The SB-Bus Driver
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.
You can see some examples of SB-Bus drivers with varying drive capacity elsewhere on my site.
The Slave Interface Hardware
Now it's time to describe the Slave side of the interface. Let's consider what needs to be done:
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.
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.
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.
The SB-Bus Connector
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.
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.
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.
If I were to re-invent the SB-Bus all over again today I probably would use the cheaper and easier to apply RJ-11 connectors. Crimp tools are very affordable today, but in the late eighties they were still rather expensive. Thus, if you want to implement the SB-Bus on your own equipment, feel free to use RJ-11 instead of the Mini-Din plugs.