AWINFHAH@HMARL5.BITNET (08/07/89)
There has been a lot of upgrading stuff on the net lately. Many of them discuss some type of piggy back method. This may be somewhat tricky. The address lines have a 33 ohm series resistor between the MMU and the RAM array. Somebody noted that they are there to limit the current drawn from the MMU. This only partially true. Another (and probably the main) purpose of the resistors is to avoid ringing on the address lines. Due to the length of the traces on the PCB and capacitive loading by the inputs of the RAM chips, this gives some LC circuit behaviour. The 33 ohm resistor damps oscillations, and thus the resistor value is more or less critical. Now there are three possibilities. 1. Atari picked the right value for 1 bank installed. In this case the resistors are a bit too large when you install the second bank. This can cause address setup time problems (causing unexplainable crashes). 2. Atari picked the right value for 2 banks installed. In this case the RAM chips that are at the far end from the MMU can be in danger due to ringing on the address lines. I haven't seen any blown RAMs in a 520, but it was a well known problem in the early PC cloning days. 3. Atari picked a value that is nearly good in both cases. I think number 1 is true for the old 520s (in which case they probably have 68 ohm resistors), while number 2 should be true for the 520+, the 1040 and the Mega. I invite inside-the-ST experts to comment.