Sinclair ZX80 / ZX81 / Z88 Forums

MaFli wrote: ↑Fri Jun 02, 2017 6:15 pmEach 1K of RAM consists of 2 of these chips. One for the high nibble, the other for the low nibble. But why is one half done with an 2114LC and the other with a 2114LC-1. The -1 version has a 300ns access time, whereas the unmarked is 450ns. Same was done in the original ZX80.
The only thing I can think of, is because of dataline D6' that is used in the NOP code generation during video display and has to be fast enough. That's probably also the reason why it is the only dataline with an additional pull-up resistor to speed it up a bit more. The slower device is less critical and was probably cheaper.

Indeed, that is the only reason that I can think of, the requirement for the RAM to be fast enough for the video/refresh access. In order to keep costs low, a slower chip was permissible for the other data bits. That's why some instructions say it's okay to fit the faster chips in both positions.
If you have both types of 2114 SRAM, you could try swapping them around in both the ZX80 and the RAM expansion...

Mark

1024MAK wrote: ↑Fri Jun 02, 2017 9:06 pm Indeed, that is the only reason that I can think of, the requirement for the RAM to be fast enough for the video/refresh access. In order to keep costs low, a slower chip was permissible for the other data bits. That's why some instructions say it's okay to fit the faster chips in both positions.
If you have both types of 2114 SRAM, you could try swapping them around in both the ZX80 and the RAM expansion...

Mark

Good idea! I will give that a try. I have a few 2114LC and 2114LC-1 socketed in my RAM pack that I can use.

Incidentally, the ZX81 that I was looking at earlier today had two 2114LC-1 fitted.
My ZX80 has one 2114LC (IC3) and one 2114LC-1 (IC4).

Mark

1024MAK wrote: ↑Sat Jun 03, 2017 9:33 pm Incidentally, the ZX81 that I was looking at earlier today had two 2114LC-1 fitted.
My ZX80 has one 2114LC (IC3) and one 2114LC-1 (IC4).
Mark

Yup, same here! Just realised my 2114's are soldered in my ZX80... But the ones in my RAM pack are socketed. I could use those and test/swap these in my ZX80 clone I managed to get working today Got some National Semi MM2114N-2L from Ebay and they work as well. These are even faster: 200ns. Have 36 Toshiba (150ns) versions coming and 10 more MM2114N-2L from the same seller. Let's mix and match

MaFli wrote: ↑Fri Jun 02, 2017 6:19 pm And I just noticed the RS logo on the 74LS138. Weird Never knew RS Components had their own 'brand' of logic devices. And no date code either. Fascinating...

While looking through my stock of spare chips, I remembered the comment above, so I thought a photo was in order... Mark

1024MAK wrote: ↑Thu Jun 29, 2017 3:42 pm While looking through my stock of spare chips, I remembered the comment above, so I thought a photo was in order...

Mark

Haha, nice! And an LS138 with a date code that sort of matches the age of the RAM pack!

I would recommend better antistatic material than plain styropor.

Many years ago i used to use polystryne covered with tin foil... don't really remember killing that many chips. i guess however the old TTL chips were way tougher than todays whimpy Cmos chaps...

It's only my bipolar 74 series and 74LS series chips, and some LED segment displays that are stored like that...
All the CMOS 4000 series, 74HC and 74HCT etc, plus NMOS and CMOS logic, memory, micro-controller, CPUs etc are all either stored in anti-static tubes, or on black anti-static foam

Not managed to kill a logic chip yet

Whoops, going even further off topic...

Indecently, does anyone here remember how suppliers used to package chips when you bought them by mail order (yes, before the age of the internet...!). I know Greenweld (of Southampton, UK) used polystyrene for bipolar 74 series and 74LS series chips and bipolar analogue chips. For NMOS and CMOS, Greenweld did like Andy described, aluminium foil on top of polystyrene.
But I can't remember if Maplin did. As later, they used plastic "tray" type packaging mounted on thin card that had an even thinner layer of aluminium metal foil (well, for the NMOS and CMOS chips at least to start with, later they used this packaging for all chips).

I forget what RS and Farnell used for small quantities...
Except that one of them (RS I think), used those individual black plastic chip carriers. Of course, if you ordered loads of chips, they came in tubes.

Mark

Hi,

Sorry a few months have passed since this thread but I was just wondering if the PCB you used for your clone RAM pack could be sourced from somewhere ?

Thanks