IBM 5150 All 256KB Filled - No activity other than power supply fan on

[KurtBuscus]

Well, I have an IBM 5150 revision 3 with 128 out of 256k occupied, I have the ram for the other banks it's just that when I insert it and configure the switches to be what I think is correct all that works is the power supply. I can run the computer with just the current banks filled and it will read as 28636 bytes, so roughly 3.5kb, which is obviously an issue hence I want to fully occupy it to 256kb. Not sure how much of an effect this could have but it is also using a Maxconn W86855AF Hercules clone card as the graphics card. If I fill ram bank 2 it stays in a parity check 1 loop. Bank 0 is the default ram which is NEC 833IM6225, bank 1 is M3764-20RS, banks 2 and 3 when I fill them are MN4164P-15A.

 

[Gary C]

might be worth looking here http://minuszerodegrees.net/5150/ram/5150_ram_64_256.htm

If you populate the board with the RAM, but dont change the settings of SW1 & SW2, does it still boot ? if not then something is probably taking the PSU down, maybe a faulty chip. Worth checking if the PSU is actually still working in that config. Ie the fan may still be running, but is there volts to the board ?

 

[modem7]

Well,

Welcome to these forums.

I have an IBM 5150 revision 3 ...

Just to be sure (i.e. no misunderstandings), confirm for us that the motherboard is a '64KB-256KB' one, fitted with the 10/27/82 revision of BIOS.
You can establish that from the information at minuszerodegrees.net

... with 128 out of 256k occupied, I have the ram for the other banks it's just that when I insert it and configure the switches to be what I think is correct all that works is the power supply.

Either:
* Power supply overloaded, and has shut down, but because the fan in the particular power supply is AC powered, the fan still spins.
* Power supply operating okay, producing all output voltage and signals, but something is going wrong related to the motherboard.

Are you able to measure the +5V line on the motherboard? If so, that will inform you as to which of the above two is the scenario that you are in. If no +5V, then the power supply is being overloaded, which would suggest a bad RAM chip (bad in a way that overloads the power supply).

I can run the computer with just the current banks filled and it will read as 28636 bytes, so roughly 3.5kb, ...

28636 bytes = roughly 28 KB.

Right there, "28636 bytes" is not making sense. Two banks of 64 KB totals 128 KB.

I know where you are you getting "28636 bytes" from? It is from Cassette BASIC. The BIOS on the motherboard believes that 32 KB is fitted, and when Cassette BASIC starts, Cassette BASIC takes some of that RAM, then Cassette BASIC displays "28636 Bytes free".

There are bugs in the 10/27/82 revision of motherboard BIOS that result in the BIOS incorrectly determining that amount of RAM if four banks of RAM are not fitted and the motherboard switches set accordingly. And sometimes we see people not set the SW1 and SW2 switch banks properly.

I have just now brought out a 64KB-256KB type of 5150 motherboard. RAM banks 0 and 1 populated. Switches 3 and 4 on SW1 set to OFF/ON. Switches 1 through 5 on SW2 set to ON/OFF/ON/ON/ON. At power up, the POST displays "2055 201", then Cassette BASIC starts and among other things, Cassette BASIC displays "28636 Bytes free". That unexpected behaviour is due to the aforementioned bugs.

If I fill ram bank 2 it stays in a parity check 1 loop.

For me: RAM banks 0 and 1 and 2 populated. Switches 3 and 4 on SW1 set to ON/OFF. Switches 1 through 5 on SW2 set to ON/ON/OFF/ON/ON. At power up, the POST displays "3055 201", then Cassette BASIC starts and among other things, Cassette BASIC displays "45020 Bytes free". That unexpected behaviour will be due to the aforementioned bugs.

Your "parity check 1 loop" may be due to incorrect switch settings, but again, one or two or three bank population with the 10/27/82 revision BIOS is an invalid configuration (due to the bugs).

Bank 0 is the default ram which is NEC 833IM6225, bank 1 is M3764-20RS, banks 2 and 3 when I fill them are MN4164P-15A.

MN4164P-15A (good ones) are not expected to be a problem. Plugging them in the correct orientation? Is one or more getting hot to touch?

The aforementioned +5V measurement would be a good thing to do.

 

[modem7]

BTW.

Later, when you get the motherboard going with four banks of RAM, i.e. 256 KB, Cassette BASIC will show "62940 Bytes free". That is because the most RAM that Cassette BASIC accesses is a segment, 64 KB.

If you want some sort of verification that the motherboard BIOS is registering 256 KB, enter the following two lines into Cassette BASIC, expecting to see "256" appear as a result.
def seg = &h40
print peek(&h14)*256 + peek(&h13)

 

[MauriceH]

To test your new RAMs you can also do the following.

Set up with original working RAM set in the working condition.
Then just remove the working BANK 0 with a new set of RAMs.
Test again if still power-up with no failure.
If so test the next set of RAMs.
That way you know the new RAMs are OK.

If you encounter a bad set, just change back half the bank and test again.
And so on, So quickly find your bad RAM IC.

 

[cwathen]

To test your new RAMs you can also do the following.
Then just remove the working BANK 0 with a new set of RAMs.
Test again if still power-up with no failure.
If so test the next set of RAMs.
That way you know the new RAMs are OK.

That's not a simple job...Bank 0 on a 5150 is soldered in

 

[MauriceH]

ah yes checked with mine bank 0 indeed soldered,MB 64k-256k, but bank 1-2-3 aren't,
So take bank 1.
Just see the idea.