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IBM 5150 Ram Error: 406E 201

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Compgeke

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Sep 30, 2011
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Fairfield, CA, USA
Having fun tonight with an IBM 5150 with 64-256 KB board and apparently fault memory. The error is 406E 201, a 64-256 board and no ram expansion cards. The switches for the memory are on-off-off-on-on and the motherboard has 4 banks of 64 KB.

Does anyone know which ram chip(s) are at fault for this error?
 
Compgeke said:
Having fun tonight with an IBM 5150 with 64-256 KB board and apparently fault memory. The error is 406E 201, a 64-256 board and no ram expansion cards. The switches for the memory are on-off-off-on-on and the motherboard has 4 banks of 64 KB.
Click to expand...
The "406E 201" RAM error produced by your 64KB-256KB 5150 motherboard equates to:

Address: 256K
Bits in error: 1,2,3,5,6

That can be determined by the information at:
1. Goto www.minuszerodegrees.net
2. Choose 'IBM 5150 - RAM Information'
3. Choose '64KB-256KB Motherboard'

So, for whatever reason, your 64KB-256KB 5150 motherboard (with no RAM cards fitted) believes that more than 256K is present and has attempted to test RAM that doesn't exist.

For your 64KB-256KB 5150 motherboard (with no RAM cards fitted), the motherboard switches should be:
SW1: Switches 3 and 4 both off.
SW2: 1=on, 2=off, 3=off, 4=on, 5=on

Compgeke said:
The switches for the memory are on-off-off-on-on
Click to expand...
That is right for SW2, but the symptoms suggest a problem with SW2, either incorrect settings, faulty switch within SW2, or a switch not quite positioned 100%.
Try switching each of the SW2 switches to the alternate position and then back.
Just in case, also take a look at http://www.minuszerodegrees.net/5150_5160/misc/5150_5160_on_versus_off.htm
 
I flipped all switches to the wrong position and then back again, and that seemed to be the trick. Being as there is no bootdisk it loads Basic automatically and it sees around 63 KB ram, so not sure if that's BASIC limitation or system problem.
 
Compgeke said:
Being as there is no bootdisk it loads Basic automatically and it sees around 63 KB ram, so not sure if that's BASIC limitation or system problem.
Click to expand...
The 5150's cassette BASIC uses 64K of RAM.

If you want to see how much 'conventional' RAM that the POST found at start-up, then type in the following two lines in BASIC:
def seg = &h40
print peek(20)*256 + peek(19)
 
I've just had a 5150 mainboard give me this exact error "406E 201", even when SW2 was set for 64k. The PRINT PEEK above showed that the POST thought there was 320k rather than 64k, and a few experiments showed that switch 4 of SW2 is registering as 'off' in both positions.

With a RAM expansion card present and the switches set for 640k, the error was "A055 201", because with SW2-4 registering as always 'off' that gave the switch combination for 896k of RAM. Now to desolder the switch and find a replacement, I suppose...
 
Some contact cleaner sprayed into the dip switch might work. Grabbing a can of Deoxit or similar cleaner is easier than soldering. Mine was probably just a case if dirty contacts rather than physical damage.
 
modem7 said:
But perhaps the cause is the associated switch circuitry rather than the swtch itself? Did one of your "a few experiments" prove that it was the switch?
Click to expand...

I put a multimeter across the switch pins. Switch 4 has infinite resistance when 'on'; other switches have zero.
 
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