SRAM PCB modification difficulties

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Boydie
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SRAM PCB modification difficulties

Postby Boydie » Wed Sep 06, 2017 4:34 pm

I'm having a very frustrating problem, trying to modify my Issue 4 motherboard to take 32K/64K ROMs and SRAM.

I've diligently followed the instructions on both Retroclinic and MDFS sites (they're subtly different in where they tell you to connect Pin 1 of the SRAM socket), but it stops my user port from working.

I've found that if I've got links to both pins 11 and 12 on IC76, then the user port stops working (MMC card gives Card? and ?&FE60 gives EB insetad of FF). Disconnecting pin 12 of IC76 (connected to pin 27 on the sockets designated for ROM) returns ?&FE60 to FF, but obviously stops the ROM sockets working properly.

As an incidental finding, I had to replace IC76 on the board with a spare, as the mod wouldn't work at all at first and was traced back to pins 11 and 12 being shorted together. This is no longer the case. I can't see any obvious PCB damage resulting from this.

Any suggestions as to where to look next would be gratefully received.

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hoglet
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Re: SRAM PCB modification difficulties

Postby hoglet » Wed Sep 06, 2017 4:55 pm

Were these the instructions?
http://www.retroclinic.com/acorn/swr/swr.htm
http://mdfs.net/Info/Comp/BBC/SROMs/32K/

What ROM/RAM device are you using?

Boydie
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Re: SRAM PCB modification difficulties

Postby Boydie » Wed Sep 06, 2017 5:32 pm

Yeah, they're the ones. They differ very slightly in how they say to connect the pins, in a way I suspect doesn't make a whole lot of difference, but I've tried both to make certain. I've currently wired it the Retroclinic way.

So far, I've only being using various combinations of the original BASIC ROM, and vontage 27128s and 27256s. I've tried both with and without a modern 6C62256 in the dedicated SRAM socket. In all cases, the ROMs appear to work fine in any combination. It's not got a 1770 disc interface fitted, so I've tried using Solidisk's Toolkit, ADT, and JGH's SRAM utils off MDFS. Toolkit and ADT list all filled slots (for both 16K and 32K chips) appropriately. For SRAM utils, if it's a 16K chip it lists that rom image for all slots associated with that socket (eg BASIC in 15,11,7,3), if it's a 32K chip it alternates the two images across the four associated slots (eg SMART SPI in 14 and 6, SRAM in 10 and 2). I've not tried a 64K device yet.

From what I can tell, the sockets work fine if both pins 11 and 12 of IC76 are connected, but this scuppers the user port. If only pin 11 is connected, the sockets don't work properly but the user port would appear to.

I've also tried swapping the 6522 for several others. This makes no difference.

dp11
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Re: SRAM PCB modification difficulties

Postby dp11 » Wed Sep 06, 2017 5:37 pm

Is there a chance you haven't actually connected to IC76 ? can you post a photo ?

dp11
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Re: SRAM PCB modification difficulties

Postby dp11 » Wed Sep 06, 2017 5:47 pm

If you have shorted IC24 pin12 or IC23 Pin11 you would break the User Port

Boydie
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Re: SRAM PCB modification difficulties

Postby Boydie » Wed Sep 06, 2017 6:00 pm

It's worse than I thought.

I've just completely reversed all the mods to the ROM sockets, so now the only difference between now and when I started is the replaced IC76.

ROM sockets seem to work fine.

If nothing's plugged into the user port ?&FE60 gives FF. If an MMC card (I've tried several, all previously good) is inserted, it gives the card? error, and ?&FE60 gives EB.

Could a broken IC76 cause this?

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1024MAK
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Re: SRAM PCB modification difficulties

Postby 1024MAK » Thu Sep 07, 2017 7:35 am

IC76 (74LS163) only latches the nybble (bits 0 to 3) from the data bus when the CPU writes to the ROMSEL address (0xFE30). It is known as the ROM select latch.

It has no other function and I can't see how it would affect the user port VIA. If IC76 is damaged, or there is damage to it's pins or connections, then that may prevent some ROM chips from being selected.

For more on sideways ROMs/RAM read this thread.

The user port is provided by IC59 (a 6522 VIA).

Reading 0xFE60 will read the state of the VIA port B pins, so the result will depend on what device (if any) is attached to the user port.

Mark
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Boydie
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Re: SRAM PCB modification difficulties

Postby Boydie » Thu Sep 07, 2017 9:16 am

Thanks. I was assuming that IC76 was purely for ROM selection and it's nice to have it confirmed. That was how I worked out that the IC76 was broken - I originally tried to wire things up (as documented on mdfs sram guidelines) so the sockets were 0123, 4567, etc - the two middle sockets vanished!

Thanks for the thread link. I'd read it before starting any of this, along with Michelle's web page. So I thought I'd prepared for this, and what could possibly go wrong?

Current plan:

Working on the theory it's something I've done, I've removed IC76 (again). I can now check the area for any inadvertent damage (damaged and repaired a couple of adjacent tracks (to FDC I think) this time so there may be other stuff lurking). I'm ordering replacement 74LS163AN from Farnell. When I'm happy, I'll socket IC76 and hope it boots.
I've found the thread for testing the user port, so when I've got a booting machine I'll fully test it.
Hopefully this may shed some light as to what is wrong.

Will hopefully feed back in a few days.

Are there any relevant tracks passing near IC76 which I may have damaged and should be paying particular attention to?

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1024MAK
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Re: SRAM PCB modification difficulties

Postby 1024MAK » Thu Sep 07, 2017 9:46 am

Boydie wrote:Are there any relevant tracks passing near IC76 which I may have damaged and should be paying particular attention to?

Without taking a Beeb with the same issue board as yours apart, it's not easy to say...

Mark
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Boydie
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Re: SRAM PCB modification difficulties

Postby Boydie » Thu Sep 07, 2017 6:27 pm

I may have solved it!
Remembering advice from a previous problem, I decided to check continuity between the shoulders of the pins on IC69 and the socket pins. Pin 11 didn't have continuity, which turned out to be due to a small piece of wire stuck in that pin's hole (previously had a Solidisk SWR board fitted). Removing the fragment has restored continuity. Checking the circuit diagram, pin 11 is PB1, which is indeed used in the MMC interface, so it's looking hopeful.

Shame I didn't think of it last night, before I desoldered IC76 again, but that's what you get if you try to do jobs after an on-call.

Would someone mind quickly checking over a couple of photos of the carnage I caused around IC76, just to make sure there's nothing I've missed? I've fixed a couple of tracks next to pins 9-12 - one was a simple solder bridge, the other required a run of wire, and both have continuity. But I'm not sure I trust my judgement on the rest. The melting of IC81's socket was there when I acquired the board, honest!

As for whether it's all cured, we'll know after I bob to Maplin tomorrow for a 16-pin socket...
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1024MAK
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Re: SRAM PCB modification difficulties

Postby 1024MAK » Thu Sep 07, 2017 8:33 pm

It would help if you cleaned the old flux off. Use either a flux cleaner or IPA. A clean toothbrush will help.

Mark
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CMcDougall
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Re: SRAM PCB modification difficulties

Postby CMcDougall » Thu Sep 07, 2017 8:56 pm

exposed copper on some holes, and with blurry pic, bottom right maybe copper gone, but should get away with it as
it's connected solder side anyways!

hole3 needs cleaned out, or the new socket won't 'fall' in

nice job with blue jumper wire, but next to bottom 1cm up of it, looks like another track about to fall apart, so would also do that.
don't want to be ripping your hair out 1+yrs later, & thats the problem....
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Boydie
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Re: SRAM PCB modification difficulties

Postby Boydie » Fri Sep 08, 2017 11:38 am

Thanks everyone for all the advice, particularly cleaning with IPA - made everything a lot easier to see.

New socket has been fitted for IC76 and 16K ROMs appear to work okay in all sockets. I'm happy to report that MMC card is now working.

Many thanks again for all the help. I've learnt a lot, but then again you always do from the things that go wrong.

Now to try again to modify for 32/64K chips. What's the worst that can possibly happen?

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BeebMaster
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Re: SRAM PCB modification difficulties

Postby BeebMaster » Fri Sep 08, 2017 12:23 pm

It isn't necessary to modify the PCB, you can instead modify the chip by soldering flying wires to the pins and attaching these permanently (by soldering) or temporarily (using a spring-loaded probe clip or similar) to the points on the PCB.

That way there's no permanent change to your BBC, there's no need to take the PCB out of the Beeb, and the only thing which is likely to be damaged in the process is the 32K EPROM/RAM chip, which is (probably) expendable.
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Boydie
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Re: SRAM PCB modification difficulties

Postby Boydie » Fri Sep 08, 2017 4:19 pm

True, but my past experiences with soldering to IC pins have been even worse; soldering to be back of the PCB has been a joy by comparison. Since I'm modifying all 4 sockets it's saved having a lot of flying leads all over the shop - my encounters with Solidisk SWR32-128 boards have been horrible.

I now have 64K ROMs in ICs 52, 100, and 101, and IC 88 has been modded to accept 32K SRAM as standard. I'm a happy bunny.

I'm now planning on fitting a 128K 6C1008 into IC88 to give 64K SRAM. Since the socket's already been modified for 32K, and on the basis of viewtopic.php?t=8980, am I correct in assuming:

Pin 1 remains NC
Pin 3 (=Pin 1 in motherboard) has already been modified to give Bit 2 of the ROM select latch as it is connected to IC76 pin 12
Pin 29 (=Pin 27 in m/b) has already been modified to write-enable on IC77
Pin 30 needs no modification
Pin 32 needs connecting to Pin 30
Either Pin 2 or Pin 31 needs to be connected to IC76 pin 11 for Bit 3, and the other is redundant because Pin 3 is being used instead. Does the other, redundant pin need to be made high by connecting to Pin 30?

Have I missed anything?


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