512kb x 16 (8 Mbit) EPROM  40pin needed? - Page 1
Forum section: electronics / beginners

author: Mp3
author_rank: Frequent Contributor
timestamp: July 20, 2018, 04:15:11 am
content: Guys,I am having so much trouble finding a replacement ROM for this.40 pin DIP package, 512kb x 16 (8 MBIT)I am finding a lot in different packages, finding plenty 32 pin and 42 pin, but none 40 pin?does anyone know a compatible part? Been looking online for a long time. Some one bay listed as 40 pin, but when you look at the picture they're 42 pin

author: amyk
author_rank: Super Contributor
timestamp: July 20, 2018, 11:55:11 am
content: What is the original part? Is it a mask ROM?There's the 27C4096 in DIP40 but that's only 4Mbit.

author: Cliff Matthews
author_rank: Unknown
timestamp: July 20, 2018, 12:54:29 pm
content: What about two ($7usd) AT27C4096 piggy-backed? Vpp may not be needed (once programmed). Perhaps fold it under and route pin-2 of the top chip to pin 1 in the socket to determine which tri-state chip you'll read from?

author: Mp3
author_rank: Frequent Contributor
timestamp: July 21, 2018, 08:32:09 am
content: Quote from: Cliff Matthews on July 20, 2018, 12:54:29 pmThanks for this idea. I haven't been able to find any currently manafactured 8mbit 40 pins, so i'm afraid my best bet might be stacking 27c4096. Thank you for showing how to properly stack them, thoughIf i can't find any replacement chips to write the images to, i will probably have to go with that, although it will not be cheap, doing 14 4096 roms! (and will have to figure out how to program them correctly.... pre stacking or post stacking?)

author: james_s
author_rank: Super Contributor
timestamp: July 22, 2018, 12:21:35 am
content: You could make a DIP carrier board to adapt a surface mount ROM to the pinout you need. If you have 14 ROMs in the same circuit you can often replace them all with a smaller number of larger ROMs on a carrier board.

author: Mp3
author_rank: Frequent Contributor
timestamp: July 22, 2018, 02:03:27 am
content: Quote from: james_s on July 22, 2018, 12:21:35 amYeah this would be far more desirable. It's 56mbit total, so that comes out to like 7mb, i would have to make the carrier board then of courseIf i did that, would it be possible to have the carrier board go in one of the sockets and send lead 2 lead wires to each other socket? as i have 7 sockets to fill.

author: oPossum
author_rank: Super Contributor
timestamp: July 22, 2018, 04:19:44 am
content: Do know how address decoding is done? It is quite common to find a 74LS138 or similar used for that.I made this to replace eight 2716 EPROMs with a single 27C128/256/512...

author: Mp3
author_rank: Frequent Contributor
timestamp: July 22, 2018, 10:51:13 am
content: Quote from: oPossum on July 22, 2018, 04:19:44 amNah i really need to look into it. Thanks for the inspiration / pointing me in the right direction. I'll see if that 74ls138 is necessary for what i want to do.

author: oPossum
author_rank: Super Contributor
timestamp: July 22, 2018, 11:35:39 am
content: That 74LS154 is a 4 to 16 decoder. (74LS138 is 3 to. Could be doing address decoding.The 74x157 are probably row/col mux for the DRAM.

author: Mp3
author_rank: Frequent Contributor
timestamp: July 23, 2018, 12:16:15 am
content: Quote from: oPossum on July 22, 2018, 11:35:39 amHmm i'll hav to study address decoding more.... if the ROMS are already being decoded i will have to figure out some fancy hackery to get it to run off a single 8 megabyte rom....

author: james_s
author_rank: Super Contributor
timestamp: July 23, 2018, 01:27:18 am
content: Look at how the ROMs are wired, if the address and data buses are the same across all of them then it should be easy to combine into one larger ROM. Look at where the CE lines go, it may be that you can just decode directly from the address bus lines.