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[Castellen]

Originally Posted by mechy

Are you sure about the type of roms the 2 sockets can use? I know the 27c220,2048 etc can replace the rom tower(which used 27c200,400 etc) on the rev 7 boards direct(u180,181) eliminating it ,but i seem to recall i tried the 27c220/2048 etc in the u182,u183 positions and it did not work. My memory may be failing me but those sockets don't seem to work for any rom/eprom pinouts i found.

Have you actually used those sockets?

All of my hardware development work was based on the U180, U181 positions so I've not personally used the U182, U183 positions.

I've seen various examples of A3000s with 27C220 ROMs in U182, U183 so I would assume they work on at least some PCB revisions.

If you're having issues, then compare the pin detail from the EPROM datasheet to what's presented on the A3000 hardware.

A potential trap for hardware newbies is that the high order address line(s) such as A17 (pin 1) is floating on certain revisions of the A3000D main board, including rev 8.9 and possibly earlier ones. Was fixed (tied to ground) in rev 9/03.

The result is that if you're using EPROMs larger than 256kB x 16 (4Mbit) then the high order address line floats and may cause unpredictable results. I know some memory devices refuse to work correctly at all if any of the address lines are floating, even if the data image is mirrored to both halves of the address range.

[TjLaZer]

My 3000 has these special ROMs in it, ordered from Software Hut back in 2000 to replace my lame rom tower.

[mechy]

Originally Posted by Castellen

All of my hardware development work was based on the U180, U181 positions so I've not personally used the U182, U183 positions.

I've seen various examples of A3000s with 27C220 ROMs in U182, U183 so I would assume they work on at least some PCB revisions.

If you're having issues, then compare the pin detail from the EPROM datasheet to what's presented on the A3000 hardware.

A potential trap for hardware newbies is that the high order address line(s) such as A17 (pin 1) is floating on certain revisions of the A3000D main board, including rev 8.9 and possibly earlier ones. Was fixed (tied to ground) in rev 9/03.

The result is that if you're using EPROMs larger than 256kB x 16 (4Mbit) then the high order address line floats and may cause unpredictable results. I know some memory devices refuse to work correctly at all if any of the address lines are floating, even if the data image is mirrored to both halves of the address range.

Good info there, it may of been a case of bigger eproms and the floating aqddress line.I was testing on a old rev7 at the time i believe. i had sworn it worked but originally i may of tested on a 9/02.

thanks for the info.

mech

[SpeedGeek]

Originally Posted by mechy

Good info there, it may of been a case of bigger eproms and the floating aqddress line.I was testing on a old rev7 at the time i believe. i had sworn it worked but originally i may of tested on a 9/02.

thanks for the info.

mech

I purchased a pair of 3.1 ROM's which were in fact 27C400s. I also have the rev. 8.9 mobo which leaves pin 1 of the ROMs floating. I Never had a bit of trouble with them even when the mobo was over-clocked and ROM timing was set for the fastest speed. They had a mirror image burned into the upper and lower 256KB of each ROM so they would have worked with pin 1 to GND or VCC although left floating is usually resolved as a logical high by the internal address buffers.

I also have a pair of TC574200 which I burned a 512KB 3.9 ROM image into using the mirror method which worked fine with pin 1 floating also. It wasn't until I did the 1MB ROM hack that I finally got around to connecting pin 1 to A19! But that's just my personal experience.

[Castellen]

Originally Posted by SpeedGeek

I purchased a pair of 3.1 ROM's which were in fact 27C400s. I also have the rev. 8.9 mobo which leaves pin 1 of the ROMs floating. I Never had a bit of trouble with them even when the mobo was over-clocked and ROM timing was set for the fastest speed. They had a mirror image burned into the upper and lower 256KB of each ROM so they would have worked with pin 1 to GND or VCC although left floating is usually resolved as a logical high by the internal address buffers.

Of course it depends on the device being used. Older UV EPROMs tend to be quite forgiving in terms of floating address lines. As you say, some of these devices have internal pullups on the address lines so you can get away with leaving these inputs floating. But again, that can vary between manufacturers.

Many modern flash based devices aren't as tolerant. I found some of these were totally unreadable over the entire address range when a high order address line was floating. That's how I first came across the floating address lines in the earlier A3000s.

[danbeaver]

Simple answer: yes and no. No, don't use the other sockets. Yes you can add and boot from different ROMs by piggy-backing them on top of each other, using a switch, a 22k ohm resister for each ROM to "pull low" the selected ROM, and a full pin schematic of the ROM. I can't tell you which pins for Amiga ROMs while typing in bed on my iPhone at 4:57 am on a Sunday morning, but I've done it on my two C128D's and an Osbourne CP/M computer. Just Google "piggy-Back ROMs" to find out the why and how and compare the Vcc pin & Ground pin on the Amiga's ROM

[cameng]

Awesome info. I hav'nt dabbled with electronics for a while and its been a long time since Amiga was my second language. I shall give it a go at some stage. FYI my moby is a rev 9/03 and I also have a C128D, which I am having trouble booting, but a topic for another thread