Relay computers - OR functionality

From: Dwight K. Elvey <>
Date: Mon Sep 27 16:00:27 2004

>From: "Steve Thatcher" <>
>you create your OR functionality with diodes - use two diodes (or as many
inputs as you need) such as the 1N4001 for your inputs. Tie the cathodes
together and attach that to the relay coil. Apply "logic high" (whatever voltage
your relays operate at) to either (or any) diode anode and the relay will turn
on. The relay can be the next stage and does not have to be there to implement
the OR.
>best regards, Steve Thatcher

 You don't need the steering diodes if the lines that provide the
terms don't cross talk to other coils. This is pure relay logic
and works with AC relays as well as DC. The diodes are only
needed when you share complex terms that use the same stack of
contact terms but would otherwise cross talk. The diodes allow
one to optimize the number of contacts used by sharing common
 Each relay contact can be used as a term in an equation. Normally
open contact can provide inverting function and normally closed
contact can provide isolation buffering ( needed for AC but can
be optimized with diodes in DC ).
 Designing with relays takes a slightly different mindset than
with normal logic, like TTL. You have to think in terms of
inverters and transmission gates. There are no NAND or NOR
gates. The function of NAND and NOR is constructed by sharing
the contacts of several relays and then using that shared
stack to control the output ( that may be another coil of
a relay ).
 Stacking contacts can provide AND like function and paralleling
contacts can provide OR like function. Choice of normally
open contacts provides OR and AND with inverted inputs,
the Boolean equivalent of NAND and NOR.
 This takes a little more thinking of design because with double
throw relays, one has to think of the source as providing the
inverting and non-inverting signals. The contacts only provide
OR and AND, depending on how they are arranged.

>Question: Wouldn't it be easier to implement an OR gate with no relays at
>all (just two wires joining together)? Or would you need to use the relays
>to keep the output voltage/amperage regulated?
Received on Mon Sep 27 2004 - 16:00:27 BST

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