STEbus backplane

The connector side is almost entirely covered with copper, which is the ground plane for all bus signals.

This is essential, because it ensures every signal has identical controlled impedance.

The STEbus specification states:

7.6.2 Characteristic Impedance

The IEEE Std 1000 Bus is designed to take into account the driving requirements of high-performance transmission-line backplanes. The transmission-line system, together with the specified maximum signal length, allow an accurate determination of the time required for a signal to be correctly received.

Backplanes should be designed using only microstrips for the signal lines, and should provide an unloaded characteristic impedance of 60 Ω ±10% including the effects of plated through holes and connectors.

Backplane signal tracks should have a constant width throughout the length of the backplane so as to keep the same characteristic impedance throughout its length.

Groundplanes are required so as to form a well-defined transmission-line environment. All groundplanes shall be continuous, allowing breaks in the groundplane only around the holes where connector pins must pass through. Under no circumstances should slot lines be allowed to exist in the groundplane, whether in the horizontal, vertical, or any other direction relative to the signal lines.

Microstrip is explained here: https://en.wikipedia.org/wiki/Microstrip

The formula for calculating microstrip impedance is given in

https://www.analog.com/media/en/training-seminars/tutorials/MT-094.pdf

and there are online calculators, e.g. here: http://pwcircuits.co.uk/microstrip-line-calculator/

https://resources.altium.com/p/fr4 says the dielectric constant of FR4 material is 3.3-4.8 (depends on weave style, resin content, and material composition).

The STEbus specification allows up to 4 amps per board on the 5V rail.

A fully-populated STEbus backplane with 21 boards could thus take up to 84 amps.

The Arcom backplanes had many M4 threaded power terminals for attaching heavy-duty power cables.

Although it is unlikely that anyone will need so much current, it is wise to make sure power cables are thick and low-resistance to reduce power rail ripple from transient high currents.

The bus termination circuit from the specification is simple but the 2.8V regulator must also sink current when required. Common voltage regulators are only designed to source current. Examination of a 5-slot backplane reveals a working circuit.

Current flows through R3 to drive the NPN transistor TR1. The voltage reaches 2.8 volts, then the junction of the feedback resistor chain (R1, R2) reaches 0.7 volts and activates TR3 to drain drive current from the base of TR1 . TR3 also activates the PNP transistor TR2, to sink current from the 2.8 volt rail. Diode D1 helps stop both transistors conducting at the same time.

The whole circuit consumes about 20 mA, unloaded.

The termination regulator circuit should be useful for other buses.

Termination can be built on boards with DIN41612 connectors, but this adds the cost of connectors and occupies one or two slots.

Termination is recommended at both ends of a long bus (e.g. 10 to 21 slots), but a short bus (e.g. 5 slots) will work safely with termination at one end.