responsible for all transmit timing, valid data identification,
and data stream buffering.
The NBEK IC is also programmed to act as a 1200
baud asynchronous modem. The modem acts as an
interface to external data terminal equipment. Note the
absence of the ever-present MAX RS-232 IC. Transmit data
in the RS-232 format is converted to PIC logic levels by
resistors R12 and R13, which are attached to NPN
transistor Q5. Received RS-232 data is converted by PNP
transistor Q2 and resistor R5. The target radio is attached
You can easily pick out the voltage regulator circuitry
which is based on an LM2940-5. Logic levels on the
carrier board are at five volts. So, if a 3. 3 volt radio is
attached, five volt to 3. 3 volt logic translation is necessary.
Voltage for the NBEK carrier board and target radio is
derived from an external power supply or a nine volt
The NBEK carrier board circuitry is better understood
when you can identify the operation of each of the PIC’s
I/O pins. Pin 1 is not connected. So, let’s begin with pin 2.
As you can see, pin 2 drives an LED. This pin is called the
STATUS pin and is driven logically high when data is
received or data is contained within the RX buffer.
The NBEK IC automatically keys the attached
transmitter when valid data is presented to the RXTXD pin
(pin 7). The keying is performed by the active-low TXE pin
Pin 4 is the PIC’s MCLR pin. For the purposes of the
NBEK carrier board, it is identified as the RESET pin.
Pins 5 and 14 are Vss and Vdd, respectively. The PIC
is powered by a five volt regulated power supply which is
under the control of IC1.
Pins 6 through 9 are shared with the four-bit data
word and the 1200 baud modem. D0 through D3 signals
reside on pins 6 through 9, respectively. The modem
transmit line is on pin 7. Pin 8 doubles as the modem
ADVANCED TECHNIQUES FOR DESIGN ENGINEERS
November 2015 61
■ Schematic 1. This
is nothing more than
an eight-bit PIC
doing what it was
designed to do.