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TTL to/from RS232 Signal Conversion

NEW! RLC-2 RS232 Level Converter in a DB9 backshell

Paul B. Webster says:

Technically, there is *no* RS-232 standard showing [how data bits are related to signal levels], and never will be because RS-232 is a level-zero = "physical" level standard. It defines wires, voltages and pins. What travels on that layer, the level-one or "transport" layer, is something else again.


Most RS232-TTL conversion is done with a IT/Maxim MAX232^, MAX3232^ or similar chip.

Peter L. Perez says:

[For an RS232 PIC interface] you don't need diodes, you need a 10K resistor for the incoming data (towards the chip) and a 220 ohm on the outgoing (from the chip). If your application is low current or sleeps you also need a zener or other clamping device across the micro power supply. I certify that this works up to 9600 Bauds at least ;-). You can use the same pin for Tx and Rx and save a resistor ;-) I can also certify that certain renowned PLL chips and other I2C peripherals do not like the -0.6V appearing on the input pin at all, and behave strangely, so your mileage will vary. Again: PICs will work like this, but other micros may not.

PS: If you reduce the input resistor from 10k to ~3k3 then it can also power the PIC temporarily (hint: set break state on RS232 and wait for filter cap across PIC power pins to charge...). You HAVE to use a clamping device for this. 5V6 Zener (0.25W low leakage) will do fine.

See also:


Ubicom demo modem board

James Michael Newton says: A SimmStick with most of what users need, plus a little power supply section to run it all. People buy them just for the TTL/RS-232 conversion to D-9 connector, which makes it very simple.



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