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16 bit ISA XMS Board Design?

SOIC is OK, for the final project, but will be difficult for most folks to hand solder. If that’s what it takes to make the board, work, though, it’s ok by me.

- Alex
 
ajacocks said:
TSOPs are easier to solder than SOICs, due to generally larger lead pitch. Both are doable, though.
Click to expand...

The TSOP RAM chips appear to have a lead pitch of 0.8mm, while the SOICs I was looking at for things like the buffers and demuxes are 1.25mm. Does the tighter spacing actually make it easier?
 
Eudimorphodon said:
The TSOP RAM chips appear to have a lead pitch of 0.8mm, while the SOICs I was looking at for things like the buffers and demuxes are 1.25mm. Does the tighter spacing actually make it easier?
Click to expand...
no lol, SOIC is easier but both are fine don't worry about it
 
Normally the SOICs that I end up using have a smaller pitch than that. Just general experience. It’s, of course, all about the pitch size.

- Alex
 
ajacocks said:
Normally the SOICs that I end up using have a smaller pitch than that. Just general experience. It’s, of course, all about the pitch size.
Click to expand...

Yeah, I was looking at the biggest-size ones for the 74x series parts. I have a *little* experience with soldering those and they don't intimidate me *too* much, but my confidence level rapidly plummets after that.

I really wish the company that made those PSRAM chips we decided to go with for this had come through with making the TSSOP version teased in the datasheet of their 8 Megabyte chip, the reduction in package count would certainly be nice. :p

FWIW, I did browse the company's website directly to see what they had on offer, and they do have a four megabyte (2Mx16) SRAM part. It sells for about $25, which would make the RAM cost about twice as much. (The 1MB PSRAM part is around $3.) This part would reduce the number of packages by 75% for a given amount of RAM, but in addition to the cost it's a .5mm pitch part instead of .8mm, and that may be an important threshold because .5 is smaller than the minimum Via diameter on the cheap 2-layer PCB boards, which would not only make soldering a bear it'd also probably make routing a lot harder.

I guess maybe I'm getting a little worried if there are going to be drive/fanout issues with this many parts. Maybe it's nothing to worry about, there's those old S-100 cards with a gazillion SRAMs on them and they seemed to work. It was just sort of making me wonder if there should be a separate set of buffers for, I dunno, each row of 8 or so chips. I guess that's what doing a prototype is supposed to find out.
 
You can always footprint for a buffer and either make traces that need to be cut to use it, or solder jumpers from A to B to replace the chip. Same as whatever part you discussed doing that on earlier in the thread.
 
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