Product Overview
The Siemens 6ES5090-8MA00 is the CPU 103 module for the SIMATIC S5-90U compact PLC family — Siemens’ entry-level, DIN-rail-mounted S5 line designed in the late 1980s/early 1990s to replace relay panels and small hard-wired control cabinets without jumping straight to the larger S5-100U/115U rack architecture. Unlike the S5-100U, which uses a separate rack, IM expansion bus, and discrete SM slices, the S5-90U packs the 24 V DC power supply, the CPU, and fixed digital I/O terminals into one compact enclosure ~50 mm wide; the Siemens 6ES5090-8MA00 is the processor core that drives this enclosure. The CPU 103 sits in the middle of the 90U lineup — above the entry CPU 102 and below the transistor-output CPU 130 and relay-output CPU 150 — offering ~2 KB of user program memory (roughly 1000+ S5 “steps”), 256 bytes of Merker (flags), 128 timers and 128 counters, and a binary instruction time of 4–6 µs. For plants still running S5-90U machines, the Siemens 6ES5090-8MA00 is the exact-replacement CPU that restores the logic core without touching the rest of the 90U chassis, the terminal wiring, or the STEP 5 project.
The Siemens 6ES5090-8MA00 communicates with the outside world through a front PG interface (Siemens MPI/AS511 protocol) used for program download, online monitoring, and forced-variable from a STEP 5 PG (or a modern PG/PC with a USB-MPI adapter and STEP 5 V7.x). It does not carry PROFIBUS or Profinet — those belong to later S7 generations and to S5-135/155U with CP modules. Instead, the Siemens 6ES5090-8MA00 focuses on what the 90U was built for: stand-alone machine logic, interlock trees, timer/counter sequencing, and small analog tasks via optional 90U analog submodule (where fitted). The integrated I/O on a 90U chassis hosting the Siemens 6ES5090-8MA00 is typically 16 digital inputs and 10–16 digital outputs (transistor or relay depending on 90U variant), with 24 V DC sensor supply sourced from the internal PSU. Scan time is approximately 10 ms per 1000 steps, so a full ~2 KB project (actually “steps”, not bytes — S5 counts program in steps, each 3–5 bytes) runs OB1 in the 15–25 ms range, easily adequate for relay-replacement duties, small conveyor interlocks, pump sequencing, and machine-guard logic. The Siemens 6ES5090-8MA00 is programmed exclusively in STEP 5 (LAD, FBD, STL — SCL and GRAPH arrived later on S5-135/155), and the project lives on an EPROM sub-module (6ES5 374 family) plugged into the 90U chassis or in RAM backed by the 3.6 V lithium cell that S5-90U carries for Merker/T/C retention.
Technical Specifications
| Parameter | Value |
|---|---|
| Product Model | 6ES5090-8MA00 |
| Manufacturer | Siemens |
| Product Type | SIMATIC S5-90U CPU 103 (Compact PLC CPU Module) |
| Program Memory | ~2 KB (≈ 1000+ S5 “steps”) |
| Data Memory | ~256 B Merker (flags) + 128 Timers + 128 Counters |
| Binary Instruction Time | ~4–6 µs/bit |
| Word Instruction Time | ~10–20 µs |
| Typical Scan | ~10 ms / 1000 steps |
| Integrated I/O (via 90U chassis) | 16 DI + 10–16 DO typical (variant-dependent; CPU module itself lives inside 90U shell) |
| Communication Interface | Front PG port (AS511 / MPI, for STEP 5 PG) |
| Expansion | Local 90U chassis only (no IM expansion bus — 90U is compact, not rack-based; 100U/115U carry IM) |
| Supply Voltage | 24 V DC (internal PSU in 90U chassis feeds the 6ES5090-8MA00) |
| Power Dissipation | ~5–8 W (CPU + chassis load) |
| Operating Temperature | 0 °C to +60 °C |
| Storage Temperature | –40 °C to +70 °C |
| Programming Software | STEP 5 V7.x (or earlier) |
| Mounting | DIN rail, inside S5-90U compact enclosure |
| Dimensions (CPU module / 90U shell context) | ~50 × 100 × 115 mm (module level); 90U shell ~90×100×75 mm approx. |
| Weight | ~0.3–0.5 kg (module + 90U shell context) |
| Backup | 3.6 V lithium cell (Merker/T/C retention, EPROM for program) |
Main Features and Advantages
Compact all-in-one footprint: The S5-90U chassis that houses the Siemens 6ES5090-8MA00 is roughly the size of a few DIN-mounted contactors — 50–90 mm wide depending on I/O count — yet it replaces an entire relay panel: 16 DI, 10–16 DO, 24 V DC sensor supply, PG port, and the CPU itself. For OEMs who originally specced the Siemens 6ES5090-8MA00 into textile sub-machines, small stamping presses, packaging infeeds, and pump-skids, there was never a rack to mount, never a separate PSU to buy, and never a backplane to wire. That compactness is why the Siemens 6ES5090-8MA00 still turns up in legacy BOMs where a modern S7-1200 would feel “too big” to retrofit without re-cutting the DIN.
STEP 5 native — no migration tax: The Siemens 6ES5090-8MA00 runs the same STEP 5 instruction set as the rest of the S5 family, meaning a 1992-vintage project downloaded from an old PG onto a replacement Siemens 6ES5090-8MA00 runs unchanged. In plants where the original ladder/FBD/STL was written by a long-retired integrator and the paper docs are thin, “drop in and go” matters more than “feature upgrade.” The Siemens 6ES5090-8MA00 preserves that asset — no rewriting to S7, no TIA Portal conversion projects, no re-validation of safety interlocks.
Battery-backed Merker + T/C + EPROM project: The Siemens 6ES5090-8MA00 sits inside a 90U shell that carries a 3.6 V lithium cell for retentive data (M-flags, timers, counters) and accepts a 6ES5 374 EPROM sub-module for the program image. This two-tier retention means a planned power-down doesn’t lose setpoints, and an unplanned one doesn’t lose the project — pop a fresh EPROM in during the last known-good burn, and the Siemens 6ES5090-8MA00 comes back exactly where it left off. For machines that run 24/7 (water treatment, continuous process auxiliaries), this is the difference between “restart in 10 minutes” and “rebuild the sequence from scratch.”
4–6 µs/bit, ~10 ms/K-step scan: The Siemens 6ES5090-8MA00 is no speed demon by 2025 standards, but for relay-replacement and small-sequencer duty it’s more than enough. A 2 KB-step project (the practical ceiling for CPU 103) scans in ~20 ms, which is 5–10× faster than the mechanical relay timing it replaced, and fast enough for guard-door veto, pump lead/lag, conveyor photo-eye, and small batch-sequencer logic. The Siemens 6ES5090-8MA00 doesn’t pretend to be a motion controller — it’s a logic-and-timer workhorse.
PG port for diagnostics without tearing the panel: The front PG socket on the Siemens 6ES5090-8MA00 (AS511/MPI) lets a maintainer plug a PG or a laptop-with-USB-MPI-adapter straight into the 90U face, go online in STEP 5, and see which rung is failing, which T-bit is stuck, which M-flag is wrong. On a machine where the Siemens 6ES5090-8MA00 is buried behind a locked door with three other 90U siblings, that faceplate access saves the “open the door, unclip, cart to bench” ritual that bigger S5 racks demand.
Legacy-availability reality: The Siemens 6ES5090-8MA00 has been out of Siemens active production for years; the current market is NOS (new-old-stock) and verified-pull/tested-surplus. For plants that standardized 90U across 30+ machines in the 1990s, keeping two spares per plant of the Siemens 6ES5090-8MA00 is cheaper than re-paneling one machine, let alone thirty. The module’s mechanical simplicity (no fan, no moving parts, DIN clip) means NOS units from 1998 still test fine today if the lithium cell is fresh.