Great series. My first computer was the Altair. I got it cheap from a disgruntled owner that bought it in kit form and tried to put it together with a soldering GUN. Yep, a 1500 watt iron. What a mess, but I got it working after a month of work. Always wanted an IMSAI for the two main reasons. First, because it has input switches. And output LEDs on the front panel. Altar did not. Secondly, the power supply was more beefer than the Altair. My first program was the C3, 00, 00. But I also loved Lionel trains. So, I program my computer to run a simple railroad signal. Blue the chips off the I/O board. Lionel AC and Altair DC don’t mix. But I did get it to work and then sold the Altair. Bought the IMSAI 8080 And never looked back.
I remember the 2114 static RAM chips fondly, using them in my first Z80 computers I made while still in school in the early 80's. They are 4096 bits arranged as 1024 x 4, so you only needed 2 chips to get a working computer. There was also a 6810 static RAM chip I used on an early design which had a whopping 128 bytes in a 24 pin DIP.
Hi! I'm impressed... Two questions: No. 1: do you happen to know what the DIP switches on the right side of the card are for? No. 2: there seem to be four voltage regulators on this card - do you know why?
the switches are for bank select. since the 8080 can only address 64K bytes, you could have several 'banks' or pages of memory and turn them on and off. the method of selections was to write (output instruction) to a bank port. the address of the bank port (0 - 255) is one set of switches. then you can program the card select for a particular bank (0 - 255) as the byte value you sent (the other set of switches) so you might have all your card bank ports set to 55h and then each card might be 01h 02h 03h.... the reason for 4 regulators is that the RAM needed lots of power. each bank of 4K RAM needed 1 amp at 5V. so 16K needed 4 amps! each 7805 would only supply 1A. The heat sinks are pretty small also. Remember this is before CMOS was invented. we have it easy now.
Too right we have it easy! All those RAM chips and more can be substituted by just one chip. It is staggering how far we have come in less than 40 years.
I've been having a problem with my IMSAI. It executes the jump instructions properly, but when I key in the "read the switches" program, the switches have no effect on the output LED's. The program doesn't work when single-stepped or run normally.
Great series.
My first computer was the Altair. I got it cheap from a disgruntled owner that bought it in kit form and tried to put it together with a soldering GUN. Yep, a 1500 watt iron.
What a mess, but I got it working after a month of work.
Always wanted an IMSAI for the two main reasons. First, because it has input switches. And output LEDs on the front panel. Altar did not. Secondly, the power supply was more beefer than the Altair.
My first program was the C3, 00, 00.
But I also loved Lionel trains. So, I program my computer to run a simple railroad signal.
Blue the chips off the I/O board. Lionel AC and Altair DC don’t mix.
But I did get it to work and then sold the Altair.
Bought the IMSAI 8080
And never looked back.
I remember the 2114 static RAM chips fondly, using them in my first Z80 computers I made while still in school in the early 80's. They are 4096 bits arranged as 1024 x 4, so you only needed 2 chips to get a working computer. There was also a 6810 static RAM chip I used on an early design which had a whopping 128 bytes in a 24 pin DIP.
Hi!
I'm impressed...
Two questions:
No. 1: do you happen to know what the DIP switches on the right side of the card are for?
No. 2: there seem to be four voltage regulators on this card - do you know why?
the switches are for bank select. since the 8080 can only address 64K bytes, you could have several 'banks' or pages of memory and turn them on and off. the method of selections was to write (output instruction) to a bank port. the address of the bank port (0 - 255) is one set of switches. then you can program the card select for a particular bank (0 - 255) as the byte value you sent (the other set of switches) so you might have all your card bank ports set to 55h and then each card might be 01h 02h 03h....
the reason for 4 regulators is that the RAM needed lots of power. each bank of 4K RAM needed 1 amp at 5V. so 16K needed 4 amps! each 7805 would only supply 1A. The heat sinks are pretty small also.
Remember this is before CMOS was invented. we have it easy now.
Too right we have it easy! All those RAM chips and more can be substituted by just one chip. It is staggering how far we have come in less than 40 years.
I've been having a problem with my IMSAI. It executes the jump instructions properly, but when I key in the "read the switches" program, the switches have no effect on the output LED's. The program doesn't work when single-stepped or run normally.
You'll have to look at the schematic for the front panel and look to see if the correct signals are enabling the switches as an input port
I assume the LEDs are working as an output port.