LM1872

Manufacturer Part NumberLM1872
DescriptionRadio Control Receiver/Decoder
ManufacturerNational Semiconductor
LM1872 datasheet
 


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Applications
(Continued)
EXPANSION TO FOUR ANALOG CHANNELS
For those applications that require more than the two ana-
log channels that are normally provided the LM1872 can
easily be expanded to 4 channels with appropriate external
circuitry This is accomplished by creating a pseudo-sync
pulse (t
) among a six channel transmitted frame from the
ps
LM1871 (Figure 10) The pseudo-sync pulse deceives the
decoder in the LM1872 causing premature recognition of
end-of-frame effectively splitting a single frame into two
The idea is to transmit analog channels 1 and 2 in the first
half of the normal frame period and analog channels 3 and
4 in the second half External logic will then steer the four
channels from the LM1872’s only two analog output pins
into four new analog outputs Steering is accomplished with
the help of one of the digital channels Inasmuch as the
digital channels respond only to the number of pulses re-
a) Transmitter Receiver and Auxiliary Decoder Timing Diagram
FIGURE 10 Deriving Four Analog Channels Through the Use of an Auxiliary Decoder
ceived between any two sync (or pseudo-sync ) pulses the
channels are capable of toggling in step with the alternating
transmission of two and three channel pulse mini-groups
occurring within each half frame Figure 10a reveals that
both digital channels A and B are high during the dual pulse
half frame and low during its triple pulse counterpart Figure
10b shows just how simple the external circuitry can be
Digital channel B drives the channel select pin of a quad 2-
input MUX that routes the LM1872 channels 1 and 2 outputs
to the four new outputs labeled analog 1 through 4
Although not the model of simplicity of Figure 10b Figure
10c is a lower cost alternative that works just as well The
diodes with the asterisk prevent a ground step from occur-
ring that could false trip an excessively edge sensitive servo
and can be eliminated in many cases
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TL H 7912 – 18