NiM1B is also available. The NiM1B family of VHF radio
modules can support a data rate of up to 5 kbps.
The NiM1B datasheet claims that the radio modules
can be reprogrammed on any frequency between 120
MHz and 175 MHz. The NBEK carrier boards have a
PROG connector. However, the only mention of
reprogramming in the datasheet points the user to the
factory for such operations.
The NiM1B transmitter subsystem is based on a sigma
delta synthesizer-based RF device, which can be
configured via an SPI portal by an internal microcontroller.
Upon assertion of the active-low transmit enable signal,
the NiM1B can transition to full output power within 5 ms.
The NiM1B receiver subsystem is based on a sigma
delta synthesizer-based local oscillator that can also be
configured via an SPI portal by the internal
microcontroller. The NiM1B receiver is a conventional
superhet design with a MOSFET mixer and crystal/ceramic
filter. I’ve included a block diagram of the NiM1B
transceiver in Figure 1. Figure 1 also applies to the
dedicated receiver and dedicated transmitter variants of
In that we have two NBEK carrier boards that are not
loaded with a transceiver module, we can only collect
data at the receiving station. If our NBEK carrier boards
were loaded with the transceiver version of the NiM1B
(like the one you see in Photos 2 and 3), we could
communicate station-to-station in half-duplex mode. Recall
that half-duplex mode is mandatory in that there are no
automatic collision detection methods in place for the
Interfacing the NBEK Carrier
The NBEK carrier boards are hard-wired to accept true RS-232 signals at
the 1200 baud modem input. If you
take another look at Schematic 1, you
will see that the electronics behind the
modem’s TXD and RXD lines are wired
as a null modem. That’s a fancy way of
saying the NBEK modem’s RXD pin is
actually sending to the other device
while its TXD pin is really receiving
from the attached device. The null
modem wiring scheme allows us to tie
the NBEK 1200 baud modem directly
to an external device’s serial port that is
configured as DTE (Data Terminal
A good example of a DTE serial
port is the nine-pin male serial ports
that used to be found on PCs prior to
the adoption of USB.
The NBEK’s null modem hookup lends itself to
attachment to embedded RS-232 serial ports, as well. The
embedded platform you see in Photo 4 happens to be
equipped with a true RS-232 serial port in the form of an
ST3232C RS-232 transceiver IC.
In reality, the NBEK’s modem interface is actually
operating at TTL logic levels. From the modem’s
perspective, Q5 can be driven with a five volt signal. In
fact, it can be driven with a 3. 3 volt logic level, as well.
With its emitter tied to + 5 VDC, transistor Q2’s collector
will toggle between 0 and + 5 VDC. If we were to attach a
3. 3 volt microcontroller UART directly to the NBEK’s
modem interface, we would only have to convert the
voltage generated by Q2. That can easily be done with a
November 2015 63
■ Photo 2. This aerial view captures the top panel of the
NiM1B narrow band FM VHF transceiver. The NiM1B
measures in at 33 mm x 23 mm x 11 mm.
■ Photo 3. This is a more interesting view of the NiM1B.