of Microprocessors

SIGNAL ROUTING 281
completely impractical. The only real sacrifice in flexibility so far has been
the limit of four signals connecting one subsystem with the others. Further
reductions can be made by further restricting interconnections, requiring
some manual patching, such as among subsystems, or permanent patching of
those connections that are nearly always made. Properly planned and backed
by an automatic signal routing program, these additional restrictions can
have a minimal effect on the utility of the system.
Mechanical Relays
In actually implementing a switching matrix, a number of switching
methods can be used. The primary split is between mechanical switches,
such as relays, and solid-state analog switches. The main advantage of relay
switching is the essentially zero contact resistance and total immunity to
signal overload conditions. Disadvantages besides· bulk and cost are slow
response times and excessive noise if a circuit is switched live.
One type of relay specifically made for matrix switching is the telephone
crossbar switch. These consist of contact assemblies arranged into rows
and columns. A relay coil is needed for every row and column rather than
every crosspoint, making the device very cost effective in the larger sizes such
as 50 X 50 or 100 X 100. Unfortunately, they are highly specialized devices,
generally only available to telephone equipment manufacturers. The
true hobbyist experimenter may be able to occasionally find them in scrap
yards, however.
A related device, also used in telephone switching systems, is the
stepping relay. The simplest type functions as a 10- or 20-position
solenoid-driven rotary switch that may be stepped into any position by
pulsing the coil. Another type has two coils and two-dimensional movement.
With proper pulsing of the coils, the armature may be connected to any of
100 points. One advantage of stepping relays is their inherent memory; they
will stay put until moved to a different position. The disadvantages are very
slow speed (1 to 2 sec to step to the desired position) and extremely noisy
operation.
One type of relay that is practical is the magnetically latched reed relay.
The contacts are sealed in glass, operation takes 1 to 2 msec, and they are
essentially inaudible. A permanent bias magnet (or separate bias winding)
gives a latching action. The bias field is insufficient to cause closure of the
relay but can hold it closed. A current pulse that aids the bias field will close
the relay, while one that opposes it will open the relay. The coils can be
wired in a matrix array just like the contacts. Thus, to establish a connection
between, .say, column 31 and row 17, one would pulse coil column 31
positive and coil row 17 negative for a millisecond to turn that relay on. Reed
relays designed for matrix operation may even have two coils in addition to
the bias coil, which eliminates the need for diodes when connected in a
matrix.