin is directly connected to ground. This provides a simple 2 wire connection through a 2 pin female SIP plug to a sensor or limit switch which would be used to pull the pins to low through the ground rail. By changing over the 10K? resistors to pull down resistors or leaving them out entirely and putting the required pull up/down resistor on the actual sensor itself, one could use this port as an output as well.
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Articles
Futaba servo signal interface
3 Understanding Circuit OperationFutaba servo signal interface
3 Understanding Circuit Operation
NOTE: During this section it may be of help to refer to Appendix B containing the schematic diagram of the interface circuit and the component layout of the PCB.
The operation of the circuit is fairly simple. At the heart of the unit is a PIC16F877 Microcontroller.
The logic voltage is taken from the relay coil supply voltage and regulated with an LM7805 +5v regulator, it is then filtered with a 3300?F capacitor to prevent brownouts during the start cycle of the relay coils inductive load. Without this capacitor the logic voltage would dip so low that it would send the Microcontroller into its reset mode, causing stuttering and errors in the circuit operation. This setup is an advantage because it only requires one power source for both the logic and relay driver voltage reducing external hardware and possible failure of a common ground link.
The microcontroller measures the width of the servo pulses from the Futaba FM servo receiver. Four input channels are available on pins 0 - 4 of port D. They are JP1 - JP4 on the component layout. These pins are separated from the signal connector with a 220 ? Resistor to prevent a direct short that could potentially and permanently damage both the receiver and the PIC16F877 if the signal lines were connected wrong.
The pin corresponding to the ground pin on the Futaba receiver cannels on the interface board is connected to the circuit's common ground. This provides a common ground between the receiver and the interface board. The center pin, normally used to supply operating voltage to a servo connected to the receiver is left unused on the interface as it is not needed. This setup provides ease of installation by only requiring a two wire direct link between each servo channel and its corresponding input on the interface.
The relay driver section of the circuit occupies the entire bus of port B. This provides a maximum of two relay drivers per servo channel. This section of the circuit, once again is separated from the PIC16F877 by a 220 ? resistor to prevent accidental short circuiting. Each pin on port B is pulled to a low state by a 10K ? pull down resistor to prevent the pin from floating, causing the relay to fail to be disengaged. The opposite end of the 220? resistor is then connected to the base of a TIP122 Darlington transistor. Due to the architecture of Darlington transistors no driver transistor is required to drive the TIP122 into its conductive state. This aids in simplicity of both the circuit and PCB. The emitter of the TIP122 is connected directly to ground to properly bias the transistor. I failed to realize this as I mentioned earlier on my first attempt of the circuit design. Between the collector and the unregulated positive supply is where the load for the driver will be placed. A Two pin SIP connector was placed here for connection to a relay. Paralleled with this connector, a 2.2v LED in series with a 1K? resistor was added to aid in visual testing. These LEDs were added to allow for testing code without actually having the device attached to relays and motors. When a pin on port B is driven High, it triggers the base of the TIP122 and sends it into its conductive stage. Acting like a switch it allows current to pass through a load connected to its collector and out of its emitter to ground.
The port A input section is once again separated from the PIC16F877 with 220? resistors to prevent short circuiting. The section was added for use with external sensors and limit switches to send feedback and hardware states to the Microcontroller. Port A was chosen for this particularly because it can also be used as an Analogue to Digital Converter. However it was never used in the Skills Canada Robot design. Each pin is pulled High by a 10K? pull up resistor. The six pins of port A (0 - 5) are connected to a 12 pin male ICD connector. Each pin adjacent to the input pin is directly connected to ground. This provides a simple 2 wire connection through a 2 pin female SIP plug to a sensor or limit switch which would be used to pull the pins to low through the ground rail. By changing over the 10K? resistors to pull down resistors or leaving them out entirely and putting the required pull up/down resistor on the actual sensor itself, one could use this port as an output as well.
