SB-Bus, What Is It?

The SB-Bus is a serial bus system which enables you to connect up to 125 devices to one serial port on any type of computer with an RS-232 interface.
On the software side the SB-Bus is only a protocol that describes the behaviour of the devices. On the hardware side it is a very cheap interface containing only 2 opto-couplers a few resistors and one transistor per Slave device. The SB-Bus is a single Master system, which means that only the computer controls all data traffic, all other devices on the bus are just Slaves, which simply obey commands given by the Master.

Why Did I Develop This SB-Bus?

Back in the late eighties computers used to have just one so called COM ports, two at the most. USB 1.0 came in 1997. I wanted to connect multiple home made measuring instruments and other devices to one serial connection of any given type of computer. Existing bus systems at the time, like GP-IB, were far too expensive for me. As you will soon see, the implementation of the SB-Bus system can hardly be any cheaper!
Another requirement was that controlling the devices had to be possible without the need for dedicated programs on the Master computer. The SB-Bus can be controlled by typing normal English commands on a simple terminal emulation program.
To my knowledge the SCPI standard was not defined yet when I started to develop my SB-Bus. That is why I didn't go for that protocol standard. I did however borrow some ideas from the Fluke 45 multi meter.
As a result the SB-Bus is a very affordable bus system, both on the hardware side and the required system resources. The full implementation of the bus system usually takes about 2kB of program ROM on the Slave devices.

What Are The Features?

  • Single Master system
  • Up to 125 addressable devices on one bus
  • General call address, to which all Slaves will listen
  • Communication in plain English commands
  • Communication by any terminal program or dedicated program on Master PC
  • Communication speed of 9600 baud, 8 data bits, no parity, 1 stop bit
  • Two flow control methods available
  • Can be connected to any standard RS-232 port
  • Slaves don't interfere with each other's communication
  • Devices are electrically isolated from each other and from the bus
  • Inexpensive hardware in each Slave device
  • Inexpensive connections between Slaves and bus driver
  • Different inexpensive driver circuits available to meet every demand