signal too weak for RS232?

hi,

i've got a simple PBPro routine to send basic ASCII commands via RS232 to another embedded device. i'm running a 12f675 at 4MHz (internal osc) and communicating at 300bps 8N2. everything is working properly - the LED blinks, LCD displays, and i've confirmed that a PC can see the ASCII as expected.

my problem is that the other embedded device doesn't react to the PIC. if i send the same ASCII with my PC (same settings), the embedded device reacts. could it be that my PIC isn't putting out enough voltage? i'm not using anything to give true CMOS voltage levels - just 0 and +5V output. this has always been enough before on other projects, but this dude isn't responding.

any similar experiences out there?

thanks!
Pat

Quote:

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Originally Posted by emptyset

hi,

i've got a simple PBPro routine to send basic ASCII commands via RS232 to another embedded device. i'm running a 12f675 at 4MHz (internal osc) and communicating at 300bps 8N2. everything is working properly - the LED blinks, LCD displays, and i've confirmed that a PC can see the ASCII as expected.

my problem is that the other embedded device doesn't react to the PIC. if i send the same ASCII with my PC (same settings), the embedded device reacts. could it be that my PIC isn't putting out enough voltage? i'm not using anything to give true CMOS voltage levels - just 0 and +5V output. this has always been enough before on other projects, but this dude isn't responding.

any similar experiences out there?

thanks!
Pat

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Hi Pat,

Welcome to the Forum.

PIC = +5V or 0V (logic rs232)
RS232 = +24V or -24V (real rs232)

IC named MAX232 MAX233 translates logic 232 to real 232 both ways

-Adam-

You don't need a MAX232 for PIC-to-PIC RS232 0-5V is adequate. All the MAX 232s will do is boost it and then drop it back to 0-5V on the other end.

You need inverted RS232 between the PICs and the PC. PIC-to-PIC doesn't matter but both send/receive need to be the same, either inverted or non-inverted..

thanks Adam and Dave for the quick response. i've used the MAX chips in the past but it is always easier to use inverted PIC-to-PC.

the PC part is just fine. the other embedded device i'm speaking to won't respond, so i'm wondering if it is waiting for the 24V levels...

i don't know what micro is in the device - maybe i'll open it up and see what is in there.

does anyone know if the MAX232 chip can handle a 0-5V input properly? if embedded device has a MAX232 inside it, and my PIC is talking to the MAX part, will it read 5V as a "1" input?

thanks again!
Pat

Max232 Data Sheet
Found under:
RS-232 RECEIVERS
Input Threshold Low
Input Threshold High
Input Hysteresis
-Adam-