Making a "piggyback" Dual Boot EPROM

How To Make A "Dual Boot" EPROM

("PIGGYBACK" STYLE)

This method describes how to make a "Dual Boot" player with two standard 27C040 EPROM's stacked on top of each other. You may wish to use this method if you have better access to 27C040 EPROM's or want to keep the chip already in your player but also add another firmware version as well.

First, the standard disclaimer:

DOING THIS MODIFICATION NEEDS SOME ELECTRONICS EXPERTISE. IF YOU FEEL YOU CAN'T DO THIS MODIFICATION BY YOURSELF, SEEK ADVICE OR HELP FROM SOMEONE WHO CAN. I WILL NOT BE HELD RESPONSIBLE!

Are you still with me? Good! Let's get on with it.

Firstly, unlike the single EPROM dual-boot method as described on the other page, you DO NOT have to make one large file with two different firmware versions. Instead, just program up two EPROM'S with the different firmware versions you require.

Now you are going to need the following items:

(2*) 27C040 4Mbit EPROMS (120ns speed rating or better).
(1) 32 pin IC socket (optional).
(1) Single Pole Double Throw toggle switch.
(2) 22K 1/4watt resistors (Color code: Red, Red, Orange).
Some hookup wire, wire wrap wire or "ribbon" wire.
Some solder, soldering iron, drill (optional).

* You only need one if you are going to keep your current firmware, because you can use your existing ROM chip and piggyback an EPROM on top of it.

A Note On Speed Rating: The smaller the number the faster the access delay time of the chip. For example, an 80ns chip is faster than a 120ns chip. You can use as fast as you like (or your budget can handle!) but DO NOT USE SLOWER THAN 120ns. People have tried 150ns chips and they do not work very well.

First of all you are going to have to program your EPROM'S. If you have a programmer handy then you are in luck and this is easy. If not, you are either going to have to find someone who can help you out or make your own programmer. NOTE: the design on Mike's Eprom Project Page is designed for the 27C801 only, but you may be able to modify it.

Now you are going to have to familiarise yourself with the pins on the EPROM. Use this picture to guide you:

This is what an EPROM looks like. EPROM chips have a window in them and you can see the memory cells inside. The window is so you can expose the chip to UltraViolet (UV) light to erase the memory, so if you make a mistake or want to try other firmware you just erase the chip and program it again. An EPROM usually has a minimum of 10,000 erase/write cycles before something goes faulty so you can play as much as you like. Just like any other memory chip they come in all different capacities and sizes, and you must have the right type for this modification to work. The important thing to note from this picture is the notch at the top. This helps you identify where Pin 1 is. Sometimes, but not always, they can have a dimple near Pin 1 as shown by the white square.

Now get one of your EPROM chips (or your existing ROM which came with your machine) and the IC socket. Place the socket on top of the chip so the pins of the socket touch the pins on the chip and solder ALL pins as shown EXCEPT pins 24, which you bend out on both the chip and the socket as shown. If you like, you can just solder the second EPROM on top of the first one and forget the socket.

When you have done that, get the two resistors and twist one side of the resistors together. Solder this twisted side to Pin 32 of the socket. Solder the top resistor to Pin 24 of the socket and the bottom resistor to Pin 24 of the chip, as shown:

Now you wire the switch as shown in this diagram:

As can be seen in this picture, solder one side of the switch to Pin 24 of the bottom chip and the other side of the switch to Pin 24 of the socket. The middle wire has a short length of solid wire soldered to it. The scrap bits of wire from the trimmed resistor legs is ideal.

And here is the complete package!

Pretty neat, huh? Now all you need to do is fit it!

First, let's have a look at where the BIOS chip is....

All that is left to do now is simply carefully put in your "Dual Boot" package. Take care when insering the centre switch wire into the socket on the circuit board. You will most likely have to insert this wire first and then the rest of the package on top afterwards. Also remember to note the notch in one end if the chip as indicated by the yellow highlighted picture above. You must use this as a guide because even though the chip can go in the socket both ways, only one way is correct and remember:

Putting the chip in the wrong way will damage your player, so be careful before you power up!

Once you are sure you have installed the chip correctly you can test your firmware before mounting the switch. Turn on your player and make sure it is working correctly. If it is then turn off your player, flip the toggle switch to the other firmware and turn your player on again. If this firmware is also working correctly you can now mount the switch in the back of the unit. Some people may want to just leave the switch hanging out the case. This is OK, but put some insulation tape around the switch to stop the bare solder connections shorting out on the case or any other metal! But I think it looks better like this:

Just remember you must turn off your player before touching the firmware selection switch!

For the technically minded, what this modification does is select two firmware versions from different EPROM's. This is done by switching Pin 24 which is the Output Enable line (OE). When OE is low (0 Volts) the EPROM will allow data to flow out of the chip. When OE is high (+5 Volts) the output data pins of the EPROM are switched off and no data can flow out of the chip.

The idea here is to have one EPROM connected to the processor chip at a time, and the switch to select between them. This is done by having the switch select which EPROM to connect to the processor, while the other EPROM is kept at +5 Volts via the 22K resistor. We call this resistor a "Pull Up" resistor because it "Pulls Up" the pin to +5V so no data can flow from it. You must have these resistors because if the pin is just left unconnected it can "float" and maybe sometimes allow data to flow from it even when you don't want it to.

What about the "Pull Up" resistor when you connect the chip to the processor? Does this effect the operation of the chip? No. That is why I have chosen 22K. It is a resistor value high enough that still allows the processor to normally control the chip without the "Pull Up" resistor current causing any problems to normal operation.