Siemens LOGO! 12/24RCE and KTP700 HMI Network Limits for Hotel BMS

1. Application Overview

This reference covers the engineering constraints encountered when deploying a large-scale room-automation architecture based on Siemens LOGO! 12/24RCE controllers supervised by a SIMATIC KTP700 HMI panel. The target use case is a hotel building management system (BMS) where each guest room is assigned an autonomous LOGO! 12/24RCE acting as a stand-alone room controller (no LOGO!-to-LOGO! communication), and the HMI acts as a single-head-end visualization client that polls and writes the room setpoints.

For a building of 220 rooms (22 rooms per floor × 10 floors), the design challenge is not the LOGO! program itself but the network-layer capacity of the HMI panel and the LOGO! project limits inside LOGO! Soft Comfort and TIA Portal / WinCC. The following sections define each limit, show the math that bounds the architecture, and propose a workable topology that survives the published constraints.

2. LOGO! 12/24RCE Hardware Reference

The LOGO! 12/24RCE (Siemens article number 6ED1052-1MD08-0BA1, LOGO! 8.FS4 generation) is the relay-output variant with integrated Ethernet. The key specifications for a hotel room application are summarized below.

Source: LOGO! 8 System Manual (Siemens Support entry ID 109743415) and the LOGO! product page on the Siemens Industry Online Support portal.

3. I/O Allocation per Hotel Room

A single LOGO! 12/24RCE handles the full room I/O budget. The allocation used in the reference application is:

Two physical analog inputs cover two sensors. Two digital inputs cover the setpoint buttons, two more cover SOS and door status. Four relay outputs cover two solenoid valves, fan enable, and one spare. Total: 2 AI + 4 DI + 4 DO, well inside the LOGO! 12/24RCE envelope.

4. HMI Panel Options and Connection Limits

The SIMATIC HMI panel family used at the head end distinguishes between Basic Panels and Comfort Panels. Each has a different ceiling on simultaneously configurable PLC connections, and this is the single most important number when sizing a multi-LOGO! architecture.

Source: SIMATIC HMI Operator Panels Manual Collection (entry ID 63481236) and the SIMATIC TIA Portal Help portal on docs.tia.siemens.cloud.

5. LOGO! Soft Comfort Project Network Limits

Inside LOGO! Soft Comfort V8.x the LOGO! network project (the topology editor that visualizes Master/Slave relationships and the IP ring) accepts up to 32 LOGO! devices per project, regardless of the HMI brand or number of connections. This limit is set by the LOGO! Soft Comfort editor and the LOGO! BM/AM remote-variable table structure, not by the panel.

Concretely, a LOGO! Soft Comfort project can contain:

• 1 × LOGO! designated as Master (network coordinator)
• ≤ 31 × LOGO! designated as Slave (remote variables BM/AM)
• 1 × external HMI peer (optional, but counted toward the 32-node ceiling in some releases)

If LOGO!-to-LOGO! communication is not required (the case in the hotel BMS where each room is autonomous), the LOGO! Soft Comfort network project is unnecessary — the HMI can be configured as point-to-point S7 client against any number of LOGO! devices by listing each IP under a separate connection.

Reference: LOGO! 8 Communication (System Manual section 7).

6. Maximum Device Math

Combining the two limits yields the absolute ceiling of a LOGO! + SIMATIC HMI architecture:

The formula is straightforward:

N_max = C_panel × N_LOGO_per_connection

where C_panel is the panel connection ceiling (4 Basic, 8 Comfort) and N_LOGO_per_connection is 32 in the LOGO! Soft Comfort project, but in practice 1 if LOGO!-to-LOGO! networking is disabled (one LOGO! IP per connection). This produces two distinct maximum-count interpretations:

• If each HMI connection is a direct S7 link to a single LOGO! (1:1 reading, no LOGO! network project): the panel ceiling itself is the absolute cap. N_max = C_panel = 4 LOGO! for a KTP700 Basic, 8 for a Comfort panel.
• If each HMI connection points at a LOGO! Soft Comfort network of 32 LOGO!s: the theoretical cap rises to 128 (Basic) or 256 (Comfort).

7. Recommended Architecture for 220 Rooms

For 220 rooms, the only clean answer is to introduce an intermediate aggregation layer:

Option A — LOGO! + S7-1200 head-end (recommended)

• 220 × LOGO! 12/24RCE in rooms, each with its own fixed IP (e.g. 192.168.10.101 through 192.168.10.220).
• 1 × SIMATIC S7-1215C DC/DC/DC (or S7-1512C for higher tag count) as the head-end PLC with 4-port switch module (CSM1277 / CM 1542-1).
• The S7-1200 acts as a Modbus/TCP or PROFINET aggregator: the LOGO! 12/24RCE exposes its VM (variable memory) area via S7-protocol on its Ethernet port; the S7-1200 opens S7 PUT/GET against each LOGO! and mirrors the data into a single DB.
• 1 × TP1200 Comfort panel reads the single DB on the S7-1200 (1 connection, ≤ 4096 tags).

This satisfies the 220-room target with one head-end panel, deterministic polling, and bounded engineering cost. See the S7-1200 Easy Book for PUT/GET programming details.

Option B — Distributed HMIs (KTP700 Basic per floor)

• 10 × KTP700 Basic PN, one per floor (22 rooms per floor).
• Each panel opens 4 S7 connections of its own; with 22 rooms per floor you need at least 22 LOGO! IPs reachable per panel.
• Result: still over the 4-connection ceiling of a Basic panel — only 4 rooms per KTP700 Basic can be polled. Not viable at 22 rooms/floor without a head-end.

Option C — Comfort panel (TP1200) + LOGO! network projects

• 1 × TP1200 Comfort panel (8 connections).
• Each connection serves 32 LOGO!s in a LOGO! Soft Comfort network project.
• Theoretical 256 LOGO! — sufficient for the 220 target with margin.
• Engineering penalty: each connection still only exchanges data with the LOGO! it points at, so the network-clustering gain is real only if the LOGO!s inter-communicate BM/AM variables (room-to-room door sharing, etc.). For a stand-alone room controller (no LOGO!-to-LOGO!), this option collapses back to 8 rooms per panel.

8. IP Address Planning

A clean /24 subnet (255.255.255.0) is sufficient for 220 LOGO!s. Recommended layout:

Subnet : 192.168.10.0 / 24
Gateway: 192.168.10.1   (router / S7-1200 head-end)
HMI   : 192.168.10.10   (or .20 for backup)
LOGO! : 192.168.10.101  ..  192.168.10.220
Netmask: 255.255.255.0
DNS   : not used

Reserve 192.168.10.1 through 192.168.10.20 for the head-end PLC, switches, and engineering laptop. Begin LOGO! IP assignments at .101 to keep the static-DHCP table small and reviewable.

In the LOGO! web server, set the subnet mask and disable DHCP. Enable SNTP if time-stamping of SOS events is required.

9. TIA Portal / WinCC Configuration Procedure

The following steps reproduce a working 4-LOGO! connection in TIA Portal V13 → V17 against a KTP700 Basic PN. The same flow scales to a TP1200 Comfort for the 8-connection limit.

1. Create the project. File → New → "HMI Station with WinCC Comfort" (Basic for KTP700). Select the panel part number exactly (e.g. 6AV2123-2GB03-0AX0 for KTP700 Basic PN).
2. Add LOGO! 8 devices as "Unspecified S7-300/400" or as "LOGO!" if the catalog contains the entry. Devices & Networks → Add new device → PLC → LOGO! 8.
3. Configure the Ethernet interface of each LOGO! with a fixed IP from the plan above. Subnet mask 255.255.255.0, no router.
4. Enable S7 communication on the LOGO!: Tools → LOGO! → Remote Access → enable "Allow S7 communication" (also reachable on the device's web page). Reference: LOGO! 8 communication manual.
5. Add an HMI connection in TIA Portal. Right-click the panel → Connections → Add S7 Connection → assign the partner IP of the first LOGO!. Repeat for the second, third, and fourth LOGO!. The number of configurable connections is visible on the panel's properties under "Connections / Maximum number" (4 for KTP700 Basic, 8 for Comfort).
6. Map LOGO! VM addresses to HMI tags. LOGO! exposes variables through the VM (Variable Memory) area at addresses starting at VW0. The simplest mapping uses the LOGO! Soft Comfort "Network Variables" feature, then references each variable's BM/AM address from the HMI tag list (e.g. DB1,BD0). For 1:1 polling of room status, only the local VM area of each LOGO! is needed; no BM/AM networking required.
7. Build the visualization. One root screen "Floor 01 – 22 rooms" with 22 numeric indicators. Wire each indicator to a tag pointing at the corresponding LOGO!'s VM word for the room temperature, setpoint, SOS latch, and door status.
8. Compile and download. Compile the HMI configuration. If the message "Maximum number of connections per HMI exceeded" appears, the configured count is above 4 (Basic) or 8 (Comfort) — remove connections until the build succeeds.
9. Set access password for the HMI connection. For each connection, open Connection → Properties → "Access Password" and enter the LOGO! access password. This is the password required by the LOGO! to accept PUT/GET from a non-trusted peer. Reference: TIA Portal help – "Access password for the HMI connection".
10. Verify with the online diagnostics. HMI → Online → Diagnostics → Connections. Each connection should show status "Connected" within 10 s of PLC scan.

10. Powerline Networking Considerations (TP-Link AV500)

Hotel retrofits rarely permit new Cat-6 cabling. TP-Link AV500 HomePlug AV adapters (e.g. TL-PA4010, TL-PA4020P with pass-through) provide a workable physical layer for LOGO! Ethernet if the following conditions are met:

• Same phase or three-phase coupling: AV500 signals propagate through a single 230 V phase. Across phases the signal must traverse the building's three-phase coupling at the distribution board, which heavily attenuates the link. Verify a working link by pairing the adapters and reading the LED status before mounting.
• Pair programming: encrypt the powerline segment with a private 16-character key to avoid accidental cross-talk with neighboring apartments.
• Bandwidth: AV500 is nominally 500 Mbit/s PHY, ~80–100 Mbit/s TCP throughput in practice. LOGO! traffic is well below 1 Mbit/s, so the headroom is large; latency is the real concern.
• Latency budget: typical round-trip per powerline hop is 5–15 ms. For 220 LOGO!s polled at 200 ms each over 4 simultaneous connections, average age of a value is ~55 s — acceptable for room-status display, marginal for SOS alarm annunciation. Plan a dedicated cyclic interrupt on the head-end to poll SOS flags at ≤ 1 s.
• EMC separation: install powerline adapters at least 0.5 m away from any VFD, elevator drive, or step-up transformer to minimize common-mode noise.

11. Tag-Budget Calculation

For 220 rooms, each exposing 8 HMI tags (room temp, bath temp, setpoint, fan state, valve 1, valve 2, SOS, door), the total tag count is:

tags_total = 220 × 8 = 1760 tags

The KTP700 Basic PN limit is 500 tags — insufficient for 220 rooms. The TP1200 Comfort supports 2048 tags — sufficient. If the Comfort panel is selected, also confirm the connection count (8) and split the LOGO!s across 8 connections of 28–32 devices each:

connections_used = ceil(220 / 32) = 7
logos_per_connection = floor(220 / 7) = 31   (last: 220 - 6×31 = 34)

12. Verification Checklist

Before declaring the architecture production-ready, run the following checks:

1. Build clean compile in TIA Portal. No "Maximum number of connections per HMI exceeded," no "tag limit exceeded" warnings.
2. Web-server confirmation. Browse to http://192.168.10.101 from a PC; the LOGO! home page shows "Operating mode: RUN," supply voltage, and current VM variables. Repeat for each room IP.
3. Single-LGO! connection test. Configure connection 1 only, download the HMI project, cycle the HMI, and confirm the room temperature, setpoint, and SOS latch update on the screen within 2 s.
4. Multi-LOGO! connection test. Repeat with all 7 (or 8) connections active. Verify update latency by toggling the SOS push-button in room .101 and confirming the indicator lights within 1.5 s.
5. Watchdog / fail-state. Disconnect the Ethernet cable of room .101 and confirm the HMI shows a "No connection" alarm after the configured timeout (default 5 s).
6. Setpoint write-back. From the HMI, push a +1 °C setpoint and confirm the room's analog input count registers the change. This exercises PUT/GET, not just GET.
7. Access-password test. Remove the LOGO! access password and re-attempt a write from the HMI — the connection should refuse, confirming the password path is active.

13. Alternative Controller Considerations

If the project scope expands beyond room control — windows, doors, blinds, room power contactors — a single LOGO! 12/24RCE may run short of outputs (only 4 relays). Alternatives:

If a single room requires more than 4 relay outputs, stack a DM8 / DM16 expansion module to the right of the base unit. The I/O bus supports up to 24 digital inputs and 20 digital outputs across all expansion modules combined.

If the head-end architecture must scale beyond a single Comfort panel (e.g. a chain of 5 hotels × 220 rooms), move the aggregation layer up to WinCC Professional / SCADA running on a server PC with S7 drivers; this lifts the connection ceiling to 256 PLC connections per OS server, suitable for thousands of LOGO!s.

14. Summary

A 220-room hotel BMS cannot be addressed by a single SIMATIC HMI without an aggregation strategy. The published numbers are:

• LOGO! Soft Comfort V8 network project: 32 LOGO!s
• KTP700 Basic PN: 4 connections, 500 tags
• TP700 / TP1200 Comfort: 8 connections, 2048 tags

The clean engineering answer is a SIMATIC S7-1200 (or S7-1500) head-end PLC acting as a poll-and-mirror concentrator, with one Comfort panel on the operator side. Direct "LOGO! ↔ HMI" topologies cannot reach 220 rooms in a single panel under the published TIA Portal limits.