So my Burgertime is stuck in test mode. I think it is a bad DIP switch. I haven’t done way too much testing with my Logic Probe so I had to find a little refresher to get me back up to speed. I happened upon this page:
Lawnmowerman’s Pac-Man Troubleshooting Page – How to use a Logic Probe
Here is the section relevant to what I am doing:
Troubleshooting faulty INPUTS
(joystick, coins, buttons, dip switches)
Player 1 Start
Note the 74LS367 has 2 sections with 2 seperate enables !
You can use the same technique as above to troubleshoot input switches such as the joystick, start buttons, Coin inputs or even the Dip Switches. You will notice that all of these switches have pull-up resistors. A pull up resistor brings the voltage up to +5 VDC so that the inputs are at a normally High state (1). On Pac-Man the pull-up resistors are RM6 (Dips), RM7 (test/table/start/player 2 controls), and RM8 (coin, credit, player 1 controls). The RM is a designation for Resistor Network or Resistor Array. You will notice above that RM7 is described as a 1K x 8 (1 K by 8), meaning that this resistor array has 8 resistors of 1K ohm value with a common lead.
You will also see a small .1 micro Farad capacitor (C17) tied to ground, this is to lengthen the momentary pulse (a low state as the switch is closed connecting it to Ground) as the capacitor recharges and enable the logic to actually have enough time to register a input. This also acts as a “debounce circuit” eliminating the circuit from registering many switch closures and opens that occur in the tiny time window as the switch changes state. (Notice the Dip Switches don’t use these capacitors).
You can “see” the controls/ dips at work with your logic probe from the switch. Place your logic probe at the 74LS367 (Hex Buffer Tri-State) 8H pin 6. It will be normally High until you press the player 1 START down, when it will change to Low. So if I look at the output of this circuit, I’ll see the same thing at 8H pin 7 right? WRONG. Notice the output is labeled DB5 (Data Bus 5), but DB5 is also at 8E pin 7, 8D pin 5, 3F pin 6, 4D pin 7, and 6D pin 6. We’re on the BUS here man! Not only is this now the path for information about the joystick being pressed down but it could also be a Player 1 Down, or Dip 5 or info to the Graphic ROM’s or Sound Section etc… The BUS is the pathway for data to travel in either direction. In our case the Data BUS is 8 BITs wide, our joystick closure is only one bit of information in a system that can directly access 64K bits.
So how do we keep up with this information? The same way the post office delivers your mail, you have a address that is explicitly defined to you. Okay, what’s the address for “Player 1 Start”? We’ll save that for latter. For now it is important to notice that if you are on the bus, things will be going too fast to see what you want. If you notice the 74LS367 chip at 8H has a line at pin 1 IN1 which is an input (see the arrow pointing to our chip?). This is used to ADDRESS our chip 8H and tell it “Hey man, what data do you have for me on DB4~DB7″ (it also reads DB0~ DB3 from 8F concurrently) at which time the Central Processor Unit (CPU) is only looking at these particular inputs at this given address, which includes our switch Player 1 START. So things are too fast for us to know if they are working properly on the BUS side but we can check to see if they are working properly before the Hex Buffer and we can also be sly & check to see that the Hex Buffer is being used by the CPU if we look at the IN1 signal at pin 1, it must go LOW to enable a read from chip 8H.
This is going to be a big help troubleshooting my DIP switch issue. Once this is resolved I will have a 100% working Burgertime!
