FANUC A20B-2900-0681 Daughter Board A2OB-29OO-O681 A20B29000681

A20B-2900-0681 | FANUC 512K SRAM Module — Battery-Backed Parameter and Programme Storage / Plug-In CNC Memory Module

SRAM in FANUC CNC Systems — Persistent Operational Data

While FROM (Flash ROM) stores the CNC system software and DRAM provides the CPU's working memory, SRAM fills a distinct third role: it stores the data that defines how the CNC behaves on its specific machine and what programmes are loaded in its memory.

CNC parameters: Axis gains, software limits, acceleration/deceleration rates, spindle speed limits, communication settings, and hundreds of other machine-specific values. Without these parameters, the CNC software cannot operate correctly on the machine it is installed on.

Part programmes: The G-code programmes created by the operator or programmer for producing parts. The SRAM holds the programme library — all stored programmes are accessible without external media.

Tool data: Tool length offsets, tool radius offsets, and tool life management data.

Work offsets: Work coordinate zero point settings for each programme fixture setup.

All of this data lives in the SRAM — retained by the lithium battery backup even when the machine is powered down.

Key Specifications

Battery Backup — The Critical Dependency

The SRAM retains its content through lithium battery power. The battery's role is simple but critical: if it fails, all SRAM content is lost the next time machine power is removed.

Preventive battery maintenance is the most important routine for SRAM-equipped CNC systems:

• Check battery voltage annually
• Replace before voltage drops below the specified threshold (typically 3V for a 3.6V lithium cell)
• Always perform a full SRAM backup before replacing the battery

A failed SRAM battery does not cause immediate loss — data is retained while the main power is on. The loss occurs when the machine is powered down with a dead battery installed.

Application Scenarios

SRAM module replacement: A FANUC CNC develops memory errors — RAM parity alarms or corrupted parameter values that cannot be corrected by re-entry. The A20B-2900-0681 SRAM module is identified as faulty. After backing up all data, the replacement module is fitted and all parameters and programmes are restored from backup.

Programme memory expansion: A CNC system's existing SRAM module is at capacity — no space for additional programmes. The A20B-2900-0681 provides additional SRAM to increase the programme library capacity.

FAQ

Q1: Can A20B-2900-0681 replace or supplement the existing SRAM module in the CNC?

SRAM modules in FANUC CNC systems occupy defined slots on the main board or in the CNC card cage. The A20B-2900-0681 installs in the applicable SRAM module slot. Whether it supplements or replaces an existing module depends on the CNC's hardware configuration — some configurations support multiple SRAM modules for expanded capacity. Confirm from the specific CNC's hardware documentation.

Q2: What happens to stored programmes if the A20B-2900-0681 is replaced?

All SRAM content — programmes, parameters, offsets — is lost when the module is removed and the battery backup is disconnected. Perform a complete backup via the CNC's ALL IO function before removing the SRAM module. 

Q3: How do I check if the battery in the A20B-2900-0681 needs replacement?

The CNC displays a battery warning alarm when the battery voltage drops to the warning threshold. This alarm appears before the battery is fully exhausted — there is typically time to plan and perform the battery replacement while the main power is maintained. Never remove the module or power down the CNC after receiving the battery warning until the battery has been replaced.

Q4: Is A20B-2900-0681 compatible with Series 16, 18, and 21 CNC systems?

The A20B-2900 memory module family serves multiple FANUC CNC generations. The adjacent A20B-2900-0680 is confirmed for Series 16/18/21. The A20B-2900-0681 serves the same platform family as its 512K SRAM companion module. Confirm from the specific CNC hardware documentation that this module is specified for the installed CNC model before ordering.

Q5: How is SRAM different from FROM in FANUC CNC memory terminology?

FROM (Flash ROM): Non-volatile, no battery needed. Stores CNC system software (operating firmware). Written infrequently (software updates only). Contents load into DRAM at boot for execution.

SRAM (A20B-2900-0681): Volatile (needs battery backup for power-off retention). Stores parameters, programmes, and offsets. Written frequently (every parameter change, every programme edit). Retains working data indefinitely with battery.

Both reside in plug-in modules; both are necessary for CNC operation; neither can substitute for the other.