S-100, bus drivers and RC2014 design?

[Kande Laber]

Hello,

I am an electronic newbie.

If you look at old S-100 bus cards they seem to be full of 74244 / 74245 type of bus driver / transceiver chips. 

Recentely, I have soldered a RC2014 compatible system. I do not remember having soldered any bus driver chips.

Can anybody of the electronic cracks explain this obvious design difference to me?

Best regards

[Alan Cox]

> If you look at old S-100 bus cards they seem to be full of 74244 / 74245 type of bus driver / transceiver chips.
>
> Recentely, I have soldered a RC2014 compatible system. I do not remember having soldered any bus driver chips.
>
> Can anybody of the electronic cracks explain this obvious design difference to me?

In one word - CMOS.

The CMOS parts used run on extremely low currents. That means that the
fan out is enormous so you don't need buffers to handle the number of
devices on the bus, and it also means that the current flowing on the
bus is very low so you don't need buffers and terminators on the bus
itself. Unlike S100 the bus is also well matched (all 74HCT parts not
a mix of 74 74F 74S 74LS) and on the same 5v rail.

There are a few things that sometimes need buffers but in general
providing it is all 74HC/HCT parts connecting to the bus it all works.

Alan

[Kande Laber]

Thank you for your explanation.

If understand you correctly, this is e.g. the difference between a "ZILOG Z8400 PS Z80 CPU 8213 V" which I can find in my old Microprofessor IP (although that unit is to simple to not even have a bus driver) and a "Z80 CPU Z84C0008PEG" which I can find in my RC2014-design SC114.

Or in other words: The ""ZILOG Z8400 PS Z80 CPU 8213 V" would not work well in my SC114-based system. 

Is that correct?

[Alan Cox]

On Tue, 31 Jan 2023 at 12:50, Kande Laber <kande2...@gmail.com> wrote:
>
> Thank you for your explanation.
>
> If understand you correctly, this is e.g. the difference between a "ZILOG Z8400 PS Z80 CPU 8213 V" which I can find in my old Microprofessor IP (although that unit is to simple to not even have a bus driver) and a "Z80 CPU Z84C0008PEG" which I can find in my RC2014-design SC114.
>
> Or in other words: The ""ZILOG Z8400 PS Z80 CPU 8213 V" would not work well in my SC114-based system.
>
> Is that correct?

The older Z80 is NMOS so cannot drive as much logic as a CMOS part nor
as fast, but the most important bit is actually the fan out for the
receiving end of the signals (the I/O cards). If those are CMOS or
NMOS parts, particularly all the address decoders, then it'll be
within spec at the speeds the NMOS processor was intended to run.

The CF adapter may not. The standard one is an unbuffered interface to
the CF card which NMOS parts are often not strong enough to drive
properly. A buffered CF interface does work.

Alan

[Bill Shen]

The second reason is integration.  ICs are much more integrated so a board full of TTL logic is no longer needed.  S100 board is 50 square inch vs RC2014's 8 square inch.  Buffers between S100 boards are needed but one entire RC2014 system has the same square footage of one S100 board, so buffering is really not needed between small RC2014 boards.  I don't see significant problem with NMOS driving the rest of RC2014 because the loads are so light.
  Bill

[Bernard Oates]

The retrobrew ECB computers use bus buffers and transceivers.  They are not dependent on all-CMOS or nearly all CMOS construction.

The nhyodyne system uses a passive backplane with small modules and is fully buffered which allows use of 74LS or 74HCT

The use of buffers and transceivers can protect the capital chips from the bus backplane environment but at the expense of complexity

Buffering the system has advantages and disadvantages.  It is a trade-off that individual hobbyists have to make.

However, buffer and transceiver chips are tough and can endure more severe abuse than a CPU or UART chip can withstand in general.

 

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