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TOS rom locations/placements

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Hi am in the process of resurrecting a 520 STe with what I assume are 1.06 roms in the machine.  How are the roms placed in the sockets?  These are 28 pin, the sockets are 32 pin and pics I took before working on this one shows C301166-002A in the LOW socket but inserted with the pins backed to the left side.  That means that the 1st pin on the rom is really in the 3rd pin in the socket, is that correct?  From all my work with ICs pin 1 on the chip goes to pin 1 in the socket, or is it different on the ST?  Also, is C301166 the low rom and C301165 the high rom?  Thanks

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Yes, you’re correct you insert the chip in the leftmost position and end up with pin 1 of the IC actually being in pin3 of the socket. 
And yes, 66 is LO while 65 is HI.

 

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The reason for such positioning (pin 1 of 28 pin ROM to pin 3 of 32 pin socket) is that GND and data line positions are standardized . GND is last in bottom row (what starts with 1) .

8 data lines are located there too. So, it's not pin 1 what is with fixed function, but GND pin, and it is usually latest  one in bottom row.

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Thanks guys!  BTW, is it possible for the roms to go bad and not function correctly?  I am seeing some loss of the floppy system, even with Gotek.  I did change the memory chips to get 4 meg in the machine, the original pair of 256K looked real rough but may have to try them to see if the new memory chips are causing problems.

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Posted (edited)

If you place ROM incorrect that will not work at all, for sure. Worst is when insert rotated for 180 degrees. Then GND and +5 V pins are swapped, so even some nice burning, smoke can happen - ROM chips will be destroyed for sure, and possible damage in computer chips.  Even if just shift by pin-two ROM may be damaged.

Edited by ParanoidLittleMan
better grammar
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Yes I had 1 chip from my 6 chip TOS 1.4 go bad in my Mega ST.  I used the Diag Cart to discover the fault.  Replaced the chip and machine works perfect now.

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