DUINO PROTOCOL ANALIZER

Module

Episode 3

In this episode we are going to discover the DUINO PROTOCOL ANALYZER on our 20c computer. this board will allow us to connect an Arduino Mega so we can use it as a protocol analyzer for the 20c's three buses the ADDRESS BUS, the DATA BUS and the EXPANSION BUS.

We're going to study how the board works, how to connect our Arduino Mega, how to create a program for the Arduino that allows us to analyze the 20c buses, explaining what addresses we see and even what machine code instructions are being executed.

We'll recreate this PCB from scratch and finish with an experiment that will allow us to see how our computer's bus works.

Overview of the DUINO PROTOCOL ANALYZER

Our DUINO PROTOCOL ANALYZER board is used to interpret the electrical pulses we have on the three main buses of our 20c computer: the ADDRESS BUS, the DATA BUS, and the EXPANSION BUS.

Bus Operation Description

The buses on our 20c computer, and on every 8-bit computer based on the 6502 processor, operate by sending electrical pulses between 0V and 5V. To be precise, they operate with CMOS levels in which any voltage above 2.4 volts is taken as a 1 and anything below 0.4 as a 0. In the following table based on the 6502 processor datasheet, Vss is our ground pin, which we assume is +0V.

The computer then never sees zeros and ones but rather voltage variations. For our interpretation and to be able to apply logical functions, we will say that a voltage of Vss+2.4v is a High or 1 and that a voltage less than Vss+0.4 is a Low or 0.

Computer chips synchronize with each other using a clock, which we saw in the previous episode. When a clock pulse is marked, the values of all buses are read and assumed to be simultaneous.

For example, at a given clock pulse we can read these values on the buses:

ADDRESS BUS = 1000 0000 0000 0000 (A15-A0)

DATA BUS= 1010 1001 (D7-D0)

EXPANSION BUS= SYN sync pin to 1

Let's interpret this data, the value of the address bus converted from binary to hexadecimal is $8000

The data bus value in hexadecimal is $A9

When the SYN synchronization pin has a value of 1, the 6502 processor is in an OPCODE FETCH cycle, which means that it is searching for an instruction and the instruction will have its operation code or OPCODE on the data bus.