9.13 Mega ST Rom Table

Mega ST ROM Jumper

This is what the Mega ST schematic said:

ROM TABLE IC ; U12 (74LS11)
Jumper ; W2,W3,W4

Das Jumperfeld findet man vorne unten auf der linken Seite des MegaST. Leicht zu finden wenn man sich an den ICs U24 und U12 orientiert. Rechts davon befindet sich ein grösserer mit U30 beschrifteter Chip.

The Juperfield is located on front bottom left corner of the Mega ST. This is easily located by ICs U24 and U12 side by side. To the right of the ICs is a large square IC marked U30.

According to the schematics, in the MegaST with 2 chips (1 meg ), the Jumpers are set by default as Jumper W2 in position 2-3, Jumper W3 in position 2-3, Jumper W4 isn't used and the IC U12 is used. This is the standard factory setting.

Locate U12, in the left corner of the board. This is a 74LS11. Remove pin 12 from the board. Either cut it at board level or desolder it and bend it out of the PCB. Make sure not to break the pin, else the old Roms can only be replaced after replacing this IC. (You can also desolder the IC altogether and put it in a socket) Make jumper W4. This is normally empty. Locate Jumper resistors on position W2 and W3. These are both at 2-3. Change both to 1-2. Now 6 Eproms 27256 can be used. 150nS will do, 120nS is better.

Thanks to Guus Assmann

Im Werkszustand, im MegaST mit 2 ROM Chips (1MB), sind die Jumper default W2 = 1+2, W3= 2+3 und Jumper W4 offen, verbunden. Ebenso ist das IC U12 vorhanden und aktiv.

Soll nun ein TOS in 6 Chips verwendet werden sind die Jumper entsprechend zu ändern. Ein TOS in 6 Chips verwendet immer 256K Chips.

W2= 1+2, W3= 1+2, Jumper W4 gesetzt und U12 muss ausgelötet werden.
For TOS in 6 chips the jumpers must be removed as here:

W2 = position 1-2, W3 = position 1-2, W4 = Jumper and IC U12 needs to be removed for 6 Chip ROM.

Passende ROM Bausteine finden Sie hier:
The roms can be bought here:
http://www.best-electronics-ca.com/custom-i.htm

2 or 6

I've acquired a Mega ST2. When I opened it up, it only has 2-TOS chips (1987). But there are six sockets. I pulled the two chips and installed a set of 6-Rainbow TOS chips. But the system will not boot.

Now I know that there are 2 and 6 chip sets of TOS. But why are there six sockets on this motherboard ?

OK, I don't own the Mega but I do own an STFM which also has 6 ROM sockets. Look around the board near to the ROMs and you should find some solder pads that have 256K and 1M next to them. Unsolder the 1M ones and place solder across the 256k ones. I have three of these on my STFM, I would imagine the same as the Mega ST.