Page 120 - Anatomy of a Robot

03_200256_CH03/Bergren 4/17/03 12:27 PM Page 105
Here’s one computer bus almost everybody overlooks: www.hits.org/hits/bus/
bus5.html. Hopefully, you’ll see it coming! COMPUTER HARDWARE 105
Now let’s talk about some standard buses. More information on each can be found in
the two previous interface.com web sites. The standard buses are as follows:
Industry Standard Architecture (ISA) ISA no longer means what it says. This
bus came out with the original PC and was the mainstay of the industry for many
years, but it’s obsolete in that industry now. The bus had a limited bandwidth at 8
MBps. Don’t use it! For more info, go to www.interfacebus.com/Design_
Connector_PCAT.html.
Peripheral Component Interface (PCI) The PCI bus has taken over as the
standard bus in the PC industry. It’s a bus with a specialized type of signal that
is limited in two ways. Signals can only traverse a limited distance (roughly the
size of a PC motherboard). In addition, only about four loads (like connectors
or integrated circuit pins) can be put on the bus before it starts to load down
and fail. Bridge chips exist that can extend the PCI bus to more loads and
sockets.
A few versions of the bus exist, differentiated by the voltage, word width, and fre-
quency. The most widespread version has the following characteristics: 5 volts and
32 bits at 33 MHz. This gives a bandwidth of (32/8) 33 million 132 MBps
per second (raw speed). As a practical matter, nobody could ever get better than
about 100 Mbps out of the bus because of housekeeping tasks that take place on
the bus. The maximum size of PCI bus technology lately is 64 bits at 133 MHz
for a 1 Gbps bandwidth (raw speed).
PCI has become an industry standard. Many board manufacturers and many chip
manufacturers have adopted it. If the robot’s computer supports the PCI bus, many
third-party boards will be available to customize the design and save time (see
www.interfacebus.com/Design_Connector_PCI.html).
The PCI bus would be an excellent choice for a robot as long as the vibration prob-
lems can be addressed. The bus has around a hundred pins on each connector. It
only takes one pin to fail from a vibration to bring a system down. If reliability is a
key, look into the Compact PCI standard. It’s a bit sturdier (see www.interfacebus.
com/Design_Connector_CPCI.html).
PCMCIA cards This standard describes not so much a bus as an interface socket.
Many peripherals are available as pocket-sized PCMCIA cards, so it’s a good option
for adding memory and peripherals to a robot. Most portable laptop PCs have PCM-
CIA sockets to accommodate these cards. The transfer rate is on the order of 20
MBps (see www.interfacebus.com/Design_Connector_PCMCIA.html).