Watford Electronics 13 ROM board

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CanonMan
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Watford Electronics 13 ROM board

Postby CanonMan » Sun Feb 05, 2012 4:35 pm

Hi peeps

I've just picked up another BBC model B from Freecycle, and found a WE 13 ROM board floating around loose inside it :)

Could somebody please point me towrards some fitting or operating instructions for it?

Thanks in advance.

Cheers,

Dave
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retroclinic
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Re: Watford Electronics 13 ROM board

Postby retroclinic » Sun Feb 05, 2012 6:31 pm

By your command.....

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Mark.
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CanonMan
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Re: Watford Electronics 13 ROM board

Postby CanonMan » Sun Feb 05, 2012 7:40 pm

Thanks Mark, that should save me a load of hassle :)

I shall be putting a bit more business your way soon, too!

Cheers,

Dave

crj
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Re: Watford Electronics 13 ROM board

Postby crj » Tue Sep 26, 2017 2:26 am

Gosh. That powered 13 ROMs from a single mainboard ROM socket? And it worked reliably?

I've been idly wondering just how much power it's OK to pull from one of those before you have to start drawing it from elsewhere instead!

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danielj
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Re: Watford Electronics 13 ROM board

Postby danielj » Tue Sep 26, 2017 5:18 am

I've got one of those fully populated, and it does indeed work reliably!

d.

crj
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Re: Watford Electronics 13 ROM board

Postby crj » Tue Sep 26, 2017 6:34 am

So... um... looking at an old datasheet, a 27128 has standby power consumption of 15mA typ, 40mA max. Active power consumption is 60mA typ, 100mA max. So with 13 EPROMs, one of which is active, my maths says that's 240mA typ, 700mA max.

That latter figure is enough to make the PSU unhappy, let alone the PCB tracks to that ROM socket! I'm assuming in practice they consume closer to the "typical" number, which is still a fairly hefty current.

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danielj
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Re: Watford Electronics 13 ROM board

Postby danielj » Tue Sep 26, 2017 6:46 am

Ooh, hang on, that's not quite the same as mine (I lazily didn't click on the pictures) - mine's 12 ROM board that has two connections, one via the ADC too: http://chrisacorns.computinghistory.org ... boardA.jpg

Still ridiculous, still amazingly works.

d.

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1024MAK
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Re: Watford Electronics 13 ROM board

Postby 1024MAK » Tue Sep 26, 2017 6:52 am

I don't think I've ever seen current ratings for stamped pin DIL sockets. But some DIL chips used for power switching are rated at up to about 800mA for intermittent use.

A lot of the 0V and +5V tracks on the Beeb are fairly wide, but I have not actually checked in detail with the thought of the current handling ability...

Anyway, there have been many internal expansion boards for the Beeb and apart from broken wires and broken pins, mainly the problems are just poor contacts due to the stamped pin socket contacts becoming less springy over time/use.

The Beeb's SMPSU has enough spare capacity, so that's not a problem.

Mark
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crj
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Re: Watford Electronics 13 ROM board

Postby crj » Tue Sep 26, 2017 5:14 pm

I thought the Beeb's PSU only had a +5V headroom of about 700mA, and that was supposed to be for the use of a floppy drive?

I suspect how well a machine copes with such an expansion board might depend a very great deal on whether the drive(s) is/are independently powered.

I'd noticed the PCB traces around the ROM sockets are pretty broad for power and ground. While in one sense encouraging, in another that suggests somebody perhaps decided they needed to be, which would paradoxically imply less headroom.

(I have in mind a cunning plan that would chew about 200mA. I was assuming that would be OK, but was also contemplating buying a semi-sacrificial machine on eBay for testing, rather than risking it in the Master Turbo I've had since childhood.)

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MartinB
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Re: Watford Electronics 13 ROM board

Postby MartinB » Tue Sep 26, 2017 6:21 pm

In this PSU thread, I measured the +5v current draw of a busy Model B at 2.1A.....

Beeb 5v current draw.JPG


...so given that the stock PSU has a +5v rating of 3.75A, that suggests a theoretical headroom of over 1600mA.


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