I would have thought a 2101 was far too small. These are 4kbit chips. I agree 200ns probably isn't essential - my PET of that date uses 450ns 2114s. On the 1MHz 6502 you have 500ns between the two clock edges that govern RAM timing - one guaranteeing a valid address, the second latching in the data. (FWIW there were FOUR motherboard designs for the early PETs, based on all permutations of 2114 or 6550 RAM and 2316 or 6540 ROM. Mine is late for an old style PET, and has 2114 and 6540) That's right. When they revised the ROM code, they revised the motherboard so that all ROMs were 2332s (and the sockets would take 2732s) and all RAM was dynamic. The thing that bothers me is that it says 3071 bytes free. This is EXACTLY the number of bytes free you get on a 4K PET. My advice - Identify the suspect pair of chips (remember these are 4-bit wide parts) - I have the circuit diagrams if you want - and swap them with the corresponding pair for the top 1k of RAM. You should then get the message "6143 BYTES FREE" when you power up. (Or try swapping them with the video RAM - you'll soon see if the chip is dead!) I suspect, however, you will still get 3071 BYTES FREE. Let me explain. On these early PETS the top four address lines come from the processor and go straight into a 74154 decoder, AND NOWHERE ELSE. This outputs sixteen block select lines - one for each 4k of memory. Your PET is failing memory test at exactly the point where it passes into the second 4k block from the first. In short, I think a failure at the block boundary is too much of a coincidence. I'd trace out what this line is doing from the 74154 (there is only one, and it's 24 pin, so you can't miss it!) to the RAM. Failing this, I'd suspect something else in the chip select logic. (It could, of course be a failure that affects a whole chip, but I'd try the other things after you've swapped RAM chips if the BYTES FREE doesn't change) As I said, I can lend you my copy of the circuit diagrams if you haven't already got one. Philip. ********************************************************************** This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify the system manager. This footnote also confirms that this email message has been swept for the presence of computer viruses. Power Technology Centre, Ratcliffe-on-Soar, Nottingham, NG11 0EE, UK Tel: +44 (0)115 936 2000 http://www.powertech.co.uk **********************************************************************