Nuts & Volts

Star Link
PERSONAL ROBOTICS
sion or receipt of a message is exceptionally
fast and for this context, give
me that over bandwidth any day.
All readers with battery operated
robots will love to hear that Zigbee is
something of a power miser, as well.
The power consumption is almost
negligible during transmission and
receiving, and even less when Zibgee
is idle, dozing, or hibernating.
Transmission and receiving uses as
much as 37 mA, whereas idle uses 500
μa, doze 35 μa, and hibernate as little
as 2.3 μa. Even in the hibernate mode,
the chip still responds to attention.
So much for all these numbers, but
what does this actually mean for your
robots? Well, you could take a Zigbee
device and, depending on usage, it
might last months or even years on a
coin-shaped calculator battery.
The range of Zibgee is an
interesting thing in itself and you
might say it depends on the specific
topology which is in use. Certainly, if
the most simple network connection
is in use (peer-to-peer), then the
absolute overall range is in the 250
feet or less arena and 90 feet has been
proven indoors (hmmmm, that will
cover most homes). However, this
range may be ‘virtually’ extended if,
for example, a ‘mesh’ topology is
used, as we will discuss later.
A Zigbee network can be simple
and complex at the same time; this
mostly depending on the nodes
which are in use. You can set up three
‘node’ types within your network:
• The PAN Coordinator (PANC,
Personal Area Network Coordinator).
– Starts and ‘owns’ the network.
– Allows other devices to join.
– Provides addresses and
saves messages until they can be
delivered.
other device.
– Scans to find a network to join.
On top of this, a Zigbee device
may belong to one of two classes of
device:
• FFD — Full Function Device which
allows it to be a Coordinator or a
Router Node.
• RFD — Reduced Function Device
which means it can only be an End
Device Node.
Whew! Having gotten through
that, we can now look at some of the
topologies which Zigbee allows us to
construct.
The absolute simplest topology is a
peer-to-peer involving two devices. Take
a look at Figure 1. This is a subset of the
star topology, which we’ll discuss next.
Of these two devices, one must be a
PANC and the other may be an End
Device. One way to look at this particular
net is as if it were a point-to-point
serial connection, only without a cable.
The maximum distance the two nodes
may be apart is approximately 250 feet.
We can now imagine our PC with
the PANC Node and the robot with
the End Device Node communicating
Robot
RS232
End
Device
across the living room floor, the robot
obeying commands entered at the PC
relaying back status information, such
as battery charge level.
The next level of topology is an
extension of the simple peer or peer
network — the star (see Figure 2). The
star still has at its center — the PANC
— but may have a number of End
Device Nodes (up to 64K) with which
it communicates. The PANC may
communicate with any of the End
Devices, however, each End Device
may only communicate with the
PANC and not with each other.
In this case, we might have a
number of robots, each with an End
Device communicating with the PANC
attached to our PC. From the PC, we
might address each, in turn, issuing
control commands. The maximum
range would again be approximately
250 feet in any direction.
This is most likely the topology a
roboticist would use, as it is simple
and allows for many robots to be
connected to a central PC or, indeed,
a ‘head/lead’ robot. For the sake of
completeness, we will look at mesh
topologies (see Figure 3) (though there
are cluster or tree topologies, as
well). The main advantages a mesh
network will bring are extension of
250 foot
range
Zigbee
RFD or FFD
■ FIGURE 1
• The Router.
– Does not own the network, but
routes messages.
– Scans to find a network to join
and provides addresses.
• The End Device.
– Does not own or start a
network.
– Communicates with a single
Zigbee FFD
PANC
RS232
PC
April 2006 81