Project on Orientation Programme Project on Orientation Programme | Page 4

ABSTRACT
Although RTL stays away from all these logic families, it is still interesting to see if
there is some connection between them... some common general idea... Let’s try
to find it...
Remember that in RTL we summed voltages by converting them to currents. But
this circuit was sensitive to the magnitudes of the voltages and resistances; in
addition, the number of inputs was limited. It seems we can sum, besides voltages
and currents, why not resistances as well?
Here is the implementation.
The input logic variables turn on (at logic "1") or turn off (at logical "0") equal
reference resistances (conductance).They are summed by an analog summer
again.
Their sum is converted to voltage and compared by a threshold device (voltage
comparator) whose threshold is lower than one reference. So it is sufficient that
only one reference is turned on and the output is set at logic state "1".
This idea is taken to the extreme in the classic DL, DTL, TTL, MOS and CMOS
circuits where the reference resistances are increased up to infinity. In practice,
they are implemented by diode or transistor switches operated by the logic input
variables.
They are connected in series to sum the switch resistances or in parallel to sum
their conductance (DL, DTL and TTL use only a parallel connection).
Page 4