2-chip TOS to 6-chip TOS on MegaST

[TheAtariKing]

Hi Everybody,

 

I just installed Rainbow TOS in my Atari MegaST4 computer. Now it will not boot up. The original TOS was on 2 chips, Rainbow TOS is on 6 chips. How do I get it to work? There are 6 IC sockets on the motherboard, so there has to be a way to get it to work. Also, I have seen MegaSTs with 6 TOS 1.2 chips.

Edited September 18, 2005 by TheAtariKing

[Gunstar]

First, before I get into this, I have to inquire to the general public about something that has me confused here; I thought the Mega computers already came with Rainbow TOS 1.04/1.4, do some have earlier TOS versions? I thought they were the first to get Rainbow TOS.

 

I encountered the same problem on my 1040STf. I, like you, assumed that since there were six sockets available, that 6 chips could be used. This is true, though most would have you believe it's not, but there's more to it then just plugging them in. People on all different ST sites said I HAD to use the same as were installed; if two chip then replace with two chip, etc. I would not except this, otherwise why bother with six sockets and circuits? I KNEW there must be a way. Then, finally, IIRC, Mitch told me that there were some jumper spots that needed to be changed, but he wasn't sure where. Well, after searching I found 3 spots where jumpers needed to be changed(actually 6, in pairs of 2), these will of course be in different locations on the Mega board, but they are clearly marked if you know what to look for. Now, you had 2 chips and are changing to 6, so you will need to find 3 pairs of jumper/solder locations on the mother board that are marked '256k' and '1MB' (one of each in the 3 pairs of jumper spots). you will see that the 1MB spots are either soldered (bridged) together or they have a metal jumper and/or resister across them (just covering all possible ways to do it) you will need to remove the solder/wire/resister (whatever it is) from the 1MB spots (3 of them on the board if the mega is the same in this way to the ST) and then solder/bridge or install the jumper wire/resister onto the 256k spot that should be right next to the 1MB spot you just removed. Do this with all three spots (or as many as you can find labeled 1mb/256k) and remove and install these jumpers. If you are lucky, maybe on the Mega they are actual jumpers with know soldering/desoldering required. Just move the jumper onto the right pins (1MB-to-256k). The 6 chip versions use 6 256k chips and the 2 chip version are each 1MB chips, these jumpers are here so you can tell the ST which they are.

Edited September 18, 2005 by Gunstar

[TheAtariKing]

First, before I get into this, I have to inquire to the general public about something that has me confused here; I thought the Mega computers already came with Rainbow TOS 1.04/1.4, do some have earlier TOS versions? I thought they were the first to get Rainbow TOS.

 

I encountered the same problem on my 1040STf. I, like you, assumed that since there were six sockets available, that 6 chips could be used. This is true, though most would have you believe it's not, but there's more to it then just plugging them in. People on all different ST sites said I HAD to use the same as were installed; if two chip then replace with two chip, etc. I would not except this, otherwise why bother with six sockets and circuits? I KNEW there must be a way. Then, finally, IIRC, Mitch told me that there were some jumper spots that needed to be changed, but he wasn't sure where. Well, after searching I found 3 spots where jumpers needed to be changed(actually 6, in pairs of 2), these will of course be in different locations on the Mega board, but they are clearly marked if you know what to look for. Now, you had 2 chips and are changing to 6, so you will need to find 3 pairs of jumper/solder locations on the mother board that are marked '256k' and '1MB' (one of each in the 3 pairs of jumper spots). you will see that the 1MB spots are either soldered (bridged) together or they have a metal jumper and/or resister across them (just covering all possible ways to do it) you will need to remove the solder/wire/resister (whatever it is) from the 1MB spots (3 of them on the board if the mega is the same in this way to the ST) and then solder/bridge or install the jumper wire/resister onto the 256k spot that should be right next to the 1MB spot you just removed. Do this with all three spots (or as many as you can find labeled 1mb/256k) and remove and install these jumpers. If you are lucky, maybe on the Mega they are actual jumpers with know soldering/desoldering required. Just move the jumper onto the right pins (1MB-to-256k). The 6 chip versions use 6 256k chips and the 2 chip version are each 1MB chips, these jumpers are here so you can tell the ST which they are.

933957[/snapback]

 

Thanks for the quick reply. To my knowledge only the Atari 1040STf has the jumpers clearly marked, other STs do not. I have looked all over my MegaST for these jumpers, but could not find any. Most MegaSTs come with TOS 1.2. Rainbow TOS was an upgrade for these machines. My MegaST has 6 IC sockets, so I know that there is a way to make the 6-chip version of the Rainbow TOS work. I have attached before and after pics of the TOS upgrade that I did on my MegaST. I have high res pics if anyone needs them.

Edited September 19, 2005 by TheAtariKing