What is it?The Debugger-LPT enables you to observe the behaviour of the processor step by step. In contrast with Debugger it is controlled by the printer port of a PC.
The storyMy original debugger has three disadvantages:
- The used intelligent 7-segment displays aren't cheap.
- In quite some situations the display is not well readable due to its position in the system-under-test.
- In many situations the card is not well accessible due to its position in the system-under-test; I cannot manipulate the switches.
The PC can record the steps the system-under-test goes through. But reading the values on the address-, data- and control bus will take much more time then this system will need to perform one cycle. This means that the debugger never will be a real time analyzer.
I have thought about it to call it a sampler, but sampling means to pick only some parts/items out of the whole and the debugger still collects everything, although slow.
- Error detector
Having recorded the behaviour of a good system, the data can be compared with the behaviour of a bad system. The point in the program where the bad system starts to behave different could lead to the faulty hardware.
The hardwareThe idea is quite simple: replace all 7-segment displays with 541 buffers and replace all switches with the output of a 573 latch.
- My original debugger has no means to watch the state of the various lines of the control bus. In this case a 541 buffer can take care of it.
- I want to use this debugger for systems equipped with the 65816 processor. Therefore I need some means to watch the segment address as well.
As you know, there exist several versions of the printer port: uni-directional (the original printer port), bi-directional (also known as PS/2 printer port), EPP and ECP. Fortunately for us we only have to choose between using the uni-directional port or one of the other three.
What do we need:
- one 541 for reading the segment byte of the address bus
- one 541 for reading the high byte of the address bus
- one 541 for reading the low byte of the address bus
- one 541 for reading the data bus
- one 541 for reading the control bus
- one 573 for setting the high byte of the address selector
- one 573 for setting the low byte of the address selector
- one 573 for replacing the switches
The uni-directional versionSchematics
This version is nothing more then the automated version of the original debugger. This design lacks certain features described above. I only give the schematic as it is to show you how it can be done.
Two 573's, IC5 and IC9, replace the two 8-bit dipswitches and another 573, IC10, replaces all other switches. The data coming from the data bus of the printer port, is clocked into the IC by a 138 (IC11), 3-to-8 de-multiplexer, through some inverters in IC12A, a 74LS240. Of course you can use three inverters from a 74LS14, I only used a 240 as this occupies less space in the drawing then three single inverters :)
The 138 is directed by the four outputs of the printer port.
The uni-directional printer port cannot use its data bits to read data, therefore we use four of the five handshake lines. The data is delivered by three 74LS257 quad 2-to-1 multiplexers. The signal for telling the 257's what nibble to transfer is generated by IC10 as well.
The bi-directional versionSchematics
Roughly the schematic is the same. But the 257's have been replaced by 541 buffers and the line for selecting the right nibble has been dropped.
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