James Cross
2015-07-28 23:19:09 UTC
Hey folks,
I have been trying to get an 8088 working on a breadboard as a means to
learn how it works before attempting to build a Xi 8088. I've been
following along with Grossblatt's *8088 Project Book*. I've attached a
schematic that represents where I am at now. I have a program loaded into
ROM (IC 6) that should blink a single LED in an endless loop. This is the
same as the program in Grossblatt's book and I've verified that the hex
code from my assembler matches what is in the book. Unfortunately, the
behaviour I'm seeing is LEDs light up randomly. The pattern is usually
different each time I turn it on. Sometimes the LEDs are solid and bright,
other times dimmed from subtle flickering.
There are a few differences between the schematic and my build. Mostly it's
substitutions that will let me reuse the parts when I build the Xi 8088.
ICs 3, 4, and 10 have all been replaced with 74ACT573s for simpler wiring
(like the Xi 8088). IC5 is a 74ACT245 rather than an LS part. IC8 is a
74ALS04 rather than a 74LS04. IC9 is a 74ALS00 rather than a 74LS00. I have
also tried taking out the 74ALS00 and using a 74ALS138 with the 74ALS04 to
control all the memory and I/O in a cleaner fashion with no change in
behaviour.
I've checked over my wiring several times. Though that may not mean much in
this rat's nest of wires. I do have a two channel oscilloscope for
debugging and have checked the things that seemed obvious to me but I am
pretty new at this. I did some kit stuff as a kid and have just gotten back
into things by reading Scherz and Monk's *Practical Electronics* and then
starting this project. My only theories left are that maybe my part
substitutions were a bad idea or I'm running into capacitance issues
because this is a breadboard. I think I've bitten off way more than I can
true with this project. Any theories, suggestions, or advice would be
greatly appreciated.
Regards,
James
I have been trying to get an 8088 working on a breadboard as a means to
learn how it works before attempting to build a Xi 8088. I've been
following along with Grossblatt's *8088 Project Book*. I've attached a
schematic that represents where I am at now. I have a program loaded into
ROM (IC 6) that should blink a single LED in an endless loop. This is the
same as the program in Grossblatt's book and I've verified that the hex
code from my assembler matches what is in the book. Unfortunately, the
behaviour I'm seeing is LEDs light up randomly. The pattern is usually
different each time I turn it on. Sometimes the LEDs are solid and bright,
other times dimmed from subtle flickering.
There are a few differences between the schematic and my build. Mostly it's
substitutions that will let me reuse the parts when I build the Xi 8088.
ICs 3, 4, and 10 have all been replaced with 74ACT573s for simpler wiring
(like the Xi 8088). IC5 is a 74ACT245 rather than an LS part. IC8 is a
74ALS04 rather than a 74LS04. IC9 is a 74ALS00 rather than a 74LS00. I have
also tried taking out the 74ALS00 and using a 74ALS138 with the 74ALS04 to
control all the memory and I/O in a cleaner fashion with no change in
behaviour.
I've checked over my wiring several times. Though that may not mean much in
this rat's nest of wires. I do have a two channel oscilloscope for
debugging and have checked the things that seemed obvious to me but I am
pretty new at this. I did some kit stuff as a kid and have just gotten back
into things by reading Scherz and Monk's *Practical Electronics* and then
starting this project. My only theories left are that maybe my part
substitutions were a bad idea or I'm running into capacitance issues
because this is a breadboard. I think I've bitten off way more than I can
true with this project. Any theories, suggestions, or advice would be
greatly appreciated.
Regards,
James
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