The TTL Logic Probe is a simple diagnostic tool that can help to troubleshoot an RC2014 or give a bit more of an insight in to what is going on. It is designed to plug in to the 5v and Ground sockets in the backplane with a flying probe lead that can either be plugged in to a signal on the backplane or touched against a component leg. The 3 LEDs will give an indication of the state of that signal.
Note that this will not tell you exactly what is wrong with a non-working RC2014. However, with logical troubleshooting skills it can highlight things that are not working as expected or confirm that certain signals are present or not. It can, for example, confirm that there is an oscillating clock signal. However, it cannot tell you if it is the correct frequency. A signal will always be show as low if it is between 0 – 0.8v. Any other condition, such as open circuit or in the no-mans-land from 0.8 – 2v will show as high. It will not tell you if two devices are trying to drive a signal both high and low at the same time. For situations like these you will need more advanced tools such as an oscilloscope.
Assembly
Whilst there are only a few components, there are a few things to bare in mind when putting this together. The single pin for the probe can be awkward to get at a perfect right angle. Fitting this first with a small dab of solder and then plugging in the probe lead will let you hold it in place without burning yourself. The two pins for the power are easier
The three 1k resistors are marked brown, black, red. The 1m resistor is brown, black, green and the 10m resistor is brown, black, blue. Neither the resistors or capacitor are polarised, so the orientation does not matter. The LEDs are polarised, and have a long and a short leg. The long leg is the anode and goes through the hole marked with a + symbol. Also take note of the IC orientation so that the notch on the chip lines up with the notch on the silkscreen. As the IC is soldered directly to the PCB, do not keep your soldering iron on it for too long. If you are new to soldering then maybe do a couple of legs then give it a minute or two before doing a couple more.
Circuit
The 74HCT02 is a quad input NOR gate, however, only 1 of these gates is being used as a NOR gate. The other three have both of their inputs tied together to turn them in to NOT gates. The probe input goes in to the first gate which inverts it at the output. If the input was high then the output of this gate will be low. If the input was anything other than low (including high, high Z or even 0.8-2v ) then the output will be high.
The output from the first gate drives the high and low LEDs which connect to 5v and ground respectively. This gives a simple indication if the input was a static low or not. It then feeds in to a NOR gate which drives a simple R/C timer comprised of a 100nf capacitor and 10m resistor. The output of the capacitor goes through another inverter before going back in to the NOR gate. As the capacitor takes approximately 1 second to charge, an oscillating signal faster than that will continue to keep the output from the second inverter high. This is fed in to the final inverter which drives the activity LED which is on when there is activity on the probe.
The activity LED will stay lit for approximately 1 second after a change of state on the probe. This means that it can detect if there has been a pulse too quickly for the human eye to pick up. For example, when an RC2014 boots in to BASIC, characters are transmitted on the Tx line for a tiny fraction of a second. Whilst the red and green LEDs will have a momentary flicker, it is not perceivable to our eyes. However, the yellow activity LED will stay on for around a second which is long enough to see.
Please be aware that this is a TTL Logic Probe. The TTL means it is suitable for 5v signals only. Use on anything else may result in damage.
Usage
The anticipated use is to plug this in to a spare slot on the RC2014 backplane and use a flying lead to either plug in to another signal on the backplane or touched against a component leg on a module you are interested in. Please be careful that you get the right holes in the backplane for 5v and ground (hint – There should be a capacitor exactly in line with them). If you have a Backplane Pro then the aux power connector can also be used – but this results in the LEDs facing towards the modules. You may wish to use a couple of leads to go between the backplane and the TTL Logic Probe and then hold the probe as you push the tip towards whatever you are interested in. Please be careful not to short two pins together when probing!
If you are trying to diagnose a non-working RC2014, see the Troubleshooting With A Logic Probe section first. This will give you the expected indication for each pin of the bus when the RC2014 has been powered on and reset. Use the TTL Logic Probe to check that your LEDs signify a similar state. If you get a discrepancy, such as Data 4 being constantly low or the reset signal showing activity without pushing the button then these are the parts of the circuit you need to inspect closer.
If you are just curious about what is going on then probing the bus can be quite enlightening. Or maybe put the probe in one of the address lines towards the top of the memory whist running a program to see how often that pin or area of memory is used.
If you are trying to diagnose a module, or want to understand it better then often probing the /CS pin of the chip in question can reveal if it is being accessed or not. You may want to use a test hook for the IC leg, particularly if your tests will take a while.
Indication
Bill of materials
1 TTL Logic Tester PCB
1 74HCT02
1 2pin RA header
1 1pin RA header
3 1k resistor
1 1m resistor
1 10m resistor
1 100nf capacitor
1 3mm yellow LED
1 3mm green LED
1 3mm red LED