3458A DMM: Converting the Dallas Non-Volatile RAM to FRAM

• Introduction
• FRAM Compatibility
• PCB Gerber Files
• The Dallas NVSRAM Technology
• FRAM Technology
• Retrieving Data From the Existing Dallas RAMs
• CalRAM Upgrade (DS1220Y)
• Settings RAM Upgrade (DS1230Y)
• Final Thoughts

1. Introduction

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 3458As use battery-backed non-volatile SRAM to hold both the calibration data and the user configuration settings. These are Dallas Semiconductor parts: the DS1220Y for the calibration RAM (the CalRAM) and the DS1230Y for the settings RAM.

Once these devices are 10 years old or more, they become susceptible to failure and data loss.

Fortunately, one of the best preventative upgrades you can make to a 3458A is to replace the original Dallas NVSRAMs with modern FRAM (Ferroelectric RAM).

FRAM has major advantages over the original battery-backed SRAM. It needs no battery, has extremely high write endurance, and retains data for decades, typically around 150 years. Once fitted, a FRAM upgrade should easily outlast not only the original memory devices but quite possibly the instrument itself.

My 3458A FRAM upgrade on video: YouTube Short

2. FRAM Compatibility

The FM16W08/FM18W08 FRAM devices aren't electrically identical to conventional SRAM, but the 3458A's memory interface circuitry looks highly compatible with FRAM operation (see the FRAM datasheets).

In some simpler SRAM systems the chip enable (CE) line is permanently asserted. The 3458A instead actively decodes and controls the RAM chip-enable lines through dedicated address decode logic. That matters, because FRAM needs proper CE strobing on each memory access to latch the address correctly.

The 3458A memory subsystem also includes address latching, pull-up resistor networks and dedicated write-protection circuitry around the CalRAM. These give controlled memory access and help protect against corruption during power transitions.

In long-term practical use, FRAM replacements have proven fully reliable in the 3458A. Calibration storage, settings retention, self-calibration and normal operation all work correctly.

Supply voltage compatibility (the 3458A runs at 5 V):

3. PCB Gerber Files

This guide refers to adaptor boards I've designed. The Gerbers are free to download so you can make your own:

As well as the boards, you'll need some IC sockets and double male, round-pin SIL headers, fitted with the short pins facing downwards.

4. The Dallas NVSRAM Technology

The Dallas devices are known as NVSRAM (Non-Volatile SRAM).

Because the battery is sealed inside the epoxy package, it can't be replaced. That's why these devices are considered a major long-term reliability issue in vintage test equipment.

5. FRAM Technology

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

• no battery is required
• data remains without power
• writes are extremely fast
• endurance is enormous, typically 10 trillion to 1 quadrillion write cycles, vastly more than Flash memory

6. Retrieving Data From the Existing Dallas RAMs

Before fitting FRAM, you need copies of your existing data so you can program it onto the new chips.

Option 1: Desolder and read in a programmer

Carefully desolder the Dallas RAMs from the 3458A PCB and read each one into a file on your PC, ready to write onto the new FRAM later.

DS1220Y data being read out in the programmer:

Option 2: Download over GPIB with WinGPIB

Use WinGPIB to download your CalRAM and both settings RAMs to files on your PC, ready to write onto the new FRAM later.

WinGPIB and the 3458A CalRAM/settings RAM download utility:

My own 3458A

Here's my 3458A with the original Dallas RAMs fitted. It's a 1995 unit, and the Dallas RAMs are date-coded 1994, so they're some 32 years old! One of the first things I did on acquiring it was the FRAM upgrade.

7. CalRAM Upgrade: DS1220Y to FRAM

Unfortunately, there's no direct FRAM replacement for the DS1220Y, so an adaptor board is needed.

Option 1: Ramtron FM1608 (DIP)

With an adaptor board, the 28-pin FM1608 can be made to mimic the 24-pin DS1220Y. Program the FM1608 with your CalRAM file, plug it into the adaptor board, then fit the board into your 3458A.

FM1608 adaptor schematic (note A11 & A12 tied to 0 V):

The FM1608 adaptor PCB:

The FM1608, its adaptor board and IC socket. All that's left to add are the double male, round-pin SIL headers:

Option 2: Cypress FM16W08 (SMD)

The FM16W08 can also be programmed with your CalRAM file, fitted to the adaptor board and plugged into your 3458A. However, once it's soldered to the adaptor board, your programmer can't program it any more unless you have my programming adaptor board. Without that board, you'd have to desolder the FM16W08 to reprogram it.

My programming adaptor board maps the pins back to an FM16W08-compatible pinout, still with the top two address lines tied to 0 V. With it, you plug the assembly into your programmer, choose FM16W08 as the device and write to the first 16k (2k × 8) of memory. You must set your programmer's end address to 07FF.

FM16W08 quick guide

1. Plug your FM16W08 on its board into my programming adaptor (your 24-pin FM16W08 board goes into the 24-pin socket on my adaptor).
2. Plug the programming adaptor into your programmer, using the 28-pin header.
3. Set the device to FM16W08, not Dallas DS1220Y!
4. Set the end address to 07FF. This is vital, because the top two address lines on the FM16W08 board are tied low.
5. Hit Program. Done.

The top two address lines are tied low because the FM16W08 is 64 kbit (8K × 8), while the DS1220Y is only 16 kbit (2K × 8).

FM16W08 adaptor schematic (note A11 & A12 tied to 0 V):

The FM16W08 adaptor PCB:

The FM16W08 SMD adaptor board ready for the 3458A. Double male, round-pin SIL headers are the best type to use:

The programming adaptor PCB (pins 1 & 26 are NC, per the datasheet):

The programming adaptor board, for loading your CalRAM data onto the FM16W08 after it's fitted to its adaptor board:

The FM16W08 adaptor board on the programming adaptor, in my Dataman-40Pro programmer:

Successful programming of the FM16W08. Note the end pointer of 07FF and the device type, Cypress FM16W08:

8. Settings RAM Upgrade: DS1230Y to FRAM

Fortunately, there's a FRAM chip that's pin-compatible with the original DS1230Y.

The FM18W08 SMD version on a SOIC28 to DIP28 adaptor (available from Ebay/Amazon), awaiting its double male, round-pin SIL headers:

Alternatively (Oct. 2026), I have my own FM18W08 28-pin adaptor board:

DIP versions fitted to my current 3458A:

SMD versions of the settings RAMs in my previous 3458A:

A collection of DIP and SMD versions:

A collection of DIP and SMD versions:

SMD versions of the settings RAMs in my previous 3458A:

9. Final Thoughts

With the FRAM chips installed, your 3458A remains fully calibratable, because the replacements are completely read/write compatible with the original memory devices.

Don't wait for the original Dallas devices to fail. Upgrading the non-volatile RAM in your 3458A is highly recommended and should be considered essential preventative maintenance for any serious owner of these instruments.

The maths is stark. If your 3458A was made in the late 1980s or early 1990s, its Dallas devices are now well over 30 years old, three times their minimum rated battery life. Every day you use the instrument with the original NVSRAM fitted is a gamble, and losing your calibration data is far from trivial. A full factory recalibration needs specialist equipment and expertise, and can cost significantly more than the instrument is worth on the second-hand market. By contrast, the FRAM chips are inexpensive, and the upgrade, while it needs care, is well within reach of any competent hobbyist or technician.

The upgrade isn't merely a like-for-like swap, either. It's a genuine improvement in every measurable respect. FRAM needs no battery at all, which removes the original single point of failure entirely. Its data retention is rated at around 150 years under normal conditions, its write endurance vastly exceeds anything the original devices could offer, and there's no degradation mechanism comparable to a slowly flattening lithium cell. The FRAM will simply keep working, indefinitely, without intervention.

The 3458A isn't the only vintage instrument affected. The same Dallas NVSRAM devices, or close relatives, appear in a wide range of HP, Agilent, Tektronix and other test equipment from the same era. If you own other instruments from this period, it's worth checking whether they also carry ageing battery-backed memory. The FRAM approach described here, and the principles behind it, apply broadly across this class of equipment.

The 3458A is one of the finest metrology-grade multimeters ever made. With a little care and a modest investment, there's no reason it shouldn't keep performing at the highest level for decades to come, and the FRAM upgrade is one of the most worthwhile steps you can take toward that.

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