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Mini SB-Bus

Let's start with the easiest part of the SB-Bus, the hardware. On the Master side you need a bus driver. In case you don't understand the term Master yet, the Master is usually a PC controlling all devices (the Slaves) on the SB-Bus.
An SB-Bus driver is a simple electronic circuit which translates the electrical RS-232 signals to and from SB-Bus signals. The TxD signal from the RS-232 port must be amplified to enable it to drive multiple LED's in the opto-couplers of the Slave devices. And the signals coming back from the Slaves are transformed back to proper RS-232 levels before they can be accepted by the RxD pin of the Master's RS-232 port.

You can see some examples of SB-Bus drivers with varying drive capacity elsewhere on my site.

Now it's time to describe the Slave side of the interface. Let's consider what needs to be done:

• Signals coming in from the SB-Bus driver (the connector in the middle in the diagram below) must be fed to the LED side of an opto-coupler. This is easy enough, in the diagram you can see that a simple 390Ω resistor is all that we need. All Slaves are connected in parallel with each other, so they all receive the same signal on their incoming opto-coupler. The other side of this opto-coupler is connected to the Slave's UART RxD input.
• Signals going out from the Slave must also be electrically isolated from the SB-Bus. A second opto-coupler is used to achieve that. The output transistor of this opto-coupler is directly connected to the SB-Bus. All output signals from all the Slaves are connected in parallel to each other, which effectively creates a wired-OR function.
  The LED of the out going opto-coupler is driven by a transistor, which is used as emitter follower.

Basic SB-Bus Interface

Some remarks about this diagram:

I choose an emitter follower circuit for the output transistor. This way we can drive the LED without inverting the signal. A normal NPN transistor would invert the signal from the UART, which is high when it is in rest.
A PNP transistor on the supply side could also be used to drive the LED without inverting the signal. I deliberately didn't use this option because some UARTS have TTL levels on their outputs, which means that the high level does not exceed 3.5 Volts. This would result in a continuous base current through this transistor.

The type of the opto-coupler is not very critical. However, it should meet the most important property, which is the slew-rate. Please take care that the output ramps of the opto-coupler are steep enough to avoid signal distortion.
I have had good results with the types 4N25, 4N35 and the CNY17-1. To improve the slew-rate I connected a 100kΩ resistor between the base and emitter of the output transistor.
Please note that my experience with the CNY17-2, which is often used as feedback coupler in switched mode power supplies, was not satisfactory.

Take great care with projects that are directly connected to high voltages, e.g. measuring instruments. Electrical isolation is particularly important in those projects. Take heed to use a wide enough isolation area between the SB-Bus side and the "live" side of the instrument.

Almost 20 years ago I decided to use mini-DIN plugs for my SB-Bus devices. The same 4-way plugs are used for S-VHS connections, which somehow guaranteed availability. These plugs are both small and affordable.
If I had to redesign the SB-Bus today I probably would have used RJ-11 connectors because they are even cheaper and easier to produce.
Personally I'm not going to switch to RJ-11 connectors any more, but of course you may use whatever connectors you find suitable for your projects. Chances are very small that our projects will ever meet up on the same bus.

Master connector front view

Slave connector front view

The diagrams above show the connectors for both sides of the bus. Swapping the RxD and TxD terminals on the Master connector enables you to connect all Slaves with simple straight through cables.
Please note: The return terminals on the Slave side may not be connected together! However it is no problem to connect the return terminals on the Master side together.

It is highly recommended to implement two SB-Bus connectors per device. Both connectors should be connected in parallel with each other. This way it is extremely easy to daisy chain multiple devices to the SB-Bus driver.