It may, but it's not RS-232.
easiest way to "fake" an rs232 signal ?
May 04, 2006
46 Replies
It may not be RS-232, but reading the origional problem statement, it may perform the intended function through the required range, AKA "getter done!'
--- Of course there's always something to be said for expedience, but the real RS-232 solution will _always_ work, so when the OP asks for:
"what's the simplest circuit that can "fake" an RS232 signal ?"
and he doesn't specify any of the vagaries of his system, the _right_ answer is the one which will _always_ work. Wouldn't you agree?-)
-- John Fields Professional Circuit Designer
Can a "fake" an RS232 signal actually exist? I agree that either it is RS232 or it isn't. The same for placebo, faux, spurious, pseudo, and herbal RS232 signals. ;)
In response to what John Fields posted in news: snipped-for-privacy@4ax.com:
No. The easiest way depends on the intended listening device. Catering for all possible varieties and vagaries is certainly *not* the 'easiest way'. That is the *hardest* way.
Joe Soap.
JUNK is stuff that you keep for 20 years,
then throw away a week before you need it.
I disagree.
Since the OP asked for an RS-232 signal, all I need to generate is a
signal that rests at beween +3 and +25V and, when it's active, goes
to betweeen -3 and -25V for at least 1, and no more than 8, (or 9, I
forget...) bit times.
I can do that with a 555, two 9V batteries and three discretes, no
matter _what_ his system looks like.
Can't argue with that!-)
In response to what John Fields posted in news: snipped-for-privacy@4ax.com:
And you think /that/ is the /simplest/ way? What kind of sledgehammer do you use to crack nuts? Steam-driven?
Joe Soap.
JUNK is stuff that you keep for 20 years,
then throw away a week before you need it.
Well, since you don't know anything about his receiver except that
it's RS-232, you need a positive and a negative supply (two simple
9V batteries will work) and a means to generate a pulse that looks
something like this:
+9V ____ _______ ____
| | | |
| | | |
-9V |___| |___|
Which a 555 and a few discretes will easily do. Got an
easier/simpler way to do it within the constraints of his post(s)?
Isn't the RS232 idle state(marking, logical 1) = -3 to -25 V and space, logical 0 = +3 to -25 V? He would need a single pulse going from (-3 to-25V) to (+3 to +25V) for a period of [(character bits
+2)/baud rate] seconds. Probably wouldn't hurt to throw in a couple of bit periods just prior and after the pulse. This would simulate a "real" RS232 transmission of ASCII 0, whatever that is.___ Quidquid latine dictum sit, altum viditur. (Whatever is said in Latin sounds profound.)
No, the RS-232 idle state is spacing at 3 to 25V. The first marking
bit is the start bit which is a transition to -3 to -25V for one bit
time. That's followed by either 7 or 8 bits of data, a parity bit
(or not) and a stop bit one or two bits. I think 1-1/2 bits is also
a valid choice.
So, assuming the OP wanted to send ASCII 00 and his machine was set
up for 8 data bits, no parity and one stop bit, what he'd do would
be to send:
+V ___ ____________________________________ . . .
| |
-V |___|
1 0 0 0 0 0 0 0 0 0
| | | | | | | | | |
t0 t1 t2 t3 t4 t5 t6 t7 t8 t9
/ / / / / / / / / /
START_/ / / / / / / / / /
/ / / / / / / / /
DATA___/___/___/___/___/___/___/___/ /
/
STOP_________________________________/
each time he wanted to send ASCII 00.
Notice that the stop bit is a 0, which returns the signal to the
quiescent spacing state, where it will remain until another start
bit comes along and starts the whole process over again.
In response to what John Fields posted in news: snipped-for-privacy@4ax.com:
RS-232-C has nothing to do with start bits, stop bits or anything else connected with ASCII data. It is merely a specification of electrical characteristics.
However, it /does/ define a 'marking' state, logic ONE, as a negative voltage, An ASCII idle state is 'marking', i.e. a logic ONE - negative. A START bit is a zero and a STOP bit is a ONE - quite the opposite of what the 'Professional Circuit Designer' asserts. So an ASCII NULL, with start bit, stop bit and no parity, is ...0'00000000'1....
'Professional Circuit Designer'? Don't come here for a job.
Joe Soap.
JUNK is stuff that you keep for 20 years,
then throw away a week before you need it.
no a dormant channel should be is in the "mark" state -5..-12V the start bit is a "space" +5..+12V
Bye. Jasen
pulse durationn is everything. too long and it looks like a "break" and is ignored, too short and it gets missed.
18V peek-to-peek seems about right, the offset seems otherwise what sort of scoope were you using. are you sure it was earthed to signal ground. (chassis ground would do)I doubt the serial port hardware inside your PC is capable of producing +18V
Bye.
Jasen
Yup, I got it backwards. Thanks for the correction.
In response to what John Fields posted in news: snipped-for-privacy@4ax.com:
It isn't very professional to claim to be a 'Professional Circuit Designer' and then spout a load of rubbish. Glad to hear I won't have to chuck you off the premises.
*plonk*.
Joe Soap.
JUNK is stuff that you keep for 20 years,
then throw away a week before you need it.
Good. One less asshole I have to contend with.
John Fields spake thuswise in news: snipped-for-privacy@4ax.com:
Am I missing something here? Seems to me that Joe was right and you were wrong.
Alan Jones.
Yup, but he's still an asshole. Read the thread.
Yes, Joe was correct, and then he was stupid. You missed something.
Don
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