Quicky question: BBC Micro ROM <--> keyboard clearance

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crj
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Quicky question: BBC Micro ROM <--> keyboard clearance

Postby crj » Tue Sep 05, 2017 1:57 pm

I am scheming away at a nifty idea. Unfortunately, it's so nifty I'm trying to fit a ludicrous amount into a tiny space.

Could someone with a ruler and a BBC Micro please remind me: how much clearance is there between the top surface of one of the ROM sockets and the underside of the keyboard? (I care about the end down by pins 14/15 end, where clearance is at its lowest because of the keyboard's slope.)

I'd check myself, but unfortunately I gave the model Bs to a museum and only have my trusty Master!

cmorley
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Re: Quicky question: BBC Micro ROM <--> keyboard clearance

Postby cmorley » Tue Sep 05, 2017 3:26 pm

Not a lot! You can just fit an EPROM in a socket on a plug in board (standard 1.6mm PCB)... but only just. The first thing to hit is the pins from the components on the solder side of the keyboard - these press and scratch the device below.

My SRAM/OSRAM boards fit with a small amount of clearance with an EPROM in the piggy back socket.

The Watford Electronics Quest Paint I have doesn't actually fit! The keyboard hits it.

crj
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Re: Quicky question: BBC Micro ROM <--> keyboard clearance

Postby crj » Tue Sep 05, 2017 4:10 pm

You're talking about this product, yes?

Knowing about that is one of the factors that gives me hope what I'm up to may be feasible.

I'm wanting to make a module that fits into a ROM socket and stacks:
• 0.1" pins to fit the DIL socket
• PCB
• Minitek127 board stacking connector pair (5.8mm spacing)
• PCB
• Low-profile componentry

It looks like you have:
• Pins
• PCB
• DIP socket (4.08mm from PCB to insertion plane)
• EPROM/ROM (5.72mm above insertion plane)

That suggests I'm probably OK but just barely. I was hoping someone could provide an actual number so I didn't have to resort to a mixture of forensics and guesswork as to precisely what components other products use!

cmorley
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Re: Quicky question: BBC Micro ROM <--> keyboard clearance

Postby cmorley » Tue Sep 05, 2017 4:30 pm

Yes that's the board.

I just measured one with an eprom in and it is 20.5mm tall from pin tip to EPROM top. How far it sits into the sockets on the BBC is going to depend on the BBC sockets...

Boydie
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Re: Quicky question: BBC Micro ROM <--> keyboard clearance

Postby Boydie » Tue Sep 05, 2017 9:02 pm

18mm from top of empty socket to underside of keyboard. May get 19mm at a pinch.

crj
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Re: Quicky question: BBC Micro ROM <--> keyboard clearance

Postby crj » Wed Sep 06, 2017 1:16 am

Picking as random specimens the 3M 4824-6000-CP DIP socket and the NatSemi 27C256 EPROM, it looks like pins go about 3mm into the sockets.

So something 20.5mm from pin tip to top would be 17.5mm from ROM socket to top. That's consistent with "a small amount of clearance" in a space that's "18mm, 19mm at a pinch". (-8

Meanwhile, by tweaking things a little I might have managed to get my design down to 11mm from ROM socket to top, which would be... comfortable, if I can pull it off. (I'm not sure of all the applications for this, so it's hard to say how low is low enough.)

Thanks, guys!

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1024MAK
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Re: Quicky question: BBC Micro ROM <--> keyboard clearance

Postby 1024MAK » Wed Sep 06, 2017 10:46 am

You can get socket pins that actually fit in the PCB reducing the height of the PCB and chip. But the PCB has to be designed with this in mind as larger plated through PCB holes are needed and it makes routing the tracks just that much more interesting. Also these socket pins are not cheap :(

Mark
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crj
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Re: Quicky question: BBC Micro ROM <--> keyboard clearance

Postby crj » Wed Sep 06, 2017 1:22 pm

Do you have a part number/link?

I've looked into this a bit and the best I can find is the Omron XJ8 range, which is a little better than normal and not at all pricey.

The surface-mount options spare you the PCB real-estate on the backside of the pins which would be lost to through-hole. Unfortunately, all the ones I could find splay adjacent pads in alternating directions. This makes the board footprint of the connector wider than a ROM. That's a shame on general principles, and especially bothersome if you wanted to populate two adjacent ROM sockets with the same kind of board. If I was feeling especially devious, the boards could be "crinkle-cut" so they'd tesselate, but really!

crj
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Re: Quicky question: BBC Micro ROM <--> keyboard clearance

Postby crj » Sun Sep 17, 2017 6:24 pm

I've now found a range by Preci-dip which looks very promising.

Sourcing may be a bit of a challenge, though.


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