Logic Probe and/or Pulser
Two very good cheap pieces of kit, there is plenty of good informative information on using these devices available on the internet. Here are a few good sites to visit:
Talking Electronics
Lawnmower Man
If you are too tight to part with the relatively small amount of cash required
for these items, it is possible to build your own logic probe (well its a
little different, its a video probe). Anyway, the logic probe is useful to
check what is going on (electrically) on the inputs and outputs of each IC on
the board. Normal logic chips use only two states for operation, on and off/ 0
and 1/ high and low. These of course are all the same thing and relate to
either a logical high which is +5volts or a logical low which is active ground/
0volts. It is therefore possible with the pinouts from a site such as
The Chip Directory to see if each chip is operating correctly. For example, on 74LS
logic chips you will be able to see if the two inputs which make up an AND
logic gate have the correct resulting output, sometimes the output will be
‘floating’ meaning that its neither +5 or ground, which is quite obviously then
faulty. Care should be taken here to not bin parts which aren’t faulty as not
all inputs and outputs are connected on every IC and unconnected lines tend to
be floating rather than just off.
Before in that statement I said, “normal logic chips” referring to the TLL
type, there are also the CMOS type which rather than have a digital output they
have analogue outputs and can work at a much higher voltage. Logic probes can
be bought which have a TLL/CMOS switch to test both types.
The logic pulser is used to inject a high/low pulse into the circuit, this is useful to change the active state of an input or output. Of course, you can use a patch wire connected to ground to make a high into a low (useful for testing reset circuits) and vice versa a patch on +5v for turning a low into a high (for bringing a processor out of a HALT state springs to mind). The logic pulser is a useful tool, one of the best uses I have found for it is to be able to test speakers and amps. This is covered in greater detail in the repair pages later on.
