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In lugnet.robotics.rcx, Mark Riley wrote:
> I've opened up my RCX 1.0 and the IR receiver has the markings "TSOP 1138" on
> it. After looking this part number up, it appears it is an IR receiver
> sensitive to a 38KHz IR carrier. This is the "normal" carrier for RCX messages.
>
> However, my RCX 2.0 has an IR receiver with "TSOP 1176" markings. I couldn't
> find any info on this part, but knowing that a TSOP-1140 is an IR receiver for a
> 40KHz carrier leads me to extrapolate that the IR receiver in my RCX 2.0 is
> sensitive to a 76KHz carrier (double the 38KHz carrier).
>
> Now, as has been mentioned here before, the Spybot and Manas remotes send IR
> messages using a 76KHz IR carrier. My RCX 2.0 can receive these messages just
> fine, while my RCX 1.0 struggles to receive these messages even with the remote
> a few inches from the RCX.
>
> While the TSOP-1176 is also apparently sensitive to a 38KHz carrier (otherwise
> it couldn't receive normal RCX messages), it may be with less sensitivity than
> the native TSOP-1138. This could be the reason for the observed differences in
> IR reception between the different RCX versions.
>
> Mark
>
> BTW, the TSOP-1176 part is also used as the receiver in Spybots and Manas motor
> units.
Hello Mark,
Thanks for your answer, this will keep me from dismantling once again my RCXs !
So it seems that LEGO modified RCX2.0 to get better compatibility with
Spybots/Manas. Now a question remains, why did they increased carrier frequency
on new products? Better remote sensitivity? (I find that the remote control of
these devices do work remarquably well). Or increased communication speed?
IMHO, the choice of 76KHz carrier enables some sensitivity to older 38KHz
frequency, there is probably enough second harmonic to get through receiver
bandpass filter. Interestingly enough, 76 KHz is not an IR standard frequency,
and Vishay who manufactures TSOP1138 makes receiver only in 30 to 56 KHz range.
See documentation / tutorial here: http://www.vishay.com/docs/80069/80069.pdf
Now we should probably get better RCX2 to RCX2 transmission performance by
switching communication carrier to 76KHz as is done to speak to manas:
"SetSerialComm(SERIAL_COMM_4800|SERIAL_COMM_DUTY25|SERIAL_COMM_76KHZ);"
To be tested...
Philo
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In lugnet.robotics.rcx, Philippe Hurbain wrote:
> Thanks for your answer, this will keep me from dismantling once again my RCXs !
Mine have been apart more often than I'd like, too! :-)
> So it seems that LEGO modified RCX2.0 to get better compatibility with
> Spybots/Manas. Now a question remains, why did they increased carrier frequency
> on new products? Better remote sensitivity? (I find that the remote control of
> these devices do work remarquably well). Or increased communication speed?
My guess would be mostly for increased communication speed. With the higher
baud rate, the duration of the 2-byte Spybot/Manas packets is halved. This
allows for fewer collisions from remotes on different channels and hence a
higher update rate for the motor commands being sent (i.e. the motors will
appear more responsive).
> IMHO, the choice of 76KHz carrier enables some sensitivity to older 38KHz
> frequency, there is probably enough second harmonic to get through receiver
> bandpass filter. Interestingly enough, 76 KHz is not an IR standard frequency,
> and Vishay who manufactures TSOP1138 makes receiver only in 30 to 56 KHz range.
> See documentation / tutorial here: http://www.vishay.com/docs/80069/80069.pdf
It's interesting, the responsivity chart in the above document only covers 0.7
to 1.3 the designed carrier frequency... I wonder what the relative response is
for half the carrier frequency - at least this would give us a ballpark figure
at what reduction in sensitivity we're experiencing at 38 KHz.
Might the TSOP-1176 parts have started appearing in RCXs prior to RCX 2.0?
Could there be any RCX "1.5" units with this new receiver?
Mark
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