Product Overview
The Siemens 6DD2920-0AQ0 is a SIMADYN D SU70 signal and relay module — a DIN-rail mounted auxiliary unit (22.5 mm wide, Siemens “housing 70” form factor) that sits outside the SIMADYN D rack itself and forms the plant-facing interface between the rack’s 24 V DC domain and the site’s DCS/annunciator/relay tree. While the EB1 (6DD1681-0EB1) powers the rack internally (+5 V / +24 V backplane from 230 V AC inlet), the 6DD2920-0AQ0 takes the plant’s 24 V DC supply (typically the same 24 V that feeds the MCC’s ET 200 / S7-1500 / valve terminals), distributes it to the SIMADYN D rack’s 24 V consumers (or provides a buffered 24 V tap), and — more importantly — carries three dry-contact relays (commonly “Ready”, “Fault”, and “Acknowledge/Trip” depending on wiring) that the SIMADYN D PM6 or CS8 drives via a watchdog pulse. When the PM6’s D7-SYS task is alive, it sends a periodic pulse to the 6DD2920-0AQ0 watchdog input; if the pulse stops (PM6 crashed, EB1 died, L-bus stuck, rack power lost), the SU70 drops its relays within <100 ms, and the plant DCS sees “SIMADYN D Rack Fault” on the annunciator — no software, no OPC, just a dry contact the relay-tree understands.
The Siemens 6DD2920-0AQ0 is wired into the SIMADYN D rack’s “fault tree” as follows: the PM6 or CS8 has a digital output (often one of the CS8’s EM-bus or the PM6’s own 8 DI, or via a small EM11) that pulses “alive” at ~100–500 ms rate; that pulse lands on the 6DD2920-0AQ0 watchdog input. The SU70’s relay K1 (“Ready”) is picked when 24 V is present AND watchdog pulses; K2 (“Fault”) is the inverse NC contact — when watchdog fails, K1 drops, K2 picks, and the DCS annunciator rings. A third relay (often “Acknowledge” or “Trip latching”) can be wired so the DCS can remotely reset/latch the SU70 state, or the SU70 auto-rearms when the PM6 watchdog resumes (after a rack reboot). The 6DD2920-0AQ0 itself has no firmware — it’s pure relay logic + RC watchdog — so it never goes “obsolete” electronically, but Siemens put the SU70 family into PM500 / discontinued because SIMADYN D itself is out of support; the 6DD2920-0AQ0 is now spare-demand only.
In a full SIMADYN D rack lineup (PM6 6DD1600-0AK0 + IT41 6DD1606-3AC0 + CS8 6DD1662-0AC0 + EB1 6DD1681-0EB1), the Siemens 6DD2920-0AQ0 is the one module the plant electrician interacts with at the terminal block level — it’s where the “Stand 2 Ready” lamp, the “Stand 2 Fault” horn, and the “Stand 2 Trip to DCS” dry contact land. When a hot-strip mill says “Stand 2 dropped off — control room saw the fault contact,” the first thing the electrician checks is the 6DD2920-0AQ0 LED (if equipped) and the watchdog pulse at the PM6 end, before digging into PM6/MS52.
Technical Specifications
| Parameter Name | Parameter Value |
|---|---|
| Product Model | 6DD2920-0AQ0 |
| Manufacturer | Siemens |
| Product Type | SIMADYN D Signal & Relay Module (DIN-rail auxiliary) |
| Mounting | DIN rail (TS 35), 22.5 mm width (Siemens “70” housing family) |
| Supply Voltage | 24 V DC (plant supply, feeds internal relay coils + watchdog) |
| Relay Outputs | 3 × dry contact relays (Ready / Fault / Ack-Trip, NO+NC per relay typical) |
| Relay Contact Rating | 5 A / 250 V AC, 5 A / 30 V DC (resistive) |
| Watchdog Input | 24 V pulse train from PM6 / CS8 / EM11 (typ. 100–500 ms period) |
| Watchdog Drop-Out Delay | <100 ms (relay drop on pulse stop) |
| Distributed 24 V Output | Buffered 24 V DC tap (feeds rack-local 24 V consumers via EB1 or direct) |
| Operating Temperature | –25 … +60 °C |
| Storage Temperature | –40 … +85 °C |
| Protection Class | IP20 |
| Dimensions (W × H × D) | 22.5 × 90 × 105 mm (typical form) |
| Weight | ~0.15 kg |
| LEDs | Power / Watchdog / Relay status (model-variant dependent) |
| Certifications | CE, UL, CSA |
| Lifecycle Status | PM500 – Discontinued, deleted without replacement |
Main Features and Advantages
Plant-facing dry-contact interface for a SIMADYN D rack
The Siemens 6DD2920-0AQ0 solves a problem that PROFINET/OPC can’t: when a tandem-mill stand’s SIMADYN D rack crashes at 3 AM, the S7-1500 upstream may also be talking to that rack over DP (via CS8+SS52), and the DP drop will eventually flag a “Stand 2 Drive Fault” on the HMI — but the control room annunciator (old relay-tree, Foxboro IA, Honeywell TPS, or even a modern ET 200 digital input) needs a dry contact now, not a DP alarm delay. The 6DD2920-0AQ0 gives three relays: K1 Ready (NO — rack alive, watchdog pulsing), K2 Fault (NC — mirror of K1, so DCS sees “Fault” when K1 drops), K3 Acknowledge/Trip (configurable — can latch the fault until DCS sends a 24 V pulse to unlatch, or auto-clear when watchdog resumes). This relay-tree visibility is why every SIMADYN D rack on a hot-strip mill has an — the HMI is nice, but the roller-pulpit relay panel is what gets the foreman out of bed.
Watchdog pulse from PM6 D7-SYS task
The Siemens 6DD2920-0AQ0 watchdog input is driven by a PM6 digital output — typically one of the PM6’s own 8 DI/DO (the 8 DI include 4 interrupt-capable, but the 8th or a spare is often wired as DO to the ) or an EM11 BI/BO if the rack carries one. In D7-SYS, the PM6’s “background task” (lowest priority, 100 ms–500 ms cycle) toggles that output pin; if the PM6 crashes, L-bus freezes, or EB1 sags and the PM6 resets, the toggle stops, and the 6DD2920-0AQ0 drops relays within <100 ms. On a compressor antisurge skid, this <100 ms drop is telegraphed to the DCS’s ESD — the DCS may then trip the compressor train if “SIMADYN D rack fault” + “surge margin <5%” coincide. The itself is passive (RC timing + relay coil), so it doesn’t need firmware updates, doesn’t need a project download, and survives a PM6 MS52 corruption — it only cares that the pulse arrives.
24 V DC distribution + buffering
Although the EB1 (6DD1681-0EB1) is the rack’s internal PS (230 V AC → 24 V DC backplane + 5 V), many plants prefer the plant’s 24 V DC (from a central Phoenix/Weidmüller PSU in the MCC) to feed the , and the either (a) passes that 24 V straight to the EB1’s 24 V sense, or (b) provides a buffered 24 V tap for local 24 V loads — the 6DD2920-0AQ0 relay coils themselves are 24 V, so the plant 24 V feeds the coil side, and the ‘s contact side can switch 250 V AC / 30 V DC to the DCS dry-contact loop. This separation means a plant 24 V short doesn’t drag the EB1 down, and vice versa. On a salt-air coastal plant where the central 24 V PSU ages, the 6DD2920-0AQ0 gives a single point to measure “Is the seeing 24 V?” — if the Power LED is out, the plant 24 V feed is the first suspect, not the EB1.
Form-factor and wiring simplicity
The Siemens 6DD2920-0AQ0 is 22.5 mm DIN-rail wide (Siemens “housing 70” — the same family as the SU71/SU72 and the LOGO! AM series), so it snaps onto the same TS-35 rail that holds the EB1’s 230 V AC isolator, the itself, and maybe a Phoenix terminal block for the DCS dry-contact landing. Wiring is screw-terminal: plant 24 V in (L+/M), watchdog in from PM6 (24 V pulse), three relay NO/NC pairs out to DCS (Ready NO, Fault NC, Ack/Trip NO-NC). No Ethernet, no USB, no SD — a plant electrician who’s never seen SIMADYN D can still troubleshoot the 6DD2920-0AQ0 with a multimeter: “Is 24 V at terminal 1-2? Yes. Is watchdog pulsing 24 V at terminal 3-4? No → PM6 problem. Yes but relay not picking? → coil dead.”
