Product Overview

The 6ES5090-8MA00​ is the Siemens SIMATIC S5-90U CPU 103, the top-grade processor in the S5-90U compact micro-PLC family and one of the last “battery-free EEPROM” members of the broader SIMATIC S5 ecosystem. Where the larger S5-100U/115U/135U/155U racks all relied on backup batteries (or an external EPROM loader) to retain user programs, the 6ES5090-8MA00​ stores its 4 KB of user program in on-board EEPROM and its 1 KB of data (flags, timers, counters) in battery-less RAM with enough retentivity for typical plant duty—meaning no annual battery swap, no “BATF” LED surprise during outage, and no acid-leak risk in hot/humid cabinets. The -8MA00 suffix denotes 24 V DC supply (19.2–28.8 V range, picked up from the S5-90U base-unit PSU), distinguishing it from the 120/230 V AC variants (-8AA00/-8BA00) in the same CPU-grade ladder. The 6ES5090-8MA00​ plugs into the S5-90U base (6ES5 091-…) and can be expanded with up to three S5-90U EM (Expansion Module) slices—EM 101 digital, EM 102 analog, etc.—to build out a small machine or skid controller without a full S5 rack.Performance-wise the 6ES5090-8MA00​ (CPU 103) is the fastest of the three S5-90U grades: bit instruction ≈1.5 µs, word ≈3–5 µs, placing it ahead of the CPU 101 (~5 µs bit) and CPU 102 (~3.5 µs bit) that share the same 50 mm-wide module form factor. Communication is through a single front RS485 PG/OP port running the AS511 protocol—connect a PG720/PG740 or a PC with a CP 5611 + STEP 5 (V7.x, DOS/VM or compatible Windows host) and you have program download, online monitoring, and forced-I/O. There is no onboard Ethernet, no Profibus master, no MPI in the S7 sense—this is pure late-era S5: compact, deterministic, and deliberately minimal. The 6ES5090-8MA00​ was Siemens’s answer to “I need an S5 but I only have 120 mm of DIN rail and 24 V DC,” and it found its way onto packaging machines, conveyor zone controllers, pump-skid local logic, analyzer-house RTUs, and OEM skid panels throughout the 1990s–2000s.Today the 6ES5090-8MA00​ is long discontinued (SIMATIC S5 hit end-of-life globally in the mid-2000s, with S5-90U among the last S5 sub-families to go), so the commercial reality is legacy sparing: plants that still run S5-90U cells because the machine OEM went bust, the logic is undocumented, or the re-qualification cost to S7-1200/S7-1500 isn’t justified yet. In that world the 6ES5090-8MA00​ is a strategic spare, not a new-design SKU, and the description below reflects that—performance that was respectable in 1998, lifecycle positioning that is pure obsolescence management, and an upgrade path note (S7-1200) for when the panel finally does get retired.

Technical Specifications

Main Features and Advantages

The 6ES5090-8MA00​ sells on three things that mattered in the S5-90U era and still matter for legacy owners: battery-free EEPROM, CPU-grade headroom, and compact DIN footprint. The EEPROM program store is the headline—4 KB doesn’t sound like much in 2025 terms, but for a machine-zone controller doing DI interlocks, timer/counter sequencing, a handful of analog compares, and maybe a simple PID loop, 4 KB (≈1 000–1 500 STEP 5 statements) is plenty. More importantly, EEPROM means the 6ES5090-8MA00​ survives a cabinet power-off for months and comes back with the program intact—no backup battery to leak, no “memory lost” fault on restart, no weekend callout because someone left the disconnect open too long. For plants with S5-90U scattered across remote pump houses or analyzer shelters, that autonomy is the real value.CPU 103 grade is the second lever. The S5-90U family splits three ways: CPU 101 (2 KB program, ~5 µs bit), CPU 102 (3 KB, ~3.5 µs), CPU 103 (4 KB, ~1.5 µs). If you inherited a panel with a 101 that’s hitting scan-time ceiling as the machine OEM kept adding logic over two decades, dropping a 6ES5090-8MA00​ (CPU 103) into the same base gives double the program envelope and ~3× the bit speed with zero rewiring. The STEP 5 project recompiles clean—same instruction set, same addressing, same PG cable—so the upgrade is a “swap-and-download” not a rewrite. That’s unusually graceful for an S5-era part.Compactness is the third win. The S5-90U base + 6ES5090-8MA00​ + up to 3 EMs fits in roughly 120 × 120 × 75 mm of DIN row—less than a shoebox—yet delivers a standalone PLC with 24 V DC supply, PG/OP port, and expandable I/O. For OEMs that standardized S5-90U on packaging wrappers, palletizer cells, conveyor merges, and skid-mounted chemical-dosing units, the 6ES5090-8MA00​ was the default because it didn’t need a rack, a separate PSU module, or a backplane extender—just clip the base to DIN, plug the CPU, add EMs, and go. The 24 V DC version (-8MA00) is especially popular in mobile/vehicle and solar-shelter panels where only DC is available; the 120/230 V AC variants (-8AA00/-8BA00) serve mains-fed machinery panels.The honest downside: the 6ES5090-8MA00​ is S5, not S7. No Profinet, no Profibus-DP master without an add-on CP (and S5-90U expansion bandwidth barely supports one), no structured text natively (STL/LAD/FBD in STEP 5 only), and the PG/OP port is AS511 serial—you’ll need a PG or a USB-to-AS511 adapter + STEP 5 license to touch it. For new designs nobody should pick a 6ES5090-8MA00; for keeping a 25-year-old packaging line running until the next capex window, it’s exactly the right part. Siemens’s forward path is S7-1200 (6ES7 212-1…) for compact-duty replacement, but that requires rewiring the base, re-licensing STEP 7, and re-qualifying the logic—which is why most plants stock 6ES5090-8MA00​ spares instead.