MAX232N converter circuit idels at 5v

[sean87]

Hi all,

For a project I needed to make a RS232 to CMOS level converter. So I went ahead with MAX232 and 4 1uF caps. Just followed the schemtic here:



This is the result from my PC's old school RS232 port (Yes I have a RS232 port on my PC!):



After I fed the same signal into my MAX232N chip, I noticed that when I do not send something, it is idle at +5V....is this what it should it be? is this cmos level (Considering 5v PP) if not, what can be wrong?!


for the 5v Vcc for the chip I used a LM7805 to convert 12v from a wall adapter to 5v and it works ok (I checked with multimeter).

[kxenos]

MAX232 has inverted buffers in it so I think it is correct. Doesn't it work?

[baljemmett]

Yes, that looks right -- RS232 idles in the logic high ('mark') state, so will be at 5V on the logic-level side of the MAX232.  The RS232 data signals themselves should be negative (between -3V and -15V, with -12V being a common standard) for a high, and positive (again, between 3V and 15V) for the logic low ('space') state, so it looks inverted compared to the logic-level signals your uC will be working with.  The MAX232 handles this inversion for you, in addition to the level shifting.

Just to make life interesting the RS232 control signals (DSR, DTR etc.) are not inverted, so a logic high is positive and a logic low is negative.  Since you aren't handling those at all you don't need to care about this, but it's worth bearing in mind if you do need to probe them at some point.

[sean87]

Thanks guys, I think its working fine since I managed send and receive correctly.

[Psi]

Most MAX232's seem to output around  +/- 8V on the RS232 side.
So not quite +/-12V but still within spec.

[sean87]

Most MAX232's seem to output around  +/- 8V on the RS232 side.
So not quite +/-12V but still within spec.

The first scope shot is from PC's serial port directly.