Modern RAM Replacements and Adapter PCB Development

Download schematics and Gerber files for adapters here

6116 DIL

6116 DIP

2114 DIP

6264 DIL

Introduction

Ferroelectric RAM (FRAM) is a type of non-volatile memory that combines the speed of SRAM with the data retention capabilities of Flash memory. Unlike traditional SRAM, which requires a constant power source to maintain stored data, FRAM retains information even when power is removed. It achieves this by using a ferroelectric layer in the capacitor structure, which allows data to be stored through polarization rather than trapped charge. This mechanism enables FRAM to perform fast read and write operations while consuming low power, making it an attractive alternative to aging memory technologies.

Many vintage synthesizers, computers, and other electronic devices from the 1980s and 1990s rely on volatile memory, typically 6116 or similar SRAM, to store user settings and patches. These memory chips are backed up by batteries to retain data when the device is powered off. However, over time, these batteries degrade, often leaking and causing damage to circuit boards. Additionally, the aging CMOS RAM chips themselves can become unreliable, leading to data corruption and erratic behavior.

As a solution, modern non-volatile memory technologies, such as FRAM, offer direct replacements for aging SRAM chips. FRAM eliminates the need for a backup battery while providing high-speed, low-power operation with virtually unlimited retention cycles. This paper explores modern replacements for 6116 SRAM, the development of an adapter PCB for FRAM integration, and key advantages and disadvantages of FRAM in vintage hardware applications.

Advantages of FRAM Over Traditional SRAM

Conventional SRAM requires continuous power to retain data, making it dependent on battery backup systems. FRAM, on the other hand, is a non-volatile memory type that retains stored information even when power is removed. This unique property offers several advantages over traditional SRAM, including:

Battery-Free Operation: FRAM eliminates the need for battery backup, preventing potential leakage that can damage circuit boards and reducing long-term maintenance requirements.
High Endurance: Unlike EEPROM and Flash memory, which have limited write cycles, FRAM can endure trillions of read/write cycles, making it ideal for frequent data access applications.
JEDEC-Compatible Pinout: Many FRAM chips are designed to be compatible with standard SRAM pin configurations, allowing for drop-in replacements in some cases.
Instant Data Retention: Unlike Flash memory, which has slow write cycles and requires wear-leveling algorithms, FRAM supports instantaneous writes with no degradation over time.
Low Power Consumption: FRAM generally consumes less power than SRAM in active mode, making it suitable for battery-powered and low-energy applications.

Disadvantages of Using FRAM

Despite its many advantages, FRAM is not without its drawbacks. When considering FRAM as a replacement for 6116 SRAM, the following limitations should be taken into account:

Limited Availability and Cost: Compared to traditional SRAM, FRAM chips are less commonly available and tend to be more expensive, which may be a concern for large-scale repairs or restorations.
Compatibility Issues: While some FRAM chips have JEDEC-compatible pinouts, others require additional modifications or adapter PCBs to function correctly in older hardware.
Higher Standby Current Draw: In some cases, FRAM chips consume slightly more power in standby mode compared to low-power SRAM variants, which could impact power-sensitive applications.
Write Timing Sensitivities: Certain vintage devices may expect specific SRAM timing behavior, and while FRAM is generally fast, there could be edge cases where minor timing differences lead to unexpected issues.
Endurance Limitations: While FRAM has significantly higher write endurance than EEPROM or Flash memory, it is not truly unlimited. Most commercial FRAM chips support around 10^12 (one trillion) read/write cycles per bit. For most practical applications, this is more than sufficient, but in extremely high-frequency write operations, it may eventually wear out.

Developing an Adapter PCB for FRAM Integration

To install a modern SOIC FRAM in place of a 6116 DIP SRAM, an adapter PCB is required to bridge the differences in pin configurations. A well-designed adapter ensures proper electrical connections and seamless functionality.

Pin Mapping Considerations

The 6116 SRAM follows a 24-pin DIP package with a standardized JEDEC pinout, whereas modern FRAM modules, such as the FM16W08, typically come in a 28-pin SOIC package. The adapter must handle the following key modifications:

Address Bus Compatibility: The 6116 is a 2K x 8-bit SRAM, while FRAM chips are often larger. The adapter must properly route address lines to ensure correct memory access.
Chip Enable Handling: Traditional SRAM may have two separate chip enable lines, whereas FRAM may use a single active-low enable signal.
Power and Ground Pin Alignment: The adapter must ensure that Vcc and GND connections on the FRAM align with the original 6116 socket.

Circuit Board Design

A properly designed adapter PCB facilitates easy installation of an SOIC FRAM chip into a DIP-24 socket. Important design features include:

SOIC-28 to DIP-24 Conversion: The adapter board should allow the surface-mount FRAM chip to be soldered while interfacing with the original DIP socket.
Address Line Routing: If an 8K FRAM is used, address lines A11 and A12 must be handled to ensure only the lower 2K of the FRAM is accessed.
Chip Enable Logic Handling: If the target device uses both CE1 (active-low) and CE2 (active-high), a simple logic circuit may be required to generate correct chip enable signals.
Silkscreen Labeling: Clear labeling on the PCB assists in correct installation and orientation of the components.

Conclusion

Replacing aging 6116 SRAM with FRAM provides a reliable and maintenance-free solution for vintage synthesizers and other legacy electronics. By eliminating the need for battery backup, FRAM enhances long-term durability and reduces the risk of data loss. However, proper integration is necessary due to differences in pin configurations and potential compatibility issues. The development of an adapter PCB allows seamless installation of modern FRAM chips, ensuring continued functionality and preservation of classic hardware. With careful consideration of the advantages and disadvantages of FRAM, technicians and hobbyists can make informed decisions when upgrading legacy systems.