16 bit x 8 bit TTL RAM

[Tommy volts]

Hello,

I am planning my next project: A 16 bit (rows) x 8 bit (columns) TTL RAM.

The TTL 7489 RAM is great and is almost exactly what I need but is 16 rows by 4 columns. However, I need 8 columns to store the full 8 bit instruction.

I can use two TTL 7489 16 bit x 4 bit RAM IC's wired together to get exactly what I need. However, I don't have a PCB so there will be alot of wiring required.

To minimize wiring I'd like to use one 16 pin chip if possible. Is there a 16 bit x 8 bit RAM IC available? I'd rather not go with EEPROMS because the simplicity I am looking for gets obscured.

[dyarker]

On 1 7489 bend the data pins so they point straight out to side. Put that 7489 on top of the other with a small air gap between bodies, and solder the pins together. This parallels Vcc, address, etc. The assembly can still be plugged into a breadboard or IC socket. Tack solder 30AWG wire to data pins on upper IC.

Are you sure you want this chip in a new design? Digikey had none, and Mouser only 182 listed as replacement parts.

Cheers,

[Tommy volts]

Dale,

That is a great tip, thank you. Thanks for taking the time to look up the chip and reply. This project is going to be a rats nest as is, so anything I can do to cut back on the wiring is appreciated.

As far as the chip being outdated: I hope it is available or at least that there is an available equivalent.

I haven't checked into availability but I love what the chip does, its so simple and to the point. I found out about it in an old list of 74 series TTL IC's.

[Dean Huster]

I seem to remember a "higher" TTL number for an upgraded 7489 (74LS189, 74LS289?), but maybe I'm wrong. Of course, my newer databooks don't even list a 7489 anymore, although the NSC CMOS book DOES list a 74C89, which would be a good alternate.

As I recall, the 7489 requires pull-up resistors on the outputs AND has outputs that are inverted from the inputs, so don't forget to take that stuff into consideration. When I was last buying 7489s, they were getting a bit pricey. If you need some, let me know. I have millions of them. Well, that's a little off ... how about several tens of them, all new?

Dean

[dyarker]

You remember correctly, open collector outputs. (Jameco has a data sheet. $3.85 each, I think Mouser was around $10)

[MrAl]

Hi there,

Geeze, there are a lot of static ram chips out there that have even
more memory. I have a few different types that are at least
2048 bytes (that's 2048 x 8 bits). If you dont need that much
memory you just ground the unused address lines.

What kind of speed do you need here? Does it really have to be
TTL top speed to work ok in your application?

.

[Tommy volts]

Dean,

If I go for the 7489 I'll buy several thrown in with a large order of alot of other stuff from Jameco. The cost of the shipping will be spread out and thus negligible. But, thanks for the offer on the 7489's.

Mr. Al,

I will look into the availability of a 2048x8 bit RAM chip & data sheet. There would be 12 address pins instead of 4 but like you said I can just ground the unused pins. That would save alot of wiring. Thanks for the tip.

As for speed: input will be manual, addressing will be done with a very slow program counter/instruction register acting at about 10 machine cylcles per minute. Output will be to an 8 bit address/data bus at same slow speed.

[Dean Huster]

Ya gotta admit, though, that using the 7489 provides a lot of design experience in expanding the RAM wider and deeper. Oh and Tommy, remember that the idea with the open collector outputs on the 7489 (if you use it/them) is so that you can tie all the like data lines together when you deepen the RAM with a wider address bus; each data line still requires only one common pull-up resistor. You have to either invert the inputs, invert the outputs or play games in other areas to compensate for the internal inversion of the data in the 7849. More modern RAMs used enable pins to set the data lines to a high-impedance state since they weren't open collector and still had to be connected in common.

Dean

[MrAl]

Hi again,

Yes a good 'modern' design exercise ... how to do the same job
with less parts ... that's always good. The 7489 chip shows
us what part NOT to use he he

[Dean Huster]

Yes, but for a relatively new experimenter, it's a heck of a lot easier to check out the complete operation of a 256 bit RAM as opposed to a 256MB behemoth. And most of the learning here goes directly toward the larger RAMs. If all you want is a big RAM that'll handle an 8080 or 8085, you can opt for .... what was it ..... a 6416 (or something like that) 64KB static RAM.

Dean