An analysis of the B5x00 hardware

(by DL5UY)

The B5x00 hardware is solely made of discrete components like resistors, capacitors, diodes and transistors. No tubes are used. All transistors and diodes are germanium type.

In some places inductors, transformers and delay lines are used.

Potentiometers are used for adjustment of voltages or timing.

The central main oscillator uses crystal and a tank inductor.

The logic itself is basically diode transistor logic or DTL:

https://en.wikipedia.org/wiki/Diode%E2%80%93transistor_logic

Note that in this Wikipedia article NPN transistors are shown and the collector supply voltage is positive in regard to ground.

However, when the B5x00 Design was made in the late 1950s/early 1960s, the common and fastest Transistors were Germanium PNP transistors. Hence the collector supply voltage is negative in regard to ground level. In all circuit diagrams this fact may be confusing at first, since diode and transistor symbol arrows point in the technical current flow direction from plus to minus and not in the more “logical” electron flow direction from minus to plus.

For that reason, FALSE logic level is about 0 V, TRUE logic level is a negative voltage of about -4.8 V!

The B5x00 logic uses four different supply voltages:

• -12 V supply for the pull-ups of the diode based AND/OR gates and for the collector load resistors
• +20 V supply for the base pull-downs of the transistors
• -4.5 V sink to clamp the collector voltages with a germanium diode to around -4.8 V
• -1.2 V sink (elevated ground) to raise the emitter potential of the transistors so that the threshold for a TRUE logic level is about -1.5 V

Notes:

• the term "pull-up" refers to resistors connected to the -12 V supply
• the term "pull-down" refers to resistors connected to the +20 V supply
• the -4.5 V rail must be a current sink - current comes through the collector pull-up from the -12 V supply and through the germanium clamp diode
• likewise the -1.2 V rail must be a current sink, since it takes all the emitter currents

The core logic of a B5x00 is split into three distinct package types:

• AND/OR packages are made of diodes and pull-up resistors (to -12 V) only. As such they present a current source to the outputs of the signal source and a current source to the input of the signal destination. AND/OR packages cannot be cascaded!
• OR packages consist of diodes only and present a current sink to the output of the signal source and a current source to the input of the signal destination. OR packages are used behind AND/OR packages when many AND circuits are ORed tigether to reduce capacity. OR packages should not be cascaded because of the additional voltage drop.
• Active packages like flip-flops, buffers, inverters and other elements have pull-downs on their inputs (to +20 V) and transistor collectors with pull-ups (to -12 V) on their outputs. As such they present a current sink to the outputs of the signal source and a current sink to the input of the signal destination. Due to the pull-up they could source some current to a signal destination but this is not used. The inputs usually lead directly to the base of a transistor and therefore must be current limited!

This concept works perfect if you alternate active packages with AND/OR packages. Note:

• Active packages cannot be connected directly. The low value pull-up of the open collector output (typically 470 Ohm to -12 V) would drive roughly 25 mA into the base of the next stage input transistor
• Active packages must be connected with an AND/OR package between them. The 6.81 k pull-up in the AND/OR package will drive only about 1.7 mA into the base of the next stage input transistor
• AND/OR packages cannot be cascaded
• The outputs of AND/OR packages can be connected and result in a wired-OR configuration. When more than a few outputs are connected, the diode leakage currents and junction capacity will add up. Therefore OR packages are used to decouple groups of connected outputs
• The outputs of AND/OR packages, whether wired-or or not, can only be connected to one active package

Typical AND/OR Package

AND/OR Package

The AND/OR package has 10 pins and is equipped with six germanium diodes and two 6.81k resistors. All are connected together with the metallic bar at the top of the package. The diodes' cathodes are connected to the bar. This bar can be intentionally cut to divide the package into having two smaller AND/OR gates.

For the ease of discussion, we designate those pins as COM1, D1, R1, D2, D3, D4, D5, R2, D6, COM2.

When the bar is not cut we can realise a 5-input AND/OR gate, by connecting the input signals to 5 diodes and one of the two resistors to -12 V. The other resistor must be left unconnected.

One of the diodes is the output, which can be wire-ORed with other such outputs.

The number of AND inputs can be expanded in increments of six by just using several AND/OR packages connected at their COM1/COM2 pins. In total only one resistor should be connected to -12 V.

When the bar is cut like in the package shown above, the result is two 2-input AND/OR gates consisting of three diodes each and one resistor.

The OR package has 10 pins and is equipped with eight germanium diodes. All are connected together with the metallic bar at the top of the package. The anodes are connected to the bar.

When an OR package is used after an AND/OR package, all diodes of the AND/OR package can be used for inputs (effectively making it an AND only package) and the COM1 or COM2 pins go to inputs of the OR package.

Examples: AND/OR Package Schematics

SWITCH I Package

SWITCH I package

The SWITCH I package contains five independently useable inverters:

Since the emitter is grounded, the TRUE logic level input voltage is raised by the two diodes at the left. Those are are actually silicon diodes with roughly 0.6 V of drop each. That brings the TRUE logic level to about -1.5 V.

As stated before, in almost every active package, the inputs lead directly or through some diodes to the base of a transistor. The signal source must provide a current limitation. The TRUE level signal is clamped to about -1.5 V. The TRUE level output is clamped to about -4.8 V by a germanium diode to -4.5 V.

Here you can see a demonstration of the signal levels of a SWITCH I:

Oscilloscope of Signals

Here is the haywire test circuit used to get the signal levels:

Test Circuit

Here is the circuit diagram of that setup: SWITCH I Test Circuit Schematics

Better testing...

I made a small printed circuit board which will allow a more reliable wiring and even a working demonstration using several modules and diode sticks. The connectors are not the original ones, but do work well. Only at the “A” end some dremeling was be needed to accommodate the wider pins. The way those connectors are made makes this an easy task.

Test adapters