I am trying to drive a fairly long length of cable. What kind of additional circuitry should I consider?
US Digital makes a variety of cable drivers. For maximum noise immunity and minimum cross talk, a differential cable driver and receiver is recommended for cables longer than the 6 to 10 foot range. Our line drivers use the industry standard 26C31, which sources and sinks 20 milliamps at TTL levels. Each channel pair should drive a twisted pair in a multi-pair cable. US Digital receivers terminate each pair with a 110 Ω resistor to eliminate ringing.
I am having some problems with noise affecting my encoder signals. What should I do?
There are several ways to improve noise immunity. Separate motor driver wires
from encoder wires, and be sure to separate encoder channels from each other
(i.e., do not twist CH A with CH B or CH I). Shielded twisted pair cable is
recommended for encoder lines. Also, be sure to separate the ground lines of the
motor/amplifier and the encoder since high current switching in the motor can
cause noise in the encoder output lines. Pull-up resistors help somewhat with
noise rejection. Additional circuitry, such as line drivers/receivers and
optocouplers, is much more effective in rejecting noise. In really noisy
environments, additional circuitry may be the only way to totally eliminate a
noise problem.
Does US Digital provide encoders with differential line driver outputs?
Yes. US Digital has the E5D, E5MD, E6D, E6MD, E7PD, H15D, H5D, H6D, H6MD, HD25, PED, S5D and S6D encoders, which are US Digital encoders with a built-in line driver board. This board uses an industry standard 26LS31 line driver IC. The line driver outputs are RS-422 compatible and provide differential outputs. Both two and three channel encoders are available with the line driver option. We also have our PC4/PC5 external mini-boards that connect to the output of single-ended encoders to provide line driver outputs.
Typically, how much current do the LED and photodetector IC draw?
In the two channel modules and encoders, the LED typically draws about 15 mA and
the IC draws about 2 mA. In the three channel modules and encoders, the LED
draws about 45 mA and the IC draws about 10 to 12 mA.
What kind of output drives do US Digital encoders have?
The HEDS transmissive optical encoder module does a good job of sinking current (3.2 mA or 3.86 mA to ground), but they dont pull up to +5V as well (40 μA or 200 μA to +5V). The encoder module outputs are open collector with a 65 kΩ internal pull-up resistor. When driving a cable, they will provide good high-to-low transition times. But low-to-high times will stretch out in proportion to the cable length and capacitance. If a cable driver is not used, you can add a pull-up resistor (2.7 kΩ to 3.3 kΩ) to +5V on each output to improve these rise times.
The EM1 transmissive optical encoder module will source and sink 8 mA at TTL levels.
