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Motion Control Board:
Personal computers are ‘general purpose’ devices that perform an
enormous amount of different functions, but are not designed nor intended to be extremely powerful
in any one type of processing. For computationally intensive functions, often a separate dedicated
processor is used so that the Central Processing Unit (CPU) is free to do the dozens of tasks required
to run your computer. The dedicated processor on a graphics card is a good example.
Generating high quality, high speed digital signals is one of the tasks that a general purpose CPU is
not well suited for and Digital Signal Synthesizers (DSS) are sometimes added to the mix to perform
that task. The DSS may be a separate chip or built into a much larger chip that also contains other
dedicated processors for other tasks. However, it is impossible for a user to integrate a single chip
into the CNC control scheme, so vendors stick then on a PC board and usually add convenience and
support features as well. Some of these boards have rows of screw terminals for the signals and
therefore do not require a separate BOB.
Likewise, the InTurn™ 4
th
Axis Motor Controller has a dedicated DSS mounted on a separate small
PC board with screw terminals for the needed signal wires and also contains the chips needed for the
‘swapaxis’ function and also a line driver to create ‘differential’ signals.
Similarly, a Motion Control Board would have a dedicated processor for calculating axis moves and
generating high-speed step/dir signals.
Parallel Port:
This is both a communication standard and a physical connector standard defined by
IBM for the first personal computer
.
Because all manufacturers followed that standard, any parallel
port printer can be simply plugged into any parallel port (sometimes mistakenly called a printer port)
Single Ended Signals:
A ‘single wire’ signal that has a high state and a low state for digital data.
Susceptible to interference from RFI, EMI and any other type of stray radiation. Reliability in long runs
is questionable. Shielded, low impedance wire is required to protect the signal from interference.
USB:
The intended replacement for the aging COM ports.
Modern connectors, buss
arrangement
with hubs and switches. Very fast and with a pathway to even faster speeds. Standardized protocols.
The extreme complicity
in creating ‘native’ USB devices led many manufacturers to opt for the ‘quick
fix’ of using single chip converters that allowed the old Serial protocol to be carried over the newer
USB standard. The problem with this sch
eme is that the ‘trick’ requires drivers (small software
progra
ms to control the converter chip and create a ‘fake’ COM port in the computer).
The drivers for competing brands and even versions within the same brand often trip over each other
and the ‘emulated’ COM ports are assigned, by number, to specific USB ports, which can get
reassigned or simply lost when a new device is plugged in causing yet another ‘converter’ driver to
load.
USB carries power for USB devices and the amount of power available has climbed steadily as USB
upgraded from 1 to 2 and so on. If too much power is drawn from a USB port (a USB2 device plugged
into a USB1 port, for example), the computer will shut off the port and many computers must be
rebooted from a power off state to recover. A USB device which requires more power than the USB
spec allows must be powered separately, which often leads to ground loops and other signaling
problems.
