3458A DMM conversion of Dallas Non-Volatile Ram to FRAM

Owners of the HP/Agilent/Keysight 3458A 8.5-digit multimeter should be aware that these instruments contain an expensive ticking time bomb.

Many older versions of the 3458A use battery-backed non-volatile SRAM devices to retain both the instrument calibration data and the user configuration settings. These are Dallas Semiconductor memory devices: the DS1220Y for the calibration RAM (“CalRAM”) and the DS1230Y for the settings RAM.

Once these devices reach 10 years of age or more, they can become susceptible to failure and data loss. The most serious issue is the loss of the 3458A calibration data. If this happens, replacing the memory device alone is not enough — the instrument will require a full recalibration, which can easily cost thousands.

Fortunately, one of the best preventative upgrades you can make to a 3458A is to replace the original Dallas non-volatile SRAM devices with modern FRAM (Ferroelectric RAM) replacements.

FRAM devices offer major advantages over the original battery-backed SRAMs. They require no internal battery, have extremely high endurance, and offer data retention measured in decades — typically around 150 years. Once fitted, a FRAM upgrade should easily outlast not only the original memory devices, but quite possibly the remaining lifetime of the instrument itself.

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The Dallas Non-Volatile Ram Technology

The Dallas devices are known as NVSRAM (Non-Volatile SRAM).
Internally, they are basically:

• A standard low-power CMOS SRAM chip
• A small sealed lithium battery
• Power-monitoring and protection circuitry

all combined into a single IC package.

How it works

Under normal operation, the instrument powers the SRAM like ordinary memory.
When the instrument is switched off, or if the supply voltage drops too low:

• the internal circuitry disconnects the external power
• the internal lithium cell takes over automatically
• the SRAM contents are preserved

The chip also enables write-protection during brownout conditions to prevent corrupted data.
The weakness is the internal lithium battery.
Although Dallas specified around 10 years minimum retention, many instruments are now 20–35 years old.
Once the internal battery voltage drops:

• SRAM contents become unstable
• calibration constants may corrupt
• data can be completely lost

And because the battery is encapsulated inside the epoxy package, it is not intended to be replaced.
This is why these devices are considered a major long-term reliability issue in vintage test equipment.

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FRAM - Ferroelectric Ram Technology

FRAM (also called FeRAM or F-RAM) is a completely different technology.
Instead of storing data as electrical charge like SRAM or DRAM, FRAM stores data using a ferroelectric material whose molecular polarization represents binary data.
The important part is:

• no battery is required
• data remains without power
• writes are extremely fast
• endurance is enormous

FRAM devices can typically withstand between 1010 and 1015 write cycles, vastly higher than Flash memory.

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Getting your data from the existing Dallas Non-Volatile Ram

Option 1 - Desolder the Dallas Ram from the 3458A Pcb and read the data onto a file on your PC which you can use later to dump onto the new FRAM IC.
The Calram (calibration ram) data will be a single file (U132).
The Settings data will be two files, one for each existing settings Dallas ram IC's (U121 & U122)

Option 2 - Use WinGPIB to download your data (Calram & Settings x2 ram) onto a file on your PC which you can use later to dump onto the new FRAM IC.

The Dallas Non-Volatile Ram

Here's my own 3458A with the original Dallas ram's fitted. My 3458A is a 1995 unit, and the date code on the Dallas ram's are 1994, some 32 years!
On acquiring my 3458A just recently, one of the first things I did was carry out a FRAM upgrade.

 

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The DS1220Y Calibration ram upgrade to FRAM

Unfortunately, there is no straight replacement of the DS1220Y for a FRAM IC, so an adaptor board needs to be used.

Dallas DS1220Y - DIL, 16k (2k x 8), 24pin, non-volatile SRAM
Datasheet here.

Option 1 - Ramtron FM1608 - DIL, 64k (8k x 8), 28pin, Ferroelectric non-volatile RAM
Datasheet here.
Through the use of an adaptor board the 28pin Ramtron FM1608 FRAM IC can be made to mimic the Dallas 1220Y IC.
The FM1608 can be programmed with your Calram file, inserted into the adaptor board and then inserted into your 3458A.

The FM1608, it's adaptor board and IC socket:


Option 2 - Cypress FM16W08 - SMD, 64k (8k x 8), 28pin, Ferroelectric non-volatile RAM
Datasheet here.
Similarly, the FM16W08 can be programmed with your Calram file, inserted into the adaptor board and then inserted into your 3458A. However, once the IC is soldered to the adaptor board you can no longer program the IC in your programmer unless you have the programming adaptor board. If you don't have that board then you'll need to desolder the FM16W08 from your adaptor board in order to re-program it again.

Why is this? - The FM16W08 is a 28pin SMD IC, whilst the Dallas DS1220Y is a 24-pin IC. Both these IC's are different memory sizes, so the FM16W08 ties the top two address line to 0vdc in order to make the overall package 16k (2k x 8). Not only that but the pinout is different, so once it is adapted then your programmer will not recognise it, the mapping id now different.

My programming adaptor board helps with this by adapting the FM16W08 back to a FM16W08 compatible pinout, albeit still with the top two address lines tied to 0vdc.
So, with the programming adaptor board in use, you can insert it into a programmer, choose FM16W08 as the device and write to the first 16k (2k x 8) of the memory. You must set the top end of your programmer to 07FF.

FM16W08 Quick Guide:
- Plug your FM16W08 on it's board into my adaptor board (your 24pin 16W08 board into the 24pin socket on my adaptor)
- Plug my adaptor board into your programmer (using the 28pin header)
- Set the device to FM16W08 (not Dallas 1220Y.....!)
- Set the end address to 07FF (mega important because the top 2 address lines on the FM16W08 board are tied low)
- Hit Program.....done.
Note: The top 2 address lines are tied low because the FM16W08 is 64kbits (8Kx8), whilst the Dallas 1220Y is only 16kbits (2Kx8).

The FM16W08 SMD adaptor board ready for the 3458A:
 

The programming adaptor board:

The FM16W08 adaptor board fitted to the programming adaptor board and inserted into my Dataman-40Pro programmer:

Successful programming of the FM16W08 using the programming adaptor (note 07FF):

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The DS1230Y Settings ram upgrade to FRAM

Fortunately, there is a FRAM IC that is pin compatible with the original Dallas DS1235Y ram IC.

Dallas DS1230Y - DIL, 256k (32k x 8), 28pin, non-volatile SRAM.
Datasheet here.

Option 1 - Ramtron FM1808 - DIL, 256k (32k x 8), 28pin, Ferroelectric non-volatile RAM
Datasheet here.
Straight compatibility of the original Dallas DS1230Y to the FM1808 IC.

Option 2 - Cypress FM18W08 - SMD, 256k (32k x 8), 28pin, Ferroelectric non-volatile RAM
Datasheet here.
Compatibility through the use of a 28pin SOIC SMD to 28pin DIL adaptor (Ebay, Amazon etc).

The FM18W08 SMD version on a SOIC28 to DIP28 adaptor:

DIP versions fitted to my 3458A: