PISO register to serial port


I'm trying to get a device with 64 outputs (24 VDC) to communicate with a PC through it's serial port. I realize I need to use parallel-in serial-out shift registers to accomplish this, but there's a couple problems.

First, the output side of the circuit is borrowed from Bill Bowden's page for his (PC Serial Port Reciever) circuit. It runs fixed at 9600 baud and uses two stop bits. As far as I can tell, that means I'll need to use

11-bit hift registers.

Second, once the data's been clocked out of the registers to the port, the zero-voltages need to be converted to -10 volts and the +5 volts need to be amplified to +10 volts. Does anyone have any circuits or examples that can do this?



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Just program up a small microcontroller. For 64 bits of input, you'll need either multiplexers or shift registers. For a SR load them all in parallel with one output line, shift clock on another. Or have 8 parallel SR chains if your controller has enough IO, only one clock per set needed. Shift in groups of 8, slap them into the UART transmit register (that adds any start/ stop bits), use MAX232 or equivalent on the output. I don't know Bill Bowden's page.

Paul Burke

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Paul Burke





to be


Bill Bowden's Hobby Circuit website:

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There's a link to the PC Serial Port Reciever on that page. Actually, I was hoping to pull this off without using a microcontroller because I've never used them to build a circuit... I suppose that's not very likely at this point.

I was planning on using serially-linked PISO SR's to do the job. The trick is building eight bytes - without a microcontroller, the only way I could see it done is in the SR's themselves...

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Why don't you use a parallel port? Then you just need a few SRs, a 5V supply (can be derived from the PC with a drive connector) and some bypass caps. Plus some stuff to get the 24V inputs down to 5V (I suggest quad channel optoisolators with appropriate input networks and output pullups). Pretty simple, and no microcontroller required.

Best regards, Spehro Pefhany

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Spehro Pefhany

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