whole board with 74LS

Using, learning, programming and modding the Gigatron and anything related.
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giobbi
Posts: 30
Joined: 11 Jul 2021, 22:12

whole board with 74LS

Post by giobbi »

hi guys,

I just bought a gigatron board, can't wait for it LOL.

The board was made for 74HCT series ICs; but I have most ICs of the 74LS kind so I with to use them instead to bought all the 74HCT chips.

I know that theoretically the board should work with 74LS chips only. But did anybody test it yet? Of course it will consume more current but it should not be a big problem for me.
james7780
Posts: 6
Joined: 11 Jul 2021, 18:43

Re: whole board with 74LS

Post by james7780 »

At least clock-related IC U1 (74HCT04) must be HCT, as I read somewhere.

I tried 74LS04 on my board and it did not work.
alastair
Posts: 24
Joined: 10 Oct 2019, 14:28

Re: whole board with 74LS

Post by alastair »

The passive components of the Pierce oscillator circuit are designed for CMOS. E.g. the feedback resistor is 1M ohm. For a 74LS04 you will need to drop this significantly to around 1k ohm.

This is an example I built using F-series TTL chips. You may need to play with the values of the resistors and capacitors to get it working for 6.25 MHz. Also note, you will need to use different values in the video DAC (see schematic for TTL values).
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walter
Site Admin
Posts: 126
Joined: 13 May 2018, 08:00

Re: whole board with 74LS

Post by walter »

See viewtopic.php?f=4&t=258

You'll probably want to stick with a 74HCT04 and a 74HC595.
ImmortanJoe
Posts: 2
Joined: 31 May 2021, 04:25

Re: whole board with 74LS

Post by ImmortanJoe »

I know 74LS138s work. I had to use a couple of them until my HCTs arrived.
giobbi
Posts: 30
Joined: 11 Jul 2021, 22:12

Re: whole board with 74LS

Post by giobbi »

ok guys, thank you.
I've watched the explanation video about the presentation of the Gigatron TTL (hey, it's called TTL not CMOS, LOL) and they told it was made to work with LS series too.

Since the switch voltage of CMOS is lower that TTL, as principle if you put the switch line between 0 and 1 at about 3.5V, it should be ok for TTL and CMOS, isn't it? So the board could be engineered for TTL, still working with CMOS... am I right?
bmwtcu
Posts: 60
Joined: 01 Nov 2018, 12:02

Re: whole board with 74LS

Post by bmwtcu »

Did you use a 74LS377 or 74HCT377 for the output register? I believe you need different video DAC resistors per the schematic. The 750/1500 ohm values are meant for HCT
giobbi
Posts: 30
Joined: 11 Jul 2021, 22:12

Re: whole board with 74LS

Post by giobbi »

I used the HCT kind; but Walter built a board using just LS parts: viewtopic.php?p=1734#p1734
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