NXU-2 Operations Manual
6-4
JPS Communications, Inc.
What happens if two NXU-2's have the same IP address?
Bad things. Well, not bad things, but obviously it will not work properly. Both units will
respond at the same time, causing network errors. The extreme case would be if an NXU-2 is
programmed to have the same IP address as an existing computer on the same network. This
would likely render that computer unusable for network functions, and could cause other users
to have problems as well. The bottom line is that two devices cannot share the same IP
address, period.
What is a firewall and how does it affect NXU-2 operation?
A firewall is a device, which prevents people outside a network from accessing computers, or
devices inside the network. It's a security device. A company firewall prevents incoming
network connections, so no one outside the company can access its computers via the Internet.
This is a Good Thing, but can cause problems with a device like the NXU-2 since it requires
both incoming and outgoing connections. Fortunately, it is usually possible to program the
firewall to allow selected incoming connections so that things like the NXU-2 can work but
computers are still protected from outside attacks.
How can I program my firewall to allow the NXU-2 to work?
The NXU-2 uses two methods of communicating over IP (Internet Protocol.) These two
methods are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol.) With
both methods the NXU-2 uses IP port 1221 to communicate. If a firewall is preventing the
NXU-2 connection then the network administrator must program the firewall to allow TCP and
UDP traffic on port 1221. This is often referred to as "opening a hole in the firewall" and
should only be done with the cooperation of the network administrator.
After installing a pair of NXU-2's, the LINK ACTIVE light comes on, stays on for about 20
seconds, and then goes out. This cycle repeats endlessly. What's going on?
The most likely cause is a firewall, which is preventing UDP traffic on port 1221. See the
question above about firewall configuration.
I want to connect my NXU-2 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-2 is a straight-through cable used to connect
the NXU-2 to a PC serial port. In order to connect the NXU-2 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-2 to the ACU-1000? I don't need
one when I connect my PC.
RS-232 serial devices are divided into two types. Devices, which provide control information,
are called DTE (Data Terminal Equipment) devices. Devices, which 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-
Summary of Contents for NXU-2
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