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'[OT]: [NOT SO OT]: techamerica/wenshing rx/tx to a'
2000\10\08@202632 by Peter L. Peres

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On Sun, 8 Oct 2000, Greg Hartung wrote:

>> This should not happen imho. While I am not familiar with your brand of
>> Tx/Rx, if they are meant for data, then they will not pass DC signals. In
>> other words, you will have much better chances using them with a 555
>> oscillator set to blink a LED at 10 Hz or so than with a button.
>
>   Could you please explain why?  Are you saying if I could toggle my button a
>little faster it would be more likely to work?

A cheap Tx/Rx pair meant for data passes only AC. More exactly, the
receiver will have a comparator with automatic level following. This
means, that the mean of the signal is taken over a certain period of time,
and compared to the instant level, using a comparator (assuming AM 100%
modulation, which it usually is with cheap modules). This means that if
you press a Tx button the signal will raise (output turns on) and then the
average reaches the instant value (since it is not changing) and the
output will become flaky and turn off (or flicker very fast due to
comparator noise and residual carrier - this causes the LED to appear more
dim than when fully lit).

>> The winking LED when no signal is present is normal for this type of
>> receiver (unless it has a squelch system, which most cheap modules don't).
>
>   Shouldn't a carrier with an input signal at ground show a perfect 0 at the
>receiver?  What if my data includes long sequences of 0's?  Must I use an
>encoder?  (manchester or whatever)  Why does this matter to the transmitter
>and receiver?  Sorry for so many questions, but this concept definitely seems
>to be the root of my problem.
>   Thanks.

The answers imho are no, it should not, yes, use an encoder, and yes, it
matters (see above). You can use the 'analog output' you were talking
about to feed a comparator (which you supply). When the level is above a
threshold (which you set) you can receive data. This is the squelch (or
SQ) indication that the cheap RF module maker, err, forgot. You will use
this signal to notify the microprocessor about possibly valid signal on
the output. None of this usually applies to the Tx which will be keyed as
you wish. It moslty applies to the receiver.

There is no need to go to Manchester you can send serial data as usual, if
you do bit stuffing. Bit stuffing consists in making sure that the total
number of 1 and 0 bits in a byte (or two or three) is equal. For 8N1
serial protocol you have 1 '0' bit and 1 '1' bit built-in (the start and
stop bits), so all you have to do (this is a simplified solution) is to
send 8 data bits that satisfy the requirement. That would be sending one
nibble per byte of serial data, with the other four bits being the
complement of the first four. Or in PIC parlance:

!!! It is 0151 AM and this is off of my head (source code not accessible
now)

send_nibl:
 ; send second nibble (LSB)
 movf  INDF,w
 goto  send2

send_nibh:
 ; send first nibble (MSB)
 swapf INDF,w
send2:
 andlw H'0F' ; LSB is true nibble
 movwf Ftxreg

 ; and inverted
 swapf Ftxreg,w
 xorlw H'F0'
 iorwf Ftxreg,f

 goto send_it

This code also works with micros stuck with an internal UART (8051
derivatives f.ex., and even the ole PC serial port) and it should work
over POTS !!! It has been tried through phone line with transformer
coupling (w/o PBX or central or power - just point to point phone line).

Advantage: Some level of error detection/correction is provided ;-) No
encoder/modem is needed. This overcomes the speed disadvantage by a LARGE
margin imho. A simple comparator recovers the Rx signal to TTL level. An
optional second comparator provides SQ or BREAK signal based on 'carrier'.

Disadvantage: You send 4 bits of data in 10 bit cells, or 5 periods.
Synchronous can send 10 bits in 10 cells. This scheme is slower than
synchronous.

Peter

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