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FMA FS8 copilot hardware

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I started looking into the hardware for the FMA Copilot FS8. Its core is a PIC16LF877A -I/PT and it is connected to a Phillips HEF4015BT dual 4 bit static shift register which goes to the servo signal side of the pins.

http://www.semiconductors.philips.com/acro...4015B_CNV_3.pdf

My question is, what do you think the purpose/bennefit of the shift register (serial to parallel converter) is?

Thanks,

Walter

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It is a convenient TTL port expander. With a CD4015 or similar shift register, only two I/O pins on the PIC are needed to control up to eight outputs (servos in this case).

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Ok, but it looks like 6 I/O are needed for signal and control of it. I guess with 33 I/o pins, I did not think they were lacking in I/O, but I guess you can never have enough.

Thanks for the help.

Walter

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Ok, but it looks like 6 I/O are needed for signal and control of it.

Fortunately, only two pins are required -- Clock and Data. BTW, this chip is used in many R/C Rx's for decoding the PPM servo data.

If they did not need it for I/O expansion then they may have used it for other reasons. For example, if the PIC is operating on 3.3V, but they wanted to provide 5V servo signals, then the CD4015 could be used to do that.

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So the 4015 can be used as a level shifter? You just gave me an idea. Would the 4050, noninverting Hex buffer, do the same thing for up to 6 channels? Any idea if there is a single chip that would do up to 8?

Dan

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I don't believe either would be a good choice. There are some (not many though) 3.3V to TTL logic buffers out there, so hunt around for something that looks good.

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