PDA

View Full Version : M1 status light



8008guy
May 31st, 2017, 05:17 PM
Question for owners of older Altair's. I have an old Altair with a Rev 0 CPU card and a REV 1 front panel. When my computer is in a reset and stopped stat I only get the MEMR and WO status indicator lighted. I never see the M1 led lit. In all of the videos I see those three are lit on others computers.

I've tested the LED all the way back from the backplane and I can get it to light just fine when 5v is applied to the line. I also connected a logic probe to the SM1 signal on the buss and can see that it does get pulsed from the CPU status latch/buffer. I've "keyed" in several programs and have verified that the CPU is functional.

Is that expected behavior on an older Altair? On newer systems did they put a pulse stretcher to keep the LED on?

Cheers,

len

Marty
June 1st, 2017, 08:30 AM
Hi All;

8008Guy, I looked at my machine, and it does what Yours does NOT do..

I looked at the Schematic, and I would look at IC K, the 8212 and IC H, 8T97.. Pins 3 and 4 on the 8212, and pins 9 and 10 on the 8T97..

Take out the 8080, and then use Your +5 to see if the signal will go all the way through..

THANK YOU Marty

8008guy
June 1st, 2017, 09:53 AM
Hi All;

8008Guy, I looked at my machine, and it does what Yours does NOT do..

I looked at the Schematic, and I would look at IC K, the 8212 and IC H, 8T97.. Pins 3 and 4 on the 8212, and pins 9 and 10 on the 8T97..

Take out the 8080, and then use Your +5 to see if the signal will go all the way through..

THANK YOU Marty

Thanks for the hints, will look at it this evening.

Cheers

Len

Marty
June 2nd, 2017, 07:15 AM
Hi All;

I had a thought last night, in that both of the IC's might not be Enabled with the 8080 pulled..

So You might have to manually Enable then with Pull-ups and pull-downs, for the signal to pass through..

Also You could switch out the 8T97 IC H with another one to just check it out, leaving in the 8080.. And if You have another Board with an 8212 or a spare one from somewhere, that could be switched out as well..

Otherwise, You might want to buy some of each from a supplier, to have some spares on hand, along with as many as You can afford of all of the other types that are in the various boards that You have..

THANK YOU Marty

8008guy
June 2nd, 2017, 08:11 AM
Hi Marty,

I have ordered a few extra 8212's and 8T97's. Last night I did some basic continuity tests, those went fine. The board has had some chip changes befor I got it. Over the wkkend I will do and end to end signal test.

Thanks

Len

Mike_Z
June 3rd, 2017, 06:07 AM
If the machine is in a wait state, the M1 light will be off. A wait state is between M2 and M3. Mike