DIY cheap pitubedirect signal converter building

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duikkie
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DIY cheap pitubedirect signal converter building

Postby duikkie » Sat Jan 13, 2018 2:33 pm

veroboard from china 70*90 holes Z by 34 = 0.6 euro (free shiping)
2 chip 74lvc245a from china 1.40 euro (free shiping)
two 40 pin headers from china 0.40 euro (free shiping)
raspberry pi zero 6 euro W from holland :) (free shiping)

oeps one 40 pin header for pi 0.20 euro
.......4 female header to connect 4*0.14=0.56 from china (free shiping)

totaal about 10 euro

wires
flatcable
2 cap 100nF

work hours 5 :)
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Last edited by duikkie on Sat Jan 13, 2018 2:57 pm, edited 6 times in total.

duikkie
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Re: DIY cheap pitubedirect signal converter building

Postby duikkie » Sat Jan 13, 2018 2:35 pm

if someone what to build i upload the paper where to cut en solder the lines ;)

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BigEd
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Re: DIY cheap pitubedirect signal converter building

Postby BigEd » Sat Jan 13, 2018 2:44 pm

€10 all-in, and it works? Excellent!

duikkie
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Re: DIY cheap pitubedirect signal converter building

Postby duikkie » Sat Jan 13, 2018 2:48 pm

yes it works for pi you must have a good sd card ( not all will do :shock: )

BigEd wrote:€10 all-in, and it works? Excellent!

duikkie
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Re: DIY cheap pitubedirect signal converter building

Postby duikkie » Sun Jan 14, 2018 9:47 am

X= cut
no pins at pi at 27,28 and 31,32 do not cut (no X) pull out pins 27,28,31,32

ic1 = pin 11=TD0 ,pin 18=TD7

ic2 = pin11=TA1,pin12=TA2,pin13=gnd,pin14=TRW,pin15=T2mhz,pin16=TA0,pin17=TTUBE,pin18=TRST
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duikkie
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Re: DIY cheap pitubedirect signal converter building

Postby duikkie » Thu Jan 18, 2018 9:45 am

still playing the dutch man :)

found out that the cheaper SN74HC245N works as good as the expensive SN74LVC245AN

SN74hc245n=0.11 euro ... SN74lvc245an=0.70 euro :D


edit don't know about the speed #-o , sn74hc245n in datasheet slower , if i read well

cmorley
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Re: DIY cheap pitubedirect signal converter building

Postby cmorley » Thu Jan 18, 2018 10:35 am

You should reconsider using 74HC series in this application. I think you're powering them off 3.3v and they have input protection diodes to clamp high and low voltages. The datasheets for HC will tell you not to go 0.5v above VCC on inputs (and lower than -0.5V).

You will be making everything which drives the bus inside the BBC work harder to drive this clamping current - which will shorten the life of chips in the BBC (6502, 6522s etc). The 74HC device might fail too - but they are so cheap so maybe you don't mind.

74LVC is properly 5v tolerant when powered from 3.3v.

Also using 4245 would be better because it actually level shifts too instead of just buffering at 3.3v. Why? Because CMOS chips draw extra current from the supply when the input is not driven fully to the rail. Typically 0.3mA per input for HCT @3.3v. There aren't many CMOS chips in a B (maybe EPROMS or a 65C02 if upgraded etc), as it is mostly NMOS and 74LS, but these will all draw extra current & make extra heat (reducing life). I don't know about the Master...

duikkie
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Re: DIY cheap pitubedirect signal converter building

Postby duikkie » Fri Jan 19, 2018 5:19 am

oke easy to change if they are not soldered :) pulling 74hc out the 74lvc in. it is not that difficult :)
and the beeb cost more :)

cmorley wrote:You should reconsider using 74HC series in this application. I think you're powering them off 3.3v and they have input protection diodes to clamp high and low voltages. The datasheets for HC will tell you not to go 0.5v above VCC on inputs (and lower than -0.5V).

You will be making everything which drives the bus inside the BBC work harder to drive this clamping current - which will shorten the life of chips in the BBC (6502, 6522s etc). The 74HC device might fail too - but they are so cheap so maybe you don't mind.

74LVC is properly 5v tolerant when powered from 3.3v.

Also using 4245 would be better because it actually level shifts too instead of just buffering at 3.3v. Why? Because CMOS chips draw extra current from the supply when the input is not driven fully to the rail. Typically 0.3mA per input for HCT @3.3v. There aren't many CMOS chips in a B (maybe EPROMS or a 65C02 if upgraded etc), as it is mostly NMOS and 74LS, but these will all draw extra current & make extra heat (reducing life). I don't know about the Master...

cmorley
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Re: DIY cheap pitubedirect signal converter building

Postby cmorley » Fri Jan 19, 2018 8:35 am

duikkie wrote:oke easy to change if they are not soldered :) pulling 74hc out the 74lvc in. it is not that difficult :)
and the beeb cost more :)

The risk may be small - maybe it would work for many years.. but I just want people to be using and having fun with these original machines in another 35 years too :)


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