VOGONS


Reply 20 of 25, by dionb

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matze79 wrote:

Right, looks like my Google-fu was sadly lacking here. Your project is both vastly more elegant and mature 😀

That said, I'll still soldier on here, half the reason for doing this is learning as I go 😉

Reply 21 of 25, by VioletGiraffe

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dionb wrote:

That said, I'll still soldier on here, half the reason for doing this is learning as I go 😉

Took me some staring to figure out whether you said "soldier" or "solder" since both were equally fitting.

Reply 22 of 25, by VioletGiraffe

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So according to Wiki, RS-232 serial port uses +3 to +15 volts for logical HIGH and -3 to -15 V for LOW. I wouldn't be surprised to see +/-12 being used by PCs since -12 V is the only negative voltage produced by AT/ATX PSUs. I've also read 74HC and 74LS datasheets, and the min/max input voltage is not even listed in "Absolute maximum ratings". I assume no one in their right mind thought these parts may be used outside of the intended +5 V digital environment. Finding an optocouple device that can withstand -12 V is also challenging.

Reply 23 of 25, by dionb

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VioletGiraffe wrote:

So according to Wiki, RS-232 serial port uses +3 to +15 volts for logical HIGH and -3 to -15 V for LOW. I wouldn't be surprised to see +/-12 being used by PCs since -12 V is the only negative voltage produced by AT/ATX PSUs. I've also read 74HC and 74LS datasheets, and the min/max input voltage is not even listed in "Absolute maximum ratings". I assume no one in their right mind thought these parts may be used outside of the intended +5 V digital environment. Finding an optocouple device that can withstand -12 V is also challenging.

OK, but you're talking about input from the serial port. That's what the 7805 is for, it accepts anything up to 19V and outputs 5V (with a resistor between it and the Arduino for extra security), converting the RTS signal into something safe. The 74LS00 is on the output to the serial port, so it's not exposed to whatever voltages the other side uses to signal with. It just outputs its 5V towards the serial Rx port.

Reply 24 of 25, by VioletGiraffe

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Oh, you're right, thanks, somehow I was thinking about it backwards. So the problem is even more complex - converting 0/5 V into +/-12 V.
I think I'll try a purpose-designed MAX232 IC for this, the circuit will be even simpler.

Reply 25 of 25, by dionb

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VioletGiraffe wrote:

Oh, you're right, thanks, somehow I was thinking about it backwards. So the problem is even more complex - converting 0/5 V into +/-12 V.
I think I'll try a purpose-designed MAX232 IC for this, the circuit will be even simpler.

Don't think that's necessary - you just quoted RS232 spec as accepting anything from 3V up, so the 5V from the 74LS00 (or other source) should be fine.