Computing > Vintage Computing

Testing a M5L 2101AP Static RAM Chip


I'm trying to test a RAM chip I took out of a board which I will eventually get around to repairing...

The device is a M5L 2101AP, probably better know as 2101A which is a 256x4 Static RAM with separate input and output buses for the data.

I have attached +5V to p22, GND to p8, Driven p18 and p19 LO, p17 and p20 HI... driven a sensible address to A0..A7 and driven DI1..DI4 to sensible values...

The output is going into a uP with high impedance inputs (it's actually a PIC dsPIC33EV256GM006-I_PT) but via a 220 ohm resistor (all my digital pins have a series resistor)

What I am not seeing is nice logic levels on the Data Output Pins... instead I see values like...

0.2V, 1.8V, 0.5V, 2.0V... 

it's like it is trying to represent 0s and 1s but just at a much lower level than I expected.  I was hoping for an unambiguous output but 1.8V seems too low for a TTL '1'

Could it be that the device struggles to cope with my high impedance inputs?

I tried adding weak pull-ups or pull-downs at the PIC side but that just pulls up to 5V or down to GND respectively and I see no output.

Is there something really simple I am missing here?

Thanks in advance

I have two devices... both seem to do the same

Pin 17 should be tied high also.


--- Quote from: edavid on April 26, 2021, 09:13:16 pm ---Pin 17 should be tied high also.

--- End quote ---
Oops.. yes it is... corrected above.  Thanks

Additionally I am able to use the uP ADC to measure all the voltages of the chip under test... and I see

--- Code: ---P01 0v0, P02 4v9, P03 0v0, P04 5v0, P05 0v0, P06 5v0, P07 0v0, P08 0v0, P09 0v0, P10 0v2, P11 5v0,
P12 1v7, P13 5v0, P14 3v7, P15 5v0, P16 3v6, P17 5v0, P18 0v0, P19 0v0, P20 5v0, P21 5v0, P22 5v0

--- End code ---
You will notice anaemic voltages at the Data Out pins...
P10 0v2, P12 1v7, P14 3v7, P16 3v6

1.7V is definitely ambiguous in logic terms


--- Quote from: NivagSwerdna on April 26, 2021, 09:22:58 pm ---1.7V is definitely ambiguous in logic terms

--- End quote ---

Yes, but the other voltages are fine for NMOS.  Looks like you have one bad output.

Do you think if I made it drive a load it would help?  What sort of load?


If I drive the Inputs with a value between 0 and F and observe the outputs I get good values on DO4... totally bogus values on DO3... not much output on DO2 and small voltage swings on DO1...

... there's definitely something going on


[0] Message Index

There was an error while thanking
Go to full version