Siemens 6ES7972-0DA00-0AA0 PROFIBUS Terminator: Resistor Values, Wiring, and Bus Rules
The Siemens 6ES7972-0DA00-0AA0 is a SIMATIC DP bus terminator (active PROFIBUS/MPI terminating resistor) that is often mistaken for a simple, single-value axial resistor. The official Siemens datasheet describes it as an RS485 terminating resistor for terminating PROFIBUS/MPI networks with a 24 V DC supply input. In reality the device is a small, switchable, powered termination node that injects a defined three-resistor network into the segment. This reference covers the exact resistor values used inside the 6ES7972-0DA00-0AA0, why three resistors are required, where the unit must be installed, how it differs from a passive 120 Ω PROFIBUS connector, and how to verify correct operation on a live segment.
1. Why a PROFIBUS Terminator Is Not One Resistor
PROFIBUS DP, PROFIBUS PA, and MPI all use EIA-485 (RS-485) as their physical layer, but the PROFIBUS specification (IEC 61158-2, EN 50170 Volume 2) extends the standard RS-485 common-mode window and adds a defined idle-bus bias. A “terminator” for PROFIBUS therefore performs three jobs in one circuit:
- Provides a defined characteristic-impedance match (line termination).
- Forces a defined idle/space voltage on the bus when no driver is active (fail-safe bias).
- Sources and sinks the small leakage currents that build up across long, electrically noisy segments (active termination).
A bare 120 Ω axial resistor placed across the A and B lines will match the cable impedance and suppress reflections, but it cannot establish the bus idle state and cannot tolerate ground-potential differences seen in 24 V cabinet wiring. That is why the 6ES7972-0DA00-0AA0 is built around a 24 V-fed, three-resistor network rather than a single passive component.
2. The PROFIBUS DP Termination Resistor Network
The PROFIBUS installation guideline (PI Installation Guideline, version 1.0 / 1.1) and the EN 50170 physical-layer specification both mandate the following resistor network at each end of every DP segment:
| Reference | Value | Connection | Function |
|---|---|---|---|
| RT | 220 Ω ± 1% | Between data line A (B) and data line B (A), i.e. the differential pair | Line termination that matches the nominal 150 Ω cable characteristic impedance and damps reflections |
| RU | 390 Ω ± 5% | From data line A to VP (+5 V internal rail) | Pull-up that biases the bus to a defined “1” (idle) state when no driver is active |
| RD | 390 Ω ± 5% | From data line B to DGND (ground reference) | Pull-down that establishes the negative rail of the idle state |
The combined DC resistance measured from VP through RU, through RT, through RD to DGND is 390 + 220 + 390 = 1000 Ω. The 220 Ω across the pair is the value most engineers remember, but the 390 Ω bias resistors are equally important — they convert the segment into a fail-safe bus where a defined idle voltage is present whenever no station is driving.
3. How the 6ES7972-0DA00-0AA0 Implements the Network
The 6ES7972-0DA00-0AA0 is a D-sub (9-pin) backplane terminator that plugs into the last (or first) PROFIBUS station on a segment. According to the official Siemens data sheet, the device:
- Accepts a rated 24 V DC supply (typical range 20.4 V to 28.8 V, per Siemens S7-300/400 and ET 200 station 24 V rail tolerances).
- Generates an internal +5 V rail (VP) used to bias the two 390 Ω resistors.
- Contains a slide switch that engages or disengages the 220 Ω + 2 × 390 Ω network from the bus.
- Provides a green status LED that confirms the 24 V supply is present and the internal regulator is running.
- Carries the 9-pin D-sub pinout identical to a standard PROFIBUS connector: pin 3 = B-line, pin 8 = A-line, pin 6 = VP (5 V), pin 5 = DGND, pin 4 = RTS, pin 2 = GND, pin 7 = +24 V (in), pin 1 = shield.
Engaging the switch connects the three-resistor network directly to the bus lines through a low-impedance analog switch; disengaging the switch disconnects the network so the connector behaves as a straight-through feedthrough. The mechanical switch is the only user interface — there is no firmware, no diagnostics register, and no parameterization slot.
4. PROFIBUS Termination Topology
The 6ES7972-0DA00-0AA0 must appear at exactly two physical locations in any given segment: the first and last nodes. The rule is encoded in the PROFIBUS Installation Guideline and repeated in every Siemens SIMATIC manual.
If you place a 6ES7972-0DA00-0AA0 (or any other active terminator) on an interior node, the 220 Ω resistor is in parallel with the cable run and lowers the DC bus load to less than 100 Ω, which violates the 1000 Ω minimum DC load specified by EN 50170. The result is signal clipping, increased edge rates, and CRC errors at the master.
5. Why the Two 390 Ω Bias Resistors Are Required
Generic RS-485 networks leave the bus floating when no driver is enabled, which means the receiver output is undefined for a few hundred nanoseconds after the last bit of a frame. PROFIBUS solves this by pulling the A-line toward +5 V through RU (390 Ω) and the B-line toward ground through RD (390 Ω). The 220 Ω across the pair sits in the middle of the divider, so the steady-state differential idle voltage is:
Vidle = (220 / (390 + 220 + 390)) × 5 V ≈ 1.1 V differential (A high relative to B)
This is well above the EIA-485 receiver threshold of ±200 mV, so the receiver always reads a defined “1” (idle) the moment the last driver releases the bus. Without the 390 Ω pull-up and pull-down, a break in the cable or a stuck slave would cause a flapping idle state and trigger repeated PROFIBUS diagnostic interrupts.
6. Common-Mode Voltage and 24 V System Robustness
The 6ES7972-0DA00-0AA0 is designed for the harsher common-mode window of PROFIBUS. The EIA-485-A standard allows ground-potential differences between any two devices of −7 V to +12 V. PROFIBUS installations in 24 V cabinets routinely see ground shifts outside that window, and a plain RS-485 transceiver can latch or burn out. The Siemens terminator is built around transceivers with an extended common-mode range of approximately −25 V to +25 V referenced to the 5 V DGND, which is the same range specified for ET 200S, ET 200M, and S7-300 PROFIBUS interfaces.
The 24 V supply is not used for the bus signaling itself (PROFIBUS DP signaling is 5 V differential, levels defined by VP/5V on pin 6 of the D-sub). The 24 V input is used only to power the internal +5 V regulator that drives the bias network. The result is a terminator that survives 24 V accidental shorts to the bus lines for short durations and tolerates the large ground-potential differences that occur in long, multi-cabinet installations.
7. Mechanical Form Factor and Wiring
The 6ES7972-0DA00-0AA0 is a 9-pin D-sub plug with an integrated termination switch and a flying lead for the 24 V supply. Pinout is identical to the standard PROFIBUS connector (IEC 61158-2 / EN 50170):
| D-sub pin | Signal | Function |
|---|---|---|
| 1 | SHIELD | Cable shield, bonded to housing |
| 2 | M24V (GND) | 24 V return (internally tied to DGND through the regulator) |
| 3 | RxD/TxD-P (B-line) | Negative data line |
| 4 | RTS | Request-To-Send, used by some masters for direction control |
| 5 | DGND | 5 V reference ground for PROFIBUS signaling |
| 6 | VP (+5 V) | 5 V supply for bias network and slave electronics |
| 7 | P24V | +24 V DC supply input (20.4–28.8 V) |
| 8 | RxD/TxD-N (A-line) | Positive data line |
| 9 | NC | Not connected |
The two flying leads exit the housing as a short pigtail. Red is +24 V, blue is 24 V return. The cable end is normally terminated to a Phoenix-style two-position terminal block or, on later assemblies, to a 2-pin Mini-Combicon header that drops into the matching socket on a Siemens DP coupler.
8. Termination Placement Rules
- Install the 6ES7972-0DA00-0AA0 only at the first and last physical node of the segment. Use the slide switch to set termination ON at those two points and OFF everywhere else.
- Never run the PROFIBUS cable out of the last node and back to a separate terminating device outside the segment. The 220 Ω resistor must be at the electrical end of the line.
- Keep stub length (the drop from the trunk to a node connector) under 1.5 m at 1.5 Mbit/s, under 6.5 m at 500 kbit/s, per the PROFIBUS Installation Guideline.
- Always connect the cable shield to pin 1 of the D-sub and to the cabinet ground bar at every node. Do not rely on the terminator housing as the shield bond.
- If the segment is repeated (e.g. RS-485 repeater 6ES7972-0AA01-0XA0), each repeater segment is treated as an independent bus and each side of the repeater must be terminated at both ends.
- Maximum segment length depends on baud rate: 1200 m at 9.6/19.2/45.45 kbit/s, 1000 m at 93.75 kbit/s, 400 m at 187.5 kbit/s, 200 m at 500 kbit/s, 100 m at 1.5 Mbit/s, 100 m at 3/6/12 Mbit/s. The 6ES7972-0DA00-0AA0 has no effect on these limits; it only ensures the bus is properly biased at the end points.
9. Specifications
| Parameter | Value |
|---|---|
| Siemens part number | 6ES7972-0DA00-0AA0 |
| Product family | SIMATIC DP accessories |
| Function | Active PROFIBUS/MPI RS-485 bus terminator |
| Internal termination network | RT = 220 Ω across A/B, RU = 390 Ω A→VP, RD = 390 Ω B→DGND |
| Supply voltage (rated, DC) | 24 V (typical operating range 20.4–28.8 V) |
| Internal generated rail | +5 V (VP) for bus bias |
| Bus signaling levels | 5 V differential, idle ≈ 1.1 V diff (A high, B low) |
| Common-mode tolerance | ≈ −25 V to +25 V (PROFIBUS extended) |
| Connector | 9-pin D-sub male, PROFIBUS pinout |
| Status indicator | Green LED for 24 V / regulator OK |
| Termination switch | Slide switch on housing (ON / OFF) |
| Operating temperature | 0 °C to +60 °C (Siemens SIMATIC accessory spec) |
| Storage temperature | −40 °C to +70 °C |
| Approvals | CE, UL, cUL, PROFIBUS Nutzerorganisation e.V. conformance |
| Compatible networks | PROFIBUS DP, PROFIBUS MPI (S7), RS-485 (limited) |
10. Verification Procedure
After installation, perform these checks to confirm the 6ES7972-0DA00-0AA0 is correctly wired and active:
- Visual: The slide switch is at “ON” on the first and last node of the segment, and at “OFF” on every node in between. The green LED is lit on the two active terminators and dark on the others.
- DC voltage check (segment powered, all stations idle): measure pin 6 (VP) to pin 5 (DGND) at the D-sub of any station. Expect 4.75–5.25 V, sourced by the active terminator through the cable. If the reading is below 4.5 V, the 24 V supply to a terminator has dropped out or the cable is too long.
- Differential idle voltage (segment idle): measure pin 8 (A-line) relative to pin 3 (B-line) at the master. Expect ≈ +1.0 V to +1.2 V (A high, B low). If you read close to 0 V, the bias network is missing — one of the 6ES7972-0DA00-0AA0 units has failed or its 24 V supply is off.
- Resistance check (segment de-energized, all slaves disconnected): measure between pin 3 and pin 8 at one end of the trunk with the two terminators installed. Expect ≈ 110 Ω (two 220 Ω in parallel, one at each end). Reading 220 Ω indicates only one terminator is active; reading above 250 Ω indicates both terminators are OFF or open.
- Diagnostic integrity (live segment): with STEP 7 / TIA Portal connected, open Diagnostics → PROFIBUS Diagnostics and check for zero slave diagnostic events. Any repeating FDL/CRC errors point to missing termination or a damaged terminator board.
11. Troubleshooting Matrix
| Observed symptom | Likely cause | Corrective action |
|---|---|---|
| All slaves report “bus fault” immediately on power-up | Both terminators installed at the same end, or terminator installed at a middle node | Move terminator to the last physical node of the trunk; switch OFF on every interior node |
| Intermittent CRC errors, gets worse with longer cable | Passive 120 Ω block used instead of 6ES7972-0DA00-0AA0 (missing 390 Ω bias) | Replace passive block with active 6ES7972-0DA00-0AA0 at both ends |
| VP (pin 6) measures 0 V at the last slave | 24 V supply to the active terminator missing or open fuse | Restore 24 V to pins 7 (+24 V) and 2 (GND) of the terminator; check 24 V distribution fuse |
| Idle voltage ≈ 0 V on A/B lines | Active terminator switched OFF at one end | Set terminator slide switch to ON at both ends of the segment |
| Master reports “station failure” for one specific slave only | Defective slave interface, not a termination issue | Swap terminator with neighbouring node; if the slave still fails, replace the slave or its PROFIBUS connector |
| Green LED off on terminator but 24 V is present | Internal regulator or 5 V rail failed | Replace the 6ES7972-0DA00-0AA0; do not attempt board-level repair |
| Segment works at 1.5 Mbit/s but not at 12 Mbit/s | Cable length exceeds the 12 Mbit/s limit (100 m), or terminator distance from the end is excessive | Reduce segment length or insert a PROFIBUS repeater (6ES7972-0AA01-0XA0); re-terminate each sub-segment |
12. Comparison with Other Siemens Terminators
| Part number | Type | Supply | Network | When to use |
|---|---|---|---|---|
| 6ES7972-0DA00-0AA0 | Active D-sub terminator with switch | 24 V DC | 220 Ω + 2×390 Ω | Standard PROFIBUS DP / MPI end-of-segment, supplied from 24 V cabinet rail |
| 6ES7972-0BA30-0XA0 | Passive D-sub terminator block | None | 220 Ω only | Short, low-noise segments where the bias is provided elsewhere (e.g. directly by a master with integrated termination) |
| 6ES7972-0BA12-0XA0 | Passive 90° D-sub connector with termination switch | None | 220 Ω only | Field installation where the connector itself terminates the segment |
| 6ES7972-0AA01-0XA0 | RS-485 repeater | 24 V DC | Regenerates the bus, has two internal terminators | Extends a segment beyond the baud-rate/length limit; each side of the repeater must still be terminated externally |
| 6GK1500-0FC10 | PROFIBUS FC Terminator (no LED) | None | 220 Ω + 2×390 Ω, hard-wired | Field-stub terminator for harsh-environment IP65 segments |
13. Common Engineering Pitfalls
- Confusing the 220 Ω termination with a 120 Ω “RS-485 resistor.” The 120 Ω figure is the characteristic impedance of generic RS-485 cable; PROFIBUS cable is specified with a 150 Ω characteristic impedance and a 220 Ω matching resistor. A 120 Ω terminator on a PROFIBUS line is technically under-damped and can cause ringing at 12 Mbit/s.
- Powering the terminator from the same 24 V rail as the slaves. If a slave short-circuits the 24 V rail, the terminator will brown out and the entire segment will go into FDL error. Use a separately fused 24 V branch for the two terminators.
- Leaving the slide switch in the wrong position after a re-cable. Always re-verify the DC resistance between pins 3 and 8 at the end of the trunk after any field re-termination; expect ≈ 110 Ω with both terminators active.
- Mounting the terminator with the 24 V pigtail unsupported. The 24 V wires carry little current (< 100 mA) but mechanical strain on the pigtail can break one of the leads, which will drop the 5 V VP rail silently and create elusive CRC errors.
14. Related Standards and Documents
- IEC 61158-2 / EN 50170 Volume 2 — PROFIBUS physical layer specification, defines the 220 Ω / 390 Ω / 390 Ω termination network and the 5 V idle level.
- PROFIBUS & PROFINET International (PI) Installation Guideline — cable types, baud-rate/length matrix, shield bonding rules, and terminator placement.
- Siemens SIMATIC S7-300 / S7-400 / ET 200 system manuals — describe the 24 V supply tolerance and the 6ES7972-0DA00-0AA0 as the recommended end terminator.
- Siemens S7-1500 / ET 200MP PROFIBUS interface module manuals — specify the maximum segment length and the terminator requirements when using the integrated DP-Master interface.
FAQ
What is the resistance value of the 6ES7972-0DA00-0AA0?
It is not a single value. The device contains a 220 Ω resistor across the A and B lines plus two 390 Ω bias resistors — one from A to +5 V (VP) and one from B to DGND. The combined DC bus load at each end of the segment is 1000 Ω.
Does the 6ES7972-0DA00-0AA0 need a 24 V supply?
Yes. The rated supply is 24 V DC (operating range 20.4–28.8 V). The 24 V input powers an internal +5 V regulator that drives the 390 Ω bias network. Without the 24 V supply the terminator is electrically invisible on the bus.
Can I use a passive 120 Ω D-sub block in place of the 6ES7972-0DA00-0AA0?
You can for very short, low-speed segments, but you lose the 390 Ω bias network and the 24 V survivability. PROFIBUS DP/MPI installations longer than about 30 m at 1.5 Mbit/s will start to see intermittent FDL/CRC errors. Use the 6ES7972-0DA00-0AA0 for any production segment.
Where on the segment should I install the 6ES7972-0DA00-0AA0?
Only at the first and last physical node. Set the slide switch ON at those two positions and OFF on every node in between. Placing a terminator on an interior node puts two 220 Ω resistors in parallel and drops the bus DC load below the 1000 Ω minimum.
How do I know the terminator is working on a live segment?
Measure the differential voltage between pin 8 (A) and pin 3 (B) of the master connector with the segment idle. A correctly terminated segment will show about +1.0 V to +1.2 V (A higher than B). Measuring close to 0 V means the 24 V supply to one or both terminators has been lost.