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generator: pandoc
title: 'The Electronics of DEC''s R-Series Logic'
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---
2018-03-26T19:54:41+11:00
Introduction
============
I have tried many times, unsuccessfully, to create a discrete component
computer. My basic logic element was a NAND gate, and I was using a
popular form of DTL logic unit. I was copying the same electronic
schematic as the *Tiny Tim *DTL computer. You can find the research and
build log for this abandoned project here:
<http://www.northdownfarm.co.uk/rory/tim/tinytim.htm>. Anyway this was
my basic logic element, the NAND on the far right.
I got stuck trying to build a ring counter, which I suppose I would use
for control signals. I now see that a ring counter was not necessary,
because many TTLers have gotten away with functioning discrete component
computers without ring counters. In fact many people have successfully
built four-phase clocks, so I now don't really need to worry at all,
because the problem has been solved for me!
The reason why I got stuck was because I was attempting to build a DTL
edge-triggered D flip-flop, and no matter how I tried, I could not build
one that contained SET and RESET terminals. I could build one with just
a clock and an INPUT terminal, but I could not go any further:
Enter the Discrete-Component PDP-8
==================================
I started researching and building a discrete component CPU/computer
when I was diagnosed with schizophrenia in July 2013. The level of
detail and concentration you have to come to master prevented me from
getting any more delusional. I gave up the project when I started my PhD
in 2015. I made another attempt to make a four-phase clock in 2016, but
I failed again.
Then when I returned to this project a month ago (Feb 2018), I
remembered I came across some schematics for an old discrete-component
IBM computer back in 2013, but balked at the idea of using their basic
logic element because it used so many bizarre voltages. "It used
negative voltages!" I thought. There was no way I was going to be able
to manage a project with such whacky logic levels, like negative
voltages for positive logic signals. I went down the TTL logic route for
five years.
This time, in 2018, I was determined to get further. I *knew* discrete
component computers were built in the 50s and 60s, and I *knew* they
were able to generate clock signals and have multi-phase clocks. All I
had to do was find their schematics.
So I did. I found a wealth of resources on the DEC PDP-8. It is a
wonderful computer architecture. It has a single accumulator you use
with something like 6502 zero-page memory addressing in order to perform
calculations. It is very RISC-like with a tiny number of opcodes, and it
only contains something like 500-odd gates. So in February I thought it
would be a wonderful computer to clone.
...if only I had enough money.
When I found out I could join this TTLers group, I decided I wanted to
share my research on the strange DTL architecture of the PDP-8 in the
hope it would help somebody.
The Standard Logic Pulse of the PDP-8
=====================================
Before I start describing the electrical and logical system of the
PDP-8, I want to explain how I came to obtain all this information. I
retrieved it from [pdp8online.org](http://pdp8online.org/). Virtually
the entire library of information required in order to repair, construct
or just simply understand the PDP-8, as well as other DEC minicomputers,
is all online at that website.
All of the information I have explained here comes from the DEC 1967
Digital Logic Handbook. It is the last digital logic handbook that uses
R-series logic. From 1968 onwards, DEC uses M-series logic, which is
basically just normal TTL.
Anyway here is the standard logic pulse of the PDP-8.
The PDP-8 uses DEC's proprietary R-series logic. As the following
diagrams explain, R-series logic takes a -3V signal to be a digital ONE,
and a 0V electrical signal to be a digital 0. This is very whacky, and
may seem strange, but there's nothing theoretically incorrect about
using negative voltages in one's logic system. Virtually any voltage
differential can map onto whatever number system you want your computer
to use. By the time we get to Diode-Capacitor-Diode gates, the reasoning
for -3V = 1 will become apparent.
This diagram explains the standard rise and fall time requirements of
standard logic pulses in R-series logic:
The Standard DEC Logic Gate Symbols
===================================
So that the later diagrams and schematics make sense, it is important to
understand the symbols for different logic elements that DEC employed in
order to describe the R-series logic system. Here is the explanation:
Here, in Figure 14, we can see (from left to right, down the page) the
symbols for Diode-Capacitor-Diode gates (DCD gates), for a PNP
transistor, for NAND and NOR gates, for RS flip-flops, for monostable
multivibrators, and for general logic functions.
R-Series Logic Signal Symbols
=============================
The R-Series logic construction of the PDP-8 doesn't just use standard
-3V signals for digital ONEs. It also uses positive logic PULSES, in
addition to LEVEL logic signals (in order to edge-trigger flip-flops),
and it uses negative PULSES, as well as non-standard signals where it
isn't possible to efficiently construct logical elements electrically
using standard signals and pulses.
Please find the standard DEC R-series logic signal symbols below.
You can see why I originally dismissed this system in 2013, because it
can seem theoretically unparsimonious. It is very promiscuous with its
conceptual implementation, BUT, as I now understand, this is for the
purpose of making R-series logic computers very easy to construct by
hand. This allows the PDP-8 to be constructed with the minimal number of
electronic components, and it simplifies the overall electrical
schematics.\
\
As you can see, the DCD gates in the bottom part of the diagram are
receiving positive logic level signals, and are triggered by positive
pulses, and inverters are shown to be receiving negative pulses, or
negative-going logical level changes.
The R-Series Logic NOT-gate
===========================
This is the standard inverter of R-series logic. Virtually every other
passive logic element is constructed out of this gate:
It uses a PNP transistor, the exact electrical component equivalents I
have found, and it uses far more diodes than in other DTL electronics.
In fact, you'll find, when looking at the schematics of the R series
flip chips, which I can provide, most of the logic is actually carried
out by diodes, with signal voltages to be amplified later by
transistors. Very few transistors indeed are used in R-series logic.
This may actually be an advantage to the system.
Figure 2 above shows what a multiple input diode gate looks like in
R-series logic. With negative signals, this gate operates as a NAND, and
with positive signals, it acts as a NOR.
Edge-Triggering: Enter the Diode-Capacitor-Diode Gate
=====================================================
The diagram above explains the operation of the DCD gate in a sufficient
level of detail. Pulses with shorter levels than normal logical pulses
trigger the DCD gate, and using DCD gates allow one to be able to sample
flip-flops while they are about to be changed. ADDING capacitance to a
logical circuit seems like a crazy thing to do, but that is what DEC
did.
DCD gates are a fairly clever solution to isolating logical inputs and
outputs, as well as allowing edge-triggering. Edge-triggering using
whole NAND or NOR gates can lead to many many semiconductor components
being involved. Remember, using whole NAND gates transforms an
logical-level D flip-flop from this:
to an edge-triggered d flip-flop, this:
Not only do we now have one more gate, but we also have gates with
multiple inputs, requiring more diodes. Nevermind that we haven't dealt
with settable and resettable edge-triggered D flip-flops.
This is the symbol for a DCD gate. As you can see, they operate with
POSITIVE logic pulses, and POSITIVE logic level inputs.
Flip-Flops
==========
In much the same way as DCD gates allow input-output isolation, and edge
triggering with few electronic components, DEC standard RS flip-flops
are also very simple in their electrical construction. They use few
components, and would therefore reduce the overall size of the PDP-8, as
well as its operating power, and heat footprint. The R-series logic RS
flip-flop below reminds one of a simple RTL flip-flop.
You can also see the logical symbol for the standard R-series logic
flip-flop below.
RS flip-flops are almost always combined with DCD gates, which allow the
logic element to perform in dynamic operation. Outputs may be sampled as
inputs are waiting to be transferred into the logic element, so DCD
gates allow logical isolation.
As you can see, flip-flops require POSITIVE logic level signals, and
POSITIVE logic pulses.
Complementing Flip-Flops
========================
The power of the DCD gate is apparent here - without any extra
components, a normal RS flip-flop can be turned into a complementing JK
flip-flop.
Complex Logical Elements
========================
This is a very quick example of how R-series logic flip-flops would be
combined into a complex logical element. There are a great many examples
in the R-series logic handbook, like settable and resettable counters,
etc.
The Legacy of RTL in R-series Logic: Pulse Amplifiers
=====================================================
Because most of the logic calculation in R-series logic is performed by
diodes, R-series logic requires analogue electronic elements. The
fan-out of digital logic elements is limited, so amplifiers are needed
to restore electrical signals to their proper voltage and current
rating. All of the standard fan-out capabilities of standard R-series
logic flip-flops are in the 1967 Digital Logic handbook, if you're
looking for more information.
This is an example of a flip-chip module which is a pulse amplifier, the
R601 module:
As you can probably gather, it is just 6 DCD gates combined with a
transistor amplifier.
Component Substitution
======================
When you look at the R-series flip-chip modules, you're probably going
to end up despairing because many of the components used in the modules
are now obsolete. I have done my best to scour digikey and mouser to
find as close electrical equivalent components for all the major
elements of the flip chip modules.
That said, if this does present itself to some people as an attractive
electronic logic system, we may just reconstruct all the basic logic
elements using easily accessible components.
Webpage with original DEC substitution recommendations HERE:
(<http://so-much-stuff.com/pdp8/repair/subst.php>)
----------------------- ----------------------- -----------------------
Original DEC Component\ DEC-Specified Component My Suggested
Substitution\ Substitution\
D-662 Diode (High 1N645\ 1N4004
Forward Voltage Drop)\
D-664 Diode (Low 1N3606 SB140 Schottky Diode\
Forward Voltage Drop)\
DEC-3639 PNP 2N3639 2N4403
Transistor\
DEC-3009 NPN 2N3009 BC559/BC549
Transistor\
DC-2894 PNP Transistor\ NONE (!!)\ 2N4403 (?)\
----------------------- ----------------------- -----------------------