Add detailed troubleshooting commands

docs/troubleshooting.md

@@ -77,3 +77,90 @@ Writing the unlock code to disable Software Write Protect mode.
 #cd
 
 ```
+
+## Troubleshooting
+
+If the programmer seems to be working, but 28C256 EEPROMs cannot be written, the problem
+may simply be a locked chip.  Some chips have been received with the Software Data
+Protection (SDP) enabled, even though the data sheet states that it should be disabled
+from the factory.  If the chip does not write (try the Zap command in the TommyPROM code),
+then try an Unlock command and then retry the write.
+
+If the programmer is not working, the most common problem is a simple wiring error.  The
+hardware verify sketch can help to find missing or crossed connections.
+
+The hardware has three sections: an 8-bit read-write data bus connected to the target
+PROM's data line, a set of write-only address lines generated by shift registers and
+connected to the target PROM's address lines, and a set of write-only control lines from
+the Arduino that toggle the PROM's output enable and write lines.
+
+The first step in general testing it to remove the target chip and then toggle single
+lines from the Arduino.  The result should be measured at the target PROM footprint, not
+at the Arduino.  To fully test, use a multimeter to verify that the lines that should be
+active are active and that any other lines are not.
+
+### Testing the data bus
+
+Issue a set of Data commands from the Hardware Verify sketch and measure EACH data line at
+the PROM footprint.  **If the expected result is "D0 HIGH", verify not only that D0 reads
+HIGH, but that all of the other lines read LOW.**
+
+Data test commands
+
+|Command|Result|
+|:---   |:---  |
+|D0     |All data lines LOW|
+|D1     |D0 HIGH|
+|D2     |D1 HIGH|
+|D4     |D2 HIGH|
+|D8     |D3 HIGH|
+|D10    |D4 HIGH|
+|D20    |D5 HIGH|
+|D40    |D6 HIGH|
+|D80    |D7 HIGH|
+|Dff    |all data lines HIGH|
+
+### Testing the address lines
+
+As with the data tests above, verify not only the line with the expected result, but all
+other address lines as well.
+
+|Command|Result|
+|:---   |:---  |
+|A0     |All address lines LOW|
+|A1     |A0 HIGH|
+|A2     |A1 HIGH|
+|A4     |A2 HIGH|
+|A8     |A3 HIGH|
+|A10    |A4 HIGH|
+|A20    |A5 HIGH|
+|A40    |A6 HIGH|
+|A80    |A7 HIGH|
+|A100   |A8 HIGH|
+|A200   |A9 HIGH|
+|A400   |A10 HIGH|
+|A800   |A11 HIGH|
+|A1000  |A12 HIGH|
+|A2000  |A13 HIGH|
+|A4000  |A14 HIGH|
+|A7fff  |all address lines HIGH|
+
+### Testing the control lines
+
+The PROM control lines are active LOW, so an enable command will set the corresponding
+control line LOW and a disable will set it HIGH.  Try the commands in the following table.
+Note that they should be done in the order listed, because a test command on one control
+line will not change the state of any of the other lines.
+
+To begin, issue the following commands: D0, A0, Cd, Od, Wd.  Note that the Od command is
+the letter 'O', not the number zero.  All three control lines should read HIGH after these
+commands are complete.
+
+|Command|Result|
+|:---   |:---  |
+|Ce     | Chip Enable (CE) LOW, others HIGH|
+|Cd     | CE, OE, WE all HIGH|
+|Oe     | Output Enable (OE) LOW, others HIGH|
+|Cd     | CE, OE, WE all HIGH|
+|We     | Write Enable (WE) LOW, others HIGH|
+|Wd     | CE, OE, WE all HIGH|