Modifying a kick switcher to use EPROMs in an A500 rev5

[cypher007]

Hello, I’ve got a rev 5 A500 and an aftermarket rom switcher, from about 91, that uses the reset line to toggle the roms.

I’ve looked and tested between pin 1 and 31 and it seems no connection on the Amiga board but I can see and test a connection on the rom switcher.

What I want to do is convert one socket on the switcher to an eprom compatible one. What mods do I need to do to the underside of the rom switcher socket? I got a bit confused looking at the diagram.

[cypher007]

Anyone?

[Jope]

Remove the adapter and measure from the pins that connect to the motherboard whether it does the 1-31 jumper for you or not?

[cypher007]

Looks like it does. But when I tried last night with an eprom in it won’t post.

I’m using a tl866ii plus with an adapter for 27c400. The software says it burnt it ok.

[cypher007]

Ok some progress. If I pull pin 31 out and link it to pin 21 it posts. Gets stuck at the icon libs bit due to my harddrive having 2.04 on I think.

Anyway I’m now a bit confused as to what’s going on. Vcc is linked to 31, in the socket, from pin 1 then I’m having to pull the pin out of 31 and connect it to 21 which is vcc?

How can I mod this so I’m not pulling pins out of sockets?

[Jope]

1-31 should be connected together at the pins going to the motherboard, but they shouldn't be connected together at the socket for the ROM chip. Pin 31 is for byte/word mode and it must be high so that the chip is in word mode. The 31-21 jumper is the pullup for word mode.

Pin 1 is an address line, pin 21 is VCC. So is the EPROM socket 31 linked to 1 or 21?

[cypher007]

Yep 1 is connected to 31 on the switcher board. It’s a small wire link you can see. It’s wasn’t installed by me, it came like this.

Here is some pics.

[Jope]

To me it seems like this switcher originally took its operating voltage from pin 31. This was cut when 2.04 was installed and the wire link makes the operating voltage come from pin 21 like it should have.

The wire link connecting 1-31 might have been in the top right corner of the solder side picture, but I see no connection there now. Did you beep out the connection between 1-31 from the kick switcher's sockets with the kick switcher removed?

[cypher007]

Ok not sure why you are saying when 2.04 was installed the wire was cut the wire is still there, and I used this switcher with 1.3 and 2.04 mask roms for many years like this and never altered anything.

Anyway yes pin 1 and 31 are linked on both sockets. Pin 21 isn’t, that seems to come in from the ribbon on the first pin then links to both sockets then finally into the logic circuit using another wire link.

Thanks for all the help by the way. I struggle a bit to understand what people are trying to do with some circuits.

Here’s some close ups of the link wires on pin 1 and 21.

[cypher007]

So if I link pin 31 on the socket to pin 21 would I need to cut the link to pin 1?

Where will I need to fit the resistor networks? And after modding this will it still work with mask roms? As I might have 1.3 mask and an eprom in the other socket.

[Jope]

Thanks for the added pics, now I see the 1-31 link.

So.. what you should do is just isolate pin 31 on the socket where you want the EPROM to go by cutting the track that is going to it. Then you can solder a new wire from 21 to 31 for that socket.

[solarmon]

What about making an adapter, so that you don't need to cut anything on the switcher? Although, this will add extra height since two sockets are used for the adapter.

https://sites.google.com/one-n.co.uk...-5-rom-adapter

I use this adapter when I use a DiagROM chip in my A500 rev5 machine.

[Jope]

Why not modify the switcher to be 27c400 compliant. It's already been modified before to get the 2.04 mask ROM going. :-)

You can do both sockets at the same time if you cut the thin trace right next to the pin on the ribbon cable connector.

[cypher007]

It must have been modified by the manufacturer as I didn’t change it. Maybe they made an error in design.

Will it still work with mask roms after modification? Also will it still switch between eprom and mask rom?

Also what are the network resistors for? I’ve bought them ready to fit, but was confused by how they were fitted in the pics and diagram, as it seems to change.

[Jope]

Yep, they most likely made the design based on observed behaviour in the rev 6, then someone noticed that rev 5 exists and they had to redo it. Quite unfortunate to have chosen pin 31 as Vcc (which does actually work in a rev 6 A500), when the real Vcc is at pin 21.

The resistors in the other thread are to add stability if your machine starts crashing after installing the EPROM. Try it without and if the machine seems to run fine, you don't need to install them.

Sure, it'll work fine with mask ROMs. It will be identical to the rev 6 A500's kickstart socket after that. Just make sure after your cut that track, that pin 31 on the switcher's sockets does not go to pin 31 on the motherboard any more, then add the 31-21 jump wire on the underside of the kick switcher.

I would personally maybe also print a dymo sticker saying rev5 only and stick that on the switcher somewhere. :-)

[cypher007]

That’s great info Jope.

If I cut the feed to pin 31 as it comes into the switcher, and just let it feed the other areas, presumably it would still work on a rev 6 and up? As it wouldn’t be connecting pin 31 to 21 via the ribbon.

[Jope]

Yeah.

[cypher007]

Well even after installing the pull up resistors, which I think I’ve installed to the correct pins. I get an yellow screen briefly on warm boot. It only does it with kick 3.1. Kick 2.04 on eprom works fine.

I wonder if it’s because I have a baseboard ram expansion with 4.5mb installed. Which uses a util to give me 1mb chip and 3.5mb fast. This was an expansion designed more for kick 1.3 days. Maybe kick 3.1 is trying to auto config the ram?