Figure 8. Block
Sum using odd
parity for eight
Figure 9. An
error that can
Figure 10. Block
Sum can’t detect
computer. This port uses one wire as ground and the other
as data. Differential transmission — as that used in RS485 —
sends the data through two wires that are either driven high
or to ground, but are always opposite each other. On the
receiver end, a comparator extracts the data.
Another common problem with data transmission is
crosstalk. This can even affect short traces on a circuit board.
Crosstalk is where one wire can pick up the signals from a
nearby wire, even though there is no direct connection. If it is
possible, separate wires that carry different data as best as you
can. It also helps to put ground wires between signal wires if
you happen to be sending the data through a ribbon cable.
One final thing to think about if you are sending data
through wires is the possibility that these wires might act as
antennae and pick up radio waves from transmitters that are
close by. This can be an infuriating problem because it can be
sporadic. A way to solve this is to put a low pass filter onto
your data line. It is highly likely that the radio waves will be a
much higher frequency than your data is and can be easily
filtered out while leaving your data pulses relatively intact.
If the method that you are using to send your data is
wireless, then there are a few things to think about, as well.
The first thing would be to determine if you really need
wireless capability. Wireless communications are often noisier
than direct connections. Picking the frequency that you will
be using is important. If you choose a frequency that has a
lot of use, you are more likely to encounter interference.
The method that you use to send your data can affect
its reliability. Amplitude Modulation (AM) is prone to noise
interference, but it is inexpensive to add to your project.
Frequency Modulation (FM) is less prone to noise, but is more
You may decide to use a premanufactured radio module.
Not all radio modules are equal, so you may want to test out
When not writing for SERVO Magazine, Jack runs Buffington
Effects, a company that designs and builds animatronics and
motion control devices for the entertainment industry. Check
out his website at www.BuffingtonFX.com
a few different ones to find what fits best with your application.
One other thing to consider when selecting a radio module
is that, even though the specification for your radio module
may say that it can transmit at a certain data rate, the data
rate specified may be under ideal circumstances. By lowering
the speed that you send your data, you may find that the
number of errors you encounter decreases.
Error detection and recovery is something that you will
likely need to deal with at some point if you work long
enough with electronics. Hopefully, this article has given you
some insights into how to ensure that your data is received
reliably and, if it is not, how you can detect bad data before
it is too late. SV
STEER WINNING ROBOTS
Perform proportional speed, direction, and steering with
only two Radio/Control channels for vehicles using two
separate brush-type electric motors mounted right and left
with our mixing RDFR dual speed control. Used in many
successful competitive robots. Single joystick operation: up
goes straight ahead, down is reverse. Pure right or left twirls
vehicle as motors turn opposite directions. In between stick
positions completely proportional. Plugs in like a servo to
your Futaba, JR, Hitec, or similar radio. Compatible with gyro
steering stabilization. Various volt and amp sizes available.
The RDFR47E 55V 75A per motor unit pictured above.
SERVO 07.2004 43