Help with homebrew 64k x 1 RAM arrangement - Page 1
Forum section: electronics / general

author: RetroPhase2015
author_rank: Contributor
timestamp: February 13, 2015, 11:48:15 pm
content: Hi,I've been having some trouble registering and then my post was lost when my login timed out so I'll make this post a bit shorter.Basically, I've been doing hobby electronics for a year and I am making my first Z80 homebrew system based on the schematics for the ZAP computer in the book 'Build Your Own Z80 Computer' by Steve Ciarcia.The design uses Intel 2114 chips, which are 1K x 4 bit, arranged in pairs. The ZAP in it's test configuration has 2K provided by four chips and can be expanded to 8k.It's taken me six months to gather together the parts for this build as, being a beginner, I wanted to copy the plans directly. However, I've now decided to make the system into something a little more useful and as such have bought a tube of eight M3764A-15 64kx1 RAM chips.I understand that eight of these chips would make a 64Kb RAM size. I also understand that without paging techniques the Z80 can only address a total of any combination of RAM and ROM.The 64k x 1 RAM has a different number of pins to the 1k x 4, and I was wondering if someone could point me in the direction of some example schematics for how to connect the M3764A-15 RAM to a Z80. Most of what I have found has Motorola chips from the 6800 family which isn't helping.As a beginner hobbyist, I'm not sure I want to get into paging memory just yet - I suppose with the 2K 2816 EEPROM I have (drop in replacement for the book's 2716), I could use 48K or 56K of the RAM chips?I have attached my schematic, which has three 8T97's attached to the Z80 and the Intel 2114 RAM, directly based on drawings in Ciarcia's book.Any help would be appreciated.

author: elgonzo
author_rank: Unknown
timestamp: February 14, 2015, 12:17:14 am
content: Why exactly do you want to use M3764A?Those chips have a multiplexed address bus and separate data input and output pins - that's not exactly making your life easier...It would be much better if you would use RAM that doesnothave a multiplexed address bus and neither separate data input and output pins. (And if you would choose SRAM, you could also do away with the refresh logic...)

author: cosmicray
author_rank: Frequent Contributor
timestamp: February 14, 2015, 12:38:51 am
content: The original 2114 was static RAM, while the M3764A-15 is dynamic RAM. The 2114 has 4-wide data lines, so you need 2 per 4k bytes of storage. The M3764A-15 has 1-wide data line, so you would need a minimum of 8 to get 8-wide access (even if you do not access more than 8k of address space).

author: RetroPhase2015
author_rank: Contributor
timestamp: February 14, 2015, 09:15:59 am
content: Hi everyone,thanks for the help. I actually have a Toshiba 128k SRAM already. As it came with some CMOS logic IC's I just assumed that it couldn't be used in a TTL computer.I suppose the only reason I would use the 64k x 1 chips now I have your various replies is in fixing an existing vintage computer with no space on the PCB for mods.Thanks

author: KerryW
author_rank: Regular Contributor
timestamp: February 14, 2015, 01:51:01 pm
content: 8T97?? Do they still make those?I presume that you know that it is the same as a 74LS367 and can be replaced by a 74HCT367.

author: RetroPhase2015
author_rank: Contributor
timestamp: February 14, 2015, 05:58:59 pm
content: Quote from: KerryW on February 14, 2015, 01:51:01 pmI don't know if they still make them but I certainly bought three at some point in the past six months. The book mentions 74367's can be used so I can make the connection between those and the 74HCT367's being essentially the same chips, and I believe I have to be careful as the pinouts on the CMOS chips may differ.I have all the chips as TTL's (using a ready made 'off-the-shelf' PSU and display), and towards the end of my buying I found Jameco would have had most of the components I needed the whole time so used them for quite a few parts.

author: SeanB
author_rank: Super Contributor
timestamp: February 14, 2015, 06:05:04 pm
content: The difference between bog standard 74XX, 74C, 74L, 74LS and all others is only the fan out and the voltage levels on the inputs and outputs. All however are the same pinout and all are guaranteed to be input compatible with a standard TTL output driving it only. some are insanely fast though and have very low propogation delay times though, which can cause race conditions in mixing different families, though LS was a boon over plain TTL for decoders as you got an extra few ns per clock cycle for the slow RAM or ROM chips to output data without needing a wait cycle. You could use cheaper 350ns chips instead of the more expensive 250ns selected ones or the 150ns premium selection.

author: RetroPhase2015
author_rank: Contributor
timestamp: February 16, 2015, 08:59:37 pm
content: Quote from: KerryW on February 14, 2015, 01:51:01 pmOkay, I'm grasping this - HCT chips are CMOS and TTL compatible (hence the 'T' in 'HCT'). Sean B warns that the newer CMOS static RAM can be very fast.As I'm using an authorised Italian Z80 clone of the Z84 variety with a clock speed of 8Mhz the RAM access time may not be as much of a worry.I notice from 'Build Your Own Z80 Computer' that Steve Ciarcia says DRAM is best avoided for beginners so, I should have known that already (and that book is from 1981!)Thanks for the help, I should be able to make a fairly awesome retro-style computer now