ZX81 I/O interface for your projects

Discussions about Sinclair ZX80 and ZX81 Hardware
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1024MAK
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Re: ZX81 I/O interface for your projects

Post by 1024MAK » Sat Apr 07, 2018 2:46 pm

@rom475 this I/O interface board is mainly for interfacing circuits that only use 1 to 8 data bits. Like simple electronic circuits such as switches, photo sensors, relay contacts, LEDs, relay driver circuits and other simple circuits.

If interfacing to AY3-8910,12, or SPO256A-AL2 chips, as these can be connected directly to the ZX81 Z80 bus, a different circuit would be used. Various designs exist for AY (AY3-8910,12) sound systems. Search for 'ZX81 ZON', 'ZX81 AY' and 'MrX AY-Sound Interface' on a search engine. And there exist some designs for circuits using the SPO256A-AL2 speech chip. Here is one example.

Mark

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Re: ZX81 I/O interface for your projects

Post by rom475 » Sun Apr 08, 2018 9:11 pm

In designing any peripheral what is the logic family of choice,74LS,74HC,or 74HCT??

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Re: ZX81 I/O interface for your projects

Post by 1024MAK » Mon Apr 09, 2018 11:22 am

rom475 wrote:
Sun Apr 08, 2018 9:11 pm
In designing any peripheral what is the logic family of choice,74LS,74HC,or 74HCT??
74LSxxx were the 'standard' when the ZX81 was designed. They use bipolar transistor technology. A large number of the range are still available today. They do the job. Great for use on a breadboard for experimenting with, as they aren't easily damaged by static electricity. They are the next popular step up from the older original 74xxx series. And are compatible with this older chip design. In the past this chip range was cheaper than the equivalent 74HCxxx version.

74HCxxx are made using CMOS technology. Much lower power than the 74LSxxx range. The input logic levels are not compatible with the 74LSxxx range. But the outputs can drive the 74LSxxx range. As with all CMOS devices, it is recommended that precautions are taken to prevent damage due to static electricity.

74HCTxxx are very similar to the 74HCxxx range. In fact, apart from the input sections, they are the same. The difference is the input stages have been designed to be compatible with the 74LSxxx range.

In the 1990s, the 74LSxxx was considered to be near the end of life, and the 74HCxxx and 74HCTxxx were recommended to be used.

Although officially 74HCxxx inputs are not compatible with existing systems based on 74LSxxx logic, in practice, as long as an output is not driving too many existing logic inputs, often a 74HCxxx chip will work without problems. But if you want to be totally sure, use a 74HCTxxx between the existing system and the rest of the new logic (which can be 74HCxxx types).

Myself, where I have to buy new chips, or I want the circuit to be low power, I use 74HCxxx and/or 74HCTxxx. Otherwise I still use 74LSxxx due to having a small stock of them, plus some surplus old boards which have lots of them in sockets.

Mark

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Re: ZX81 I/O interface for your projects

Post by RetroTechie » Wed Apr 11, 2018 11:05 am

74HC -> 74LS: no problem, unless you're driving many LS inputs at once. In that case check datasheets / calculate load currents etc.
74LS -> 74HC: often works fine. In general, just check properties of IC where a signal comes from (and check datasheet of the HC part where it goes to). Better: add a pull-up to the 74HC input, or just use a 74HCT part.

There exist much faster parts (74AC(T) and 74LVC families come to mind). But using those in relatively slow machines like a ZX81 may have more drawbacks than advantages. Flip a signal fast enough, and weird effects may be seen @ the other end of a pcb track... ;) :lol:

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Re: ZX81 I/O interface for your projects

Post by rom475 » Fri Apr 13, 2018 2:51 am

Thanks for all the help...I have tons of 74LSXX and 74HCTXX,and since I'm looking for the
greatest amount of power conservation in a robotics project,74HCT Series it is...

Now to decide between the Z84C20 PIO which I have 20 or to use the 82C55 as device
controllers??

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Re: ZX81 I/O interface for your projects

Post by Paul » Fri Apr 13, 2018 7:39 am

Do you want 16 bit of IO or 24? :mrgreen:
In theory, there is no difference between theory and practice. But, in practice, there is.

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Re: ZX81 I/O interface for your projects

Post by rom475 » Sat Apr 21, 2018 1:57 am

I'll be using the 8 bit data bus so what I am looking for is 8 bits in and 8 bits out,but with the inclusion of device select codes,
one for each device to turn it off or on to send or receive data...simple,but effective...

I'm looking for a schematic of what I stated above,but if you have something I could utilize or any ideas it would be greatly appreciated...

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Re: ZX81 I/O interface for your projects

Post by rom475 » Sat Apr 21, 2018 2:04 am

I'm also looking for ideas or perhaps a schematic with docs for using the HC-SR204 Ultrasonic Transceiver Board.
I will be using 3 of the boards as part of the navigation/object avoidance system...

I already have a working Infrared system working that will provide additional control to the navigation...

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