[N8VEM: 17289] A BHE question from a wannabe computer designer

[joe.herdler-/]

Hi All,

Here is a question that has stumped me. I am attempting to design an
old-school 8086 computer using various manuals and data sheets. I have run
into one issue that read though I may, have not doped out. That is the BHE
signal from the 8086 and co-processors. According to Osborn, the BHE lines
should be connected between the 8089 and 8086. should that same line be
connected to the BHE line on the 8087, with all three processors in
parallel? Next, I know that the BHE line needs to be latched along with the
Address lines (A0-A19). I also know that the last of the three 8282 latches
that are used for the top 4 address lines is used for this purpose. What
pin should I use to latch this BHE signal? Pin one, with output on pin 19?

I know that I am no computer designer, though I really would like to be.
Any help on this would be quite appreciated. By the way, I am calling this
the "Definitely Old School Intel Machine" (DOSIM for short).

Best,

Joe

[Sergey]

Joe,

BHE pins of 8086, 8087 and 8089 should be connected together (just as all
other control, and address/data lines). Only one processor / co-processor
controls the bus at the given time, all other processors will be inactive
with these pins either in high impedance state or switched to inputs (which
is basically the same).

BHE should be latched just as multiplexed data / address signals using ALE
signal, because it only active during T1 state.
You can notice that BHE signal is actually labeled as BHE/S7. S7 is a
status signal, which is simply "LOW". By the way this signal is used by
8087 coprocessor (and I assume 8089 too) to distinguish between 8086 based
(16-bit data) and 8088 based (8-bit data) systems. Following the reset
co-processor samples this signal. In case of 8086 it will be LOW, as the
processor will try to fetch 16-bit word (first instruction from 0xFFFF0),
while 8088 running in maximum mode will always output HIGH.

I don't quite understand the second question. It doesn't really matter
which latch (of eight latches available in 8282) you use to latch what
signal... I'd just arrange them in order that will make PCB routing simpler.
Also if you want your system to be a bit cheaper, you can use 74*573 or
74*373 (a bit harder to route) latches instead of 8282. 8282 is no longer
produced, while 7400-series chips are still being made, and therefore
readily available.

Thanks,
Sergey

Post by joe.herdler-/
Hi All,
Here is a question that has stumped me. I am attempting to design an
old-school 8086 computer using various manuals and data sheets. I have run
into one issue that read though I may, have not doped out. That is the BHE
signal from the 8086 and co-processors. According to Osborn, the BHE lines
should be connected between the 8089 and 8086. should that same line be
connected to the BHE line on the 8087, with all three processors in
parallel? Next, I know that the BHE line needs to be latched along with the
Address lines (A0-A19). I also know that the last of the three 8282 latches
that are used for the top 4 address lines is used for this purpose. What
pin should I use to latch this BHE signal? Pin one, with output on pin 19?
I know that I am no computer designer, though I really would like to be.
Any help on this would be quite appreciated. By the way, I am calling this
the "Definitely Old School Intel Machine" (DOSIM for short).
Best,
Joe

[Joe Herdler]

Hi Sergey,
 
Thank you very much for the Info! I will indeed use the 74373 type latch rather than the 8282. You have helped me greatly!
 
Thank you again,
 
Joe

Consciousness affects the medium. Think happy, be happy, and happiness will follow you.



On Friday, February 7, 2014 6:28 PM, Sergey <skiselev-***@public.gmane.org> wrote:

Joe,

BHE pins of 8086, 8087 and 8089 should be connected together (just as all other control, and address/data lines). Only one processor / co-processor controls the bus at the given time, all other processors will be inactive with these pins either in high impedance state or switched to inputs (which is basically the same).

BHE should be latched just as multiplexed data / address signals using ALE signal, because it only active during T1 state.
You can notice that BHE signal is actually labeled as BHE/S7. S7 is a status signal, which is simply "LOW". By the way this signal is used by 8087 coprocessor (and I assume 8089 too) to distinguish between 8086 based (16-bit data) and 8088 based (8-bit data) systems. Following the reset co-processor samples this signal. In case of 8086 it will be LOW, as the processor will try to fetch 16-bit word (first instruction from 0xFFFF0), while 8088 running in maximum mode will always output HIGH.

I don't quite understand the second question. It doesn't really matter which latch (of eight latches available in 8282) you use to latch what signal... I'd just arrange them in order that will make PCB routing simpler.
Also if you want your system to be a bit cheaper, you can use 74*573 or 74*373 (a bit harder to route) latches instead of 8282. 8282 is no longer produced, while 7400-series chips are still being made, and therefore readily available.

Thanks,
Sergey



On Sunday, February 2, 2014 12:01:44 PM UTC-8, joe.h...-/***@public.gmane.org wrote:
Hi All,

Here is a question that has stumped me. I am attempting to design an old-school 8086 computer using various manuals and data sheets. I have run into one issue that read though I may, have not doped out. That is the BHE signal from the 8086 and co-processors. According to Osborn, the BHE lines should be connected between the 8089 and 8086. should that same line be connected to the BHE line on the 8087, with all three processors in parallel? Next, I know that the BHE line needs to be latched along with the Address lines (A0-A19). I also know that the last of the three 8282 latches that are used for the top 4 address lines is used for this purpose. What pin should I use to latch this BHE signal? Pin one, with output on pin 19?
I know that I am no computer designer, though I really would like to be. Any help on this would be quite appreciated. By the way, I am calling this the "Definitely Old School Intel Machine" (DOSIM for short).
Best,
Joe 

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[Sergey]

Just one note - Don't use 74373 part specifically, it is too slow and too
power hungry. I think 74F373 will be the best for this purpose, but
74ALS373 or 74LS373 should work as well. And again, 74F573 has exactly the
same functionality, but easier to route (all inputs on one side, and all
outputs on the other size of the chip).

Thanks,
Sergey

Post by Joe Herdler
Hi Sergey,
Thank you very much for the Info! I will indeed use the 74373 type latch
rather than the 8282. You have helped me greatly!
Thank you again,
Joe
Consciousness affects the medium. Think happy, be happy, and happiness will follow you.
Joe,
BHE pins of 8086, 8087 and 8089 should be connected together (just as all
other control, and address/data lines). Only one processor / co-processor
controls the bus at the given time, all other processors will be inactive
with these pins either in high impedance state or switched to inputs (which
is basically the same).
BHE should be latched just as multiplexed data / address signals using ALE
signal, because it only active during T1 state.
You can notice that BHE signal is actually labeled as BHE/S7. S7 is a
status signal, which is simply "LOW". By the way this signal is used by
8087 coprocessor (and I assume 8089 too) to distinguish between 8086 based
(16-bit data) and 8088 based (8-bit data) systems. Following the reset
co-processor samples this signal. In case of 8086 it will be LOW, as the
processor will try to fetch 16-bit word (first instruction from 0xFFFF0),
while 8088 running in maximum mode will always output HIGH.
I don't quite understand the second question. It doesn't really matter
which latch (of eight latches available in 8282) you use to latch what
signal... I'd just arrange them in order that will make PCB routing simpler.
Also if you want your system to be a bit cheaper, you can use 74*573 or
74*373 (a bit harder to route) latches instead of 8282. 8282 is no longer
produced, while 7400-series chips are still being made, and therefore
readily available.
Thanks,
Sergey
Hi All,
Here is a question that has stumped me. I am attempting to design an
old-school 8086 computer using various manuals and data sheets. I have run
into one issue that read though I may, have not doped out. That is the BHE
signal from the 8086 and co-processors. According to Osborn, the BHE lines
should be connected between the 8089 and 8086. should that same line be
connected to the BHE line on the 8087, with all three processors in
parallel? Next, I know that the BHE line needs to be latched along with the
Address lines (A0-A19). I also know that the last of the three 8282 latches
that are used for the top 4 address lines is used for this purpose. What
pin should I use to latch this BHE signal? Pin one, with output on pin 19?
I know that I am no computer designer, though I really would like to be.
Any help on this would be quite appreciated. By the way, I am calling this
the "Definitely Old School Intel Machine" (DOSIM for short).
Best,
Joe
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