Siemens 6ES5376-1AA21: CMOS EPROM Memory Submodule for SIMATIC S5-115U

Description

Application ScenariosA cement grinding plant in Eastern Europe had been running a SIMATIC S5-115U (CPU 928-3UA21) since the late 1990s, controlling the mill drive sequencing and bagging line interlocks. For years the plant relied on the CPU’s onboard RAM backed by a 3.6 V lithium battery—until one winter weekend the battery quietly expired, the capacitor drained, and Monday’s cold start brought the line down with a blank memory. The maintenance team spent 14 hours reconstructing the STEP 5 logic from paper prints. After that incident, they standardized on the 6ES5376-1AA21​ EPROM submodule: once the verified program was burned in, no battery could ever erase it again. Three years later, through two scheduled power outages and one battery holder corrosion event, the 6ES5376-1AA21​ kept the mill’s logic intact. The pain point it solved was simple but expensive—program loss on a battery-backed RAM architecture in a plant where a single shift’s downtime costs more than the entire S5 rack.h2

Parameter

Technical Principles and Innovative Values

• Innovation Point 1: True Non-Volatility Without Battery.​ Unlike RAM-based S5 memory submodules that demand a healthy 3.6 V lithium cell to retain user logic through power-off, the 6ES5376-1AA21​ uses CMOS EPROM cells. Once programmed via a Siemens PG (Programming Device) or a compatible programmer, the bits are trapped in floating gates—immune to battery failure, leakage corrosion, or long-term warehouse storage.
• Innovation Point 2: Quartz Glass UV Window for Field Reuse.​ The 6ES5376-1AA21​ carries a distinctive clear window over the EPROM die. Expose it to UV-C light (~253.7 nm) for 15–20 minutes and the whole array clears, ready for the next burn. In legacy S5 plants where process logic evolves slowly (maybe once every few years), this strikes a practical balance between “write-once permanence” and “occasional update.”
• Innovation Point 3: Seamless Slot Integration with CPU 928-3Ux1x.​ The submodule plugs straight into the dedicated memory slot on the 928-3UA11 / 928-3UA21 faceplate—no ribbon cables, no external carrier. The 6ES5376-1AA21​ is automatically detected by the CPU at boot; the S5 memory map simply extends, and STEP 5 treats it as the primary load memory region.

h2 Application Cases and Industry ValueIn a combined heat-and-power (CHP) plant in Central Germany, the fuel feeding and FD fan controls still run on an S5-115U with CPU 928-3UA21, originally commissioned in 1998. The plant had experienced two forced outages traced to exhausted backup batteries on the original RAM submodule—each outage cost roughly €18,000 in missed steam export plus overtime callouts. After migrating to the 6ES5376-1AA21, the site recorded zero program-loss events over a 48-month observation window. The plant’s automation lead noted: “We still have the S5 because it never gave us trouble—until the battery did. The EPROM submodule took that worry off the table. Now we only touch the logic when the process itself changes, not because a coin cell died.”From a lifecycle perspective, the 6ES5376-1AA21​ also reduces spare-parts complexity. A single EPROM submodule can sit in climate-controlled stores for years and still be field-ready, whereas battery-backed RAM modules degrade even on the shelf. For asset managers balancing “run it until it dies” versus a full DCS migration, the 6ES5376-1AA21​ is a low-cost insurance policy.