may be paralleled along the bus. The
bus can run up to 1,000 meters but
that limits the data rate to no more
than 40 kbps at that range. Each
node on the bus is a transceiver that
may send or receive data. Nodes
compete for bus usage with a
contention process that gives the bus
to the node with the highest priority.
Data is then sent byte by byte with
asynchronous start and stop bits.
There are several variations of
CAN, including CAN 2.0A and CAN
2.0B. Others are CANopen used in
industrial applications, DeviceNet
used in factory automation, and
J1939 — a Society of Automotive
Engineers (SAE) variant for trucks and
CAN may be integrated into
microcontrollers or other ICs.
However, separate CAN transceivers
and controllers are also available.
Another popular serial standard
is RS-485. Like CAN, it uses
differential shielded or unshielded
twisted pair cable to form a bus. The
differential configuration helps cancel
noise over long distances. Cable
length can be up to 4,000 feet, but it
does limit the data rate to about 100
kbps. Data rates to 10 Mbps can be
achieved at up to 40 feet. Depending
on the type of line driver and bus
length, speeds up to about 35 Mbps
can be achieved.
The bus can have up to 32 nodes
on it. Each node is a transceiver that
may send or receive. Separate IC
transceiver chips are commonly used.
As for applications, it is variable.
Mostly, RS-485 is found in industrial
applications but it also shows up in
building automation, video
surveillance, and point of sale
There is no fixed transmission
protocol, so any solution can be
used. Transmission is usually
asynchronous byte by byte using start
and stop bits.
As I said earlier, there are dozens
of other serial I/O interfaces. Just a
few are 1-Wire, BitBus, FlexRay,
IOLink, I2S, LIN, Microwire, MIDI,
OBD, and X10. Most were
developed for specific applications
and many are similar in operation.
Some of the faster interfaces are
more widely used. I will cover the
most popular of these like USB,
Ethernet, and PCIe in Part 2. NV
January 2015 65