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Topic: Problems understanding RS232 - CTS pin (Read 1 time) previous topic - next topic

darudude

I've been looking into making my ow ICSP board. The reason I want to venture into making my own in cause I want one board that I can use with both ATMEL and PIC chips while just changing the positions of a header.

Now that I got the Atmel side on the go, i started working on the PIC side. I so an interesting design and I went ahead and built it on a protoboard and it worked. However, now that I look at the schematic, I have no idea why it worked.

The schematic is below:


What I don't understand is the CTS connection. The Prog Data pin can handle a max of 5.3V, but doesn't the RS232 work at 12V? Wouldn't this destroy my chip? When does CTS normally go on? Would CTS ever go high in this design?

I know, this is not completely Arduino related (thought I plan on usnig this to bootload Arduino designs and work on the Pinguino project), but I really didn't know where else to turn.

Grumpy_Mike

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Would CTS ever go high in this design?


In this design the CTS from the DB9 connector is an input, therefore you chip is safe.

floresta

darudude

To elaborate on Grumpy_Mike's response ...

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but doesn't the RS232 work at 12V?
Normally implemented RS-232 uses -12 V to represent a logic '1' and +12 V to represent a logic '0'.

For communications between digital devices, such as in your implementation, a hybrid system is frequently used where RS-232 signals are implemented with digital logic levels.  In this case 0 V is used to represent an RS-233 logic '1' and +5 V is used to represent an RS-232 logic '0'.  

Each of the three transistors in your circuit is acting as an inverter, so when, for example, your DTR line is +5 V representing a digital logic '1',  the PROG_DATA line is 0 V, representing an RS-232 logic '1'.

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Would CTS ever go high in this design?"
Yes, when DTR goes low.  But this is only +5 V via R4.

floresta

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