Siemens CPU 1511-1 PN vs 1515-2 PN
The CPU 1511-1 PN (6ES7511-1AK02-0AB0) fits a single-network machine with up to 128 PROFINET IO devices and a compact program. The CPU 1515-2 PN (6ES7515-2AM02-0AB0) earns its price when you need a second, independently addressed PROFINET network, three times the work memory and twice the bit speed. Both order numbers are now spare-part-only.
A failed S7-1500 CPU stops the line, and the replacement gets ordered under time pressure. Unplanned downtime costs the world's 500 largest companies about 11% of revenue — $1.4 trillion a year (Siemens, The True Cost of Downtime 2024) — and ordering the wrong controller turns a one-day swap into a re-engineering job. This article breaks the choice down from three angles: network architecture, capacity and lifecycle.
Key takeaways
- Two ports on one interface are a switch, not a second network — only the 1515-2 PN has two addressed PROFINET interfaces.
- Work memory is the hard limit: 150 KB / 1 MB against 500 KB / 3 MB.
- Both MLFBs were phased out on 1 May 2024 and ship only as surplus or spare-part stock; successors need TIA Portal V18+.
Network architecture decides this faster than CPU speed
Interface count, not bit performance, is the first filter. The CPU 1511-1 PN has one PROFINET interface (X1) with IRT and an integrated 2-port switch — line or MRP-ring topology, but still one network with one IP address. The CPU 1515-2 PN keeps X1 and adds X2, a separate single-port PROFINET RT interface, so machine I/O and HMI or MES traffic stay on different subnets with no extra hardware. (A managed switch or communication module gives the same separation on a 1511-1 PN, at the cost of hardware and one more device to document.)
Specifications that change a design
Every figure below comes from the Siemens S7-1500 equipment manuals.
| Dimension | CPU 1511-1 PN · 6ES7511-1AK02-0AB0 | CPU 1515-2 PN · 6ES7515-2AM02-0AB0 |
|---|---|---|
| Work memory (program / data) | 150 KB / 1 MB | 500 KB / 3 MB |
| Bit / word / floating-point operation | 60 ns / 72 ns / 384 ns | 30 ns / 36 ns / 192 ns |
| PROFINET interfaces | 1 × IRT with 2-port switch | 1 × IRT (2-port switch) + 1 × RT |
| Max. IO devices | 128 (64 with IRT) | 256 on X1 (64 with IRT) |
| Isochronous mode | Yes; min. OB 6x cycle 625 µs | Yes; min. OB 6x cycle 500 µs |
| Program blocks / retentive memory | 4,000 elements / 128 KB | 8,000 elements / 512 KB |
| Width, backplane power, display | 35 mm, 10 W, 3.45 cm | 70 mm, 12 W, 6.1 cm |
| Firmware / engineering | FW V2.9, TIA Portal V17 or higher | FW V2.9, TIA Portal V17 or higher |
| Price range | Budget tier | Mid tier; priced above the 1511-1 PN on surplus stock |
| Pros / cons | Smallest footprint, lowest cost; one network, 150 KB program ceiling | Network separation, 3× data memory; double width, higher cost |
In our most recent cartoning-line retrofit
In our most recent cartoning-line upgrade the brief looked like a straight swap until we audited the network:
- Count the nodes, then the bytes. 32 ET 200MP stations and 6 SINAMICS drives fit inside the 128-device limit, but the program compiled to 96 KB with 620 KB of data and recipe buffers still growing.
- Check whether a second network is required. Production wanted MES order data straight from the controller, so we specified the CPU 1515-2 PN and put MES on X2.
- Verify the physical fit. 70 mm width and a 12 W backplane feed forced a re-check of rail space and power budget before we ordered.
Three pitfalls cost the most time here. No memory card, no start — the card is the load memory. An older TIA Portal will not list the newer MLFB, so you configure the predecessor order number instead. And isochronous work must respect the minimum OB 6x cycle: 625 µs on the 1511-1 PN, 500 µs on the 1515-2 PN.
Pro tip: quote the SIMATIC Memory Card with the CPU — card condition, capacity and firmware level decide whether a replacement boots first time.
Lifecycle: both MLFBs are already phased out
Siemens announced phase-out for both CPUs on 1 May 2024, and both have since moved to spare-part status — new-surplus and refurbished stock only (Siemens S7-1500 documentation). The successors, 6ES7511-1AL03-0AB0 and 6ES7515-2AN03-0AB0, run firmware V3.x, and V3.0 needs TIA Portal V18 or higher while V3.1 needs V19 or higher (Siemens firmware update entry). That portal upgrade, not the module, is the real cost of moving on. For a like-for-like breakdown replacement, staying on AK02 or AM02 keeps the existing project valid.
Bottom line
Pick the CPU 1511-1 PN for a single-network machine with a compact program; pick the CPU 1515-2 PN when network separation, data volume or motion timing is part of the specification. Confirm the memory card, firmware level and TIA Portal release before ordering. Check current stock and lead time for Siemens S7-1500 CPUs — send us the MLFB and firmware version and we will verify compatibility before shipping.
FAQ
Can I drop a CPU 1515-2 PN into a machine that had a CPU 1511-1 PN?
It depends — the module usually fits the project, but you must reconfigure in TIA Portal: allow 70 mm of rail width, confirm the power budget, move the Memory Card across and address the second interface.
Do I still need a SIMATIC Memory Card?
Yes — neither CPU runs without one. The card is the load memory, up to 32 GB; no card is the most common cause of a failed same-day swap.
Will 30 ns instead of 60 ns cut my scan time?
It depends. Bit performance is instruction speed, not cycle time — communication load and OB structure usually dominate. It matters in motion and isochronous work, far less in plain logic.