Jade Computer Products -  S-100 Bus Probe
Today with the use of multi-contact USB based logic probes and PC-based software its easy to forget how useful this S-100 bus probe by Jade was.  There were other single LED hand held logic probes but this one was unique.  Its size was a bit overwhelming and it did not do well at fast clock speeds but it often helped quickly identify a hardware problem on the bus.

The BUS PROBE provided an inexpensive tool for troubleshooting of CPU boards, S100 Temporary Bus Masters (such as disk controllers), as well as entire systems. Master boards generate most of the IEEE S100 bus signals. The BUS PROBE allows the user to quickly monitor these IEEE 696 S100 signals. Since very few front panel boards provided for display of the entire S100 bus, and most systems don't have front panels at all.  The BUS PROBE provided a cost effective and easily installed means for troubleshooting.

The BUS PROBE was a LED display board designed to be inserted into the S100 bus. This board was mostly passive to the S100 bus. It monitored each of the S100 signal lines (with minimal loading) and displayed the signal logic level on the corresponding LED. In most cases the LED was ON when the S100 signal was ASSERTED. For address and data lines this is a logic 1 or high TTL level.   Signals like INT* were ASSERTED as a logic 0 or low TTL level. The BUS PROBE was capable of displaying all IEEE 696 S100 signals. This board functioned in most S100 computer systems  designed previous  to the IEEE specification.
 
The BUS PROBE can be divided into two sections. The lower half of the board contains the S100 bus interface and display driver circuitry. The lower half of this board is about the same size as a standard SI00 board. The upper half of this board contains the display area. The display area has been double silkscreened to enhance the LED display. This section of the board extends above the top of other boards. This makes the display area visible when plugged directly  into  the  S100   bus.

This product allows selective viewing of certain bus cycles. By switch selection the user can display any one or combination of the following bus cycles: M1 instruction fetch, memory read, memory write, port input, and port output.

  
The manual for this board can be obtained here.