NXU-2A Operations Manual
Interoperability Now! 6-5
I want to connect my NXU-2A serial port to the ACU-1000 serial port, but the cable you
supplied doesn't work. What kind of cable do I need?
The RS-232 serial cable supplied with the NXU-2A is a straight-through cable used to connect
the NXU-2A to a PC serial port. In order to connect the NXU-2A port to an ACU-1000 you
need a DB-9 male-male null modem cable, also called a crossover cable.
Why do I need a null modem cable to connect the NXU-2A to the ACU-1000? I don't need
one when I connect my PC.
RS-232 serial devices are divided into two types. Devices that provide control information are
called DTE (Data Terminal Equipment) devices. Devices that accept control information are
called DCE (Data Communication Equipment) devices. In the "old days" the DTE devices were
terminals and computers, while the DCE devices were modems. The RS-232 standard
specified that DTE devices would connect to DCE devices using a straight-through cable. If
you wanted to connect two like devices (DTE to DTE or DCE to DCE) then you needed a null
modem (crossover) cable. The PC is a DTE device, and the NXU-2A and ACU-1000 are DCE
devices. That is why you need a crossover cable for connecting an ACU-1000 to an NXU-2A.
Can I change the vocoder on the NXU-2A?
Yes. This is the voice compression method used to convert the audio signal into a digital
format. There are times when the user may want or need higher voice quality or transparency to
certain kinds of signaling tones. The VOCODER command allows the selection of different
vocoders, albeit at the expense of network bandwidth.
Can I pass modem tones over the voice port of the NXU-2A?
Some types of modem tones (such as FSK) will work if you select one of the higher bit rate
vocoders, such as VOCODER 4 or VOCODER 5. Complex modem waveforms (such as 56K
modem data) will not pass no matter what vocoder is selected. Pilot tone, keying tones, and
other similar signaling tones will work properly with the 24, 32, and 64 Kbps vocoders.
In general data should be sent via the RS-232 port.
I've connected my radio to the NXU-2A, I have the levels turned all the way up, and can still
barely hear the audio. What's wrong?
The NXU-2A uses balanced audio on the input. If, instead of connecting to both balanced input
lines, you connect your audio to one audio input and ground, you'll get the effect described
above. There is enough leakage in the transformer to get some audio through, but it won't work
properly. The solution is to ground one side of the NXU-2A balanced input.
I've connected my radio to the NXU-2A and it works, but the audio sounds very "tinny" and
doesn't have much bass.
Make sure you haven't used the discriminator output, which is sometimes available on FM
receivers and transceivers. Discriminator audio is pre-emphasized, and there is no de-emphasis
circuitry in the NXU-2A. You'll need to use your regular audio output instead.
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