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DPRG: Ming Transmitters/Receivers

Subject: DPRG: Ming Transmitters/Receivers
From: MIKE MCCARTY -- 93789 JMCCARTY at DEVEL.USA.ALCATEL.COM
Date: Thu Aug 5 15:29:44 CDT 1999

I obtained some Ming transmitter/receiver pairs, and did some
experimenting with them last weekend. (I found a source at $5.95 per
transmitter, $6.95 per receiver.) I was somewhat disappointed, given
the other reports I had heard.


First, the output of the receiver was +3 VDC with no carrier present,
and the same with carrier present. When the transition from carrier to
no carrier, or no carrier to carrier was made, a pulse up to +6VDC was
observed. So DC levels cannot be transmitted. Secondly, when pulses
were being transmitted, the output still was a 2-3V P-P square wave,
riding on a 3VDC offset, never dropping below 3V.

I rigged up a 5V supply, and a DBF9 cabled to the transmitter and
receiver. Here is a quick view of the circuitry


        +5V reg o--------------+----------o Transmitter +5, Receiver +5
                               |
                               /\ 1N914
                               --
                     180 Ohms  |
        TxD     o----/\/\/\----++---------o Transmitter Data
                                |
                                /\ 1N914
                                --
                                |
        Gnd     o---------------+---------o Transmitter Gnd, Receiver Gnd


        Rcvr        1N914
        Data    o----|>|------------------o RxD


The resistor and diodes from TxD clamped the voltages to the transmitter
to -0.6V and +5.6V to prevent overdriving the transmitter. The diode on
RxD was necessary as a level translator.

With this setup, and the transmitter and receiver inches apart (on the
same solderless breadboard, actually) I was able to transmit and
receive reliably at 1200 Baud. Any higher speeds were not reliable. An
oscilloscope showed why. The receiver has a sort of one-shot output.
The output pulse begins almost immediately at the onset of carrier, but
the output holds up for a period after the cessation of carrier. At bit
rates greater than 1200 BPS (certainly at 4800 BPS) this hold-up time
encroaches on the next bit.

Next, I rigged up a 7404 hex inverter and 7474 D FlipFlop to provide me
with a constant 50% duty cycle signal at 1200 Baud. I powered the
circuit with a 9V battery and an LM7805 voltage regulator. I hooked this
up to a transmitter. I attached full wave antennas to both the
transmitter and receiver (37 1/2 inches) per the instructions.

          180 ohms 180 ohms
         +-/\/--+-/\/---+
         |      |       |               Div/2  Div/2
        ++--|>o-+--|>o--+-+-----|>o----[F/F]---[F/F]----o Data
        |                 |
        +---||------------+
           Cap, selected to provide 4x data rate error < 1%

I found that the reception of "U" was reliable up to a distance of
about 12 feet. I stationed the transmitter at about 20 feet, and found
that the reception would fade in and out as I moved about between the
transmitter and receiver. This is indoors without intervening walls.

A 'scope on the receiver output showed that when the signal was
received well, it was a nice clean square wave. When the reception
became bad, I saw a square wave as the envelope of noise. It looked
like a child had colored in a drawing with a pencil, leaving lots of
vertical marks.

Clean signal

+6      +-------+       +-------+       +-------+       +
        |       |       |       |       |       |       |
        |       |       |       |       |       |       |
        |       |       |       |       |       |       |
        |       |       |       |       |       |       |
+3      +       +-------+       +-------+       +-------+


Noisy Signal

+6      +-------+       +-------+       +-------+       +
        |||||||||       |||||||||       |||||||||       |
        |||||||||       |||||||||       |||||||||       |
        |||||||||       |||||||||       |||||||||       |
        |||||||||       |||||||||       |||||||||       |
+3      +|||||||+-------+|||||||+-------+|||||||+-------+


This is the output at the receiver terminal, not at the input to the
serial port.

This shows me that the problem was not with my simple level translator,
but rather with reception.

I purchased these hoping to use them at distances up to 50 feet or so.
It appears that I cannot use them beyond about 10 feet, which means
they are basically useless to me.

My experience seems at variance with that reported here.

I didn't try a pull-up on the output of the 7474 to see what would
happen with truly 5V on the Data line to the transmitter. Perhaps I
should try that.

Comments?

Mike
- ----
char *p="char *p=%c%s%c;main(){printf(p,34,p,34);}";main(){printf(p,34,p,34);}
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This message made from 100% recycled bits.

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