TXE433 •5
the transmit enable is held low. Once the TE pin is pulled high, the encoder
output completes its final cycle then stops. When the decoder chip receives
serial data, it compares the address pulses to its address settings and if
they are the same, it sends a high to the “data valid”, pin 18. At that point
the data is considered valid and sent to the data output pins, 11 through 14.
Each individual data line can be used as a separate output to drive
individual devices or circuits, or all can be combined using suitable logic
gates. Because there are four lines, they are capable of sending/receiving
four bits of binary information. This allows a decimal count of 0 to 15.
The data is latched, meaning that the information sent stays at the same
level until new data is sent, even when the transmit enable is no longer
present on the transmitter. If you desire a momentary output, the circuit on
the following page is an example of one way to make the data output on
your RXD916 momentary. The transmitters and receivers each have 8
address lines that can be pulled high or pulled low. To enable the address
lines on the receiver unit they must match the transmitter unit. This gives
256 possible variations in address – imagine 256 receiver units all waiting
for the proper signal and ready to be used for different applications!
Because the transmitters all use the same frequency you would run into
interference if you tried to use more than one transmitter at the same time.
We’ve all had this experience when travelling; you’re tuning the radio and
happen to be caught between two stations. Each station competes with the
other and neither of them comes in clearly. This type of situation would not
be welcome when trying to control different functions since any data
received would be corrupted. However, since each receiver is “listening for”
its own address you can have several receivers on and waiting for the
proper address signal to activate. Each receiver module will receive any
transmission on its frequency but will reject all but the proper transmission
since the address settings don’t match.
It is obvious that we must set the address lines on both the transmitter and
receiver so that they are the same. This entails tying each line either high or
low by running a jumper to either ground or +5 volts. You may want to run
wire jumpers for each individual receiver if you’re using more than one but
since you only need one transmitter to operate several receivers, the
connections you make on the transmitter module should be more easily
changed. A simple way to accomplish this is to use DIP switches. These
can be set up so that each individual address line is connected to +5 volts
through a 10K resistor when open. The other side of the switch should be
connected directly to ground. This way, any switch and its corresponding
address line that is left open is tied high via the 10K resistor.
