Overview: S7-300 Multi-Rack Architecture
The SIMATIC S7-300 organizes its I/O into a numbered rack concept (Rack 0 = central rack, Rack 1, 2, 3 = expansion racks). A single CPU 31x chassis can physically hold up to 8 modules (SM, FM, CP, etc.) plus the CPU and the IM in slot 3, and the power supply in slot 1. Once the application outgrows the eight-slot envelope, two field-proven options exist:
- Central rack expansion — add up to three additional central racks (Rack 1…3) wired back to the CPU through IM 360 (send) and IM 361 (receive) interface modules.
- Distributed I/O over PROFIBUS-DP — keep the central rack and connect an ET 200M remote station through a CP 342-5 communications processor.
Both options preserve the existing program, but they differ in addressing, scan time impact, diagnostic reachability, and the set of CPUs that can use them. The wrong IM selection on the wrong CPU is the single most common field failure when scaling an S7-300 from a single rack to two.
Prerequisites
- CPU compatibility — the firmware/hardware version of the CPU must support central rack expansion. See CPU compatibility matrix below.
- Free slot 3 in the central rack for the IM 360 send module.
- Free slot 3 in each expansion rack for the IM 361 receive module.
- Two pre-assembled 1 m or 2.4 m connecting cables (6ES7 368-3BB01-0AA0 for 1 m, 6ES7 368-3BC01-0AA0 for 2.4 m). Cable length and number of racks are bound together; see IM 360/361 topology rules below.
- STEP 7 V5.5 / V5.6 or TIA Portal V13 and newer (the S7-300 is supported in TIA Portal through V20; the cloud documentation entry point is the Configuring central racks (S7-300, S7-400, S7-1500) page).
- Power supply budget — confirm the central PS 307 can still meet the 5 V backplane budget of the new expansion rack. Each IM 361 draws 5 V / 0.8 A from the expansion rack backplane.
Critical CPU limitation: The CPU 312 and do not support rack expansion. If you intend to use IM 360/361 pairs, the CPU must be replaced with a 313C (firmware V2.6 or higher), 313C-2, 314, 314C, 315, 315-2 DP/PN, 316, 317, or 319. See the Siemens FAQ 4068543 — Interface module IM 365 for confirmation of the rack-expansion limit and the S7-300 Module Data manual, chapter "Interface Modules".
CPU Compatibility Matrix for Central Rack Expansion
| CPU | MLFB (6ES7 312/313/314/315/316-…) | Max central racks | Min firmware | IM 360/361 | IM 365 |
|---|---|---|---|---|---|
| CPU 312 | 312-1AE13-0AB0 | 1 | any | no | no |
| CPU 312C | 312-5BF04-0AB0 | 1 | any | no | no |
| CPU 313C | 313-5BF03-0AB0 | 4 | V2.6 | yes | yes (1 pair only) |
| CPU 313C-2 PtP | 313-6BF03-0AB0 | 4 | any | yes | yes |
| CPU 313C-2 DP | 313-6CF03-0AB0 | 4 | any | yes | yes |
| CPU 314 | 314-1AG13-0AB0 | 4 | any | yes | yes |
| CPU 314C-2 DP | 314-6CF02-0AB0 | 4 | any | yes | yes |
| CPU 315-2 DP | 315-2AG10-0AB0 | 4 | any | yes | yes |
| CPU 315-2 PN/DP | 315-2EH13-0AB0 | 4 | any | yes | yes |
| CPU 317-2 DP | 317-2AK14-0AB0 | 4 | any | yes | yes |
| CPU 319-3 PN/DP | 319-3EL00-0AB0 | 4 | any | yes | yes |
Interface Module Comparison: IM 365 vs. IM 360/361
| Parameter | IM 365 (pair, 6ES7 360-3BA01-0AA0 + 6ES7 361-3BA01-0AA0) | IM 360 (6ES7 360-3AA01-0AA0) + IM 361 (6ES7 361-3AA01-0AA0) |
|---|---|---|
| Max expansion racks | 1 (one fixed pair, fixed 1 m cable) | 3 (one IM 360 + three IM 361) |
| Cable length | 1 m, fixed, non-replaceable | 1 m (6ES7 368-3BB01-0AA0) or 2.4 m (6ES7 368-3BC01-0AA0), user-wired |
| Max modules per expansion rack | 8 | 8 |
| Hot-swap support | no | no |
| IM current draw (5 V) | 0.1 A (sender) / 0.1 A (receiver) | 0.3 A (IM 360) / 0.8 A (IM 361) |
| Redundant power | no | no (each rack needs its own PS 307) |
| Diagnostic capability | basic (SF LED) | basic + segment status on IM 361 |
| Typical application | low-cost single-pair extension, both racks in same cabinet | flexible 1…3 expansion racks, longer distances, mixed cabinets |
Use the IM 365 only when exactly one expansion rack is needed and the rack-to-rack distance is fixed at 1 m. The pre-assembled cable cannot be lengthened, and the second pair occupies slots 3 and 4 of the central rack (the IM 365 pair is shipped as a single pre-wired unit, but each module still consumes one slot in its own rack). For any other case — more than one expansion rack, longer cables, or a requirement to hot-swap an expansion rack independently — choose IM 360/361.
Hardware Topology: IM 360/361 Wiring Rules
- Place the IM 360 in slot 3 of the central rack (Rack 0). The IM 360 has two identical connectors labeled 1 and 2, but only one chain can be built; do not parallel-cable two chains into a single IM 360.
- Place one IM 361 in slot 3 of each expansion rack (Rack 1, 2, 3).
- Connect IM 360 → IM 361 (Rack 1) using a 1 m or 2.4 m pre-assembled cable. The cable carries 24 V, ground, and the parallel backplane bus — it is not a PROFIBUS cable; the two ends are keyed and labelled.
- Chain the next IM 361 (Rack 2) from the IM 361 (Rack 1) using a second cable on the second port of IM 361. Continue for Rack 3.
- Each expansion rack requires its own PS 307 power supply in slot 1. The 5 V rail of each expansion rack backplane is sourced by the local PS 307, not by the IM 360.
- Do not place a CPU, FM, or CP in an expansion rack — only signal modules (SM), function modules (FM), and communications processors (CP) with a passive backplane interface (e.g., CP 340) are permitted in expansion racks of a CPU 31x. CP 342-5 is only allowed in the central rack because it terminates PROFIBUS-DP.
Step-by-Step: Adding a Second Rack on a CPU 313C and Higher
1. Verify CPU and slot reservation
- Open the rack in HW Config (STEP 7) or the Device view in TIA Portal.
- Confirm that slot 3 of Rack 0 is empty. If it holds a module, rearrange so that the IM 360 is in slot 3. In an S7-300 central rack, the IM 360 is always in slot 3; it is the only module that may occupy that slot in a multi-rack configuration.
2. Insert IM 360 in Rack 0 / slot 3
- Drag the IM 360 from the catalog (SIMATIC 300 → Interface modules) into slot 3 of Rack 0.
- Double-click the module to open its properties. The I-address start is forced to 0 in STEP 7 V5.x; in TIA Portal you can let the system auto-assign.
3. Add Rack 1 and populate slot 3 with IM 361
- Right-click the central rack → "Insert rack → Rack 1" (STEP 7) or use the "Add device → SIMATIC 300 → Rack" task card (TIA Portal).
- Drag an IM 361 into slot 3 of the new rack.
- Insert a PS 307 in slot 1 of the new rack.
4. Place signal modules
- Add SM, FM, or passive CP modules into slots 4…11 of the new rack. The S7-300 supports up to 8 modules per expansion rack.
- Address allocation continues automatically. In STEP 7, the I/Q start addresses are appended to the last byte used in Rack 0. In TIA Portal, the central rack configuration procedure documents this in detail under "Specifying the input and output addresses".
5. Save, compile, and download
- Save the project, compile (Station → Consistency check in STEP 7; "Compile" in TIA Portal), and download the hardware configuration to the CPU.
- Cycle power on the CPU; the SF LED on the IM 360/361 must extinguish within 2…3 seconds if wiring and slot assignment are correct.
Alternative: Distributed I/O via CP 342-5 and ET 200M
When the application CPU is fixed at a 312 family member, a second central rack is unreachable. The supported field remedy is to keep the existing central rack and attach a PROFIBUS-DP slave in the form of an ET 200M remote station. Topology:
- Install a CP 342-5 (6GK7 342-5DA02-0XE0) in the central rack (slot 4…11).
- Configure the CP 342-5 as a PROFIBUS-DP master in HW Config.
- Add an ET 200M (6ES7 153-1AA03-0XB0 IM 153-1, or 6ES7 153-2BA10-0XB0 IM 153-2 for hot-swap) to the PROFIBUS subnet.
- Populate the ET 200M with up to 8 S7-300 SM/FM modules (the same modules that the central rack uses, so spare-parts inventory is unified).
- Configure the I/O of the ET 200M as "DP slave" in the master system — the I/Q bytes are no longer central, but appear in the process image of the CP 342-5 and must be read/written via the FC "DP_SEND" / "DP_RECV" from the SIMATIC NET library.
Cycle-time impact: data exchanged through CP 342-5 lives in the CP's process image, not the CPU's. The cycle time penalty is one PROFIBUS round-trip (typically 1…5 ms per 32-byte slot) and one FC call per direction. For most discrete-control applications this is invisible; for high-speed synchronous motion (FM 350 / FM 352) it is not acceptable — use a true central rack with IM 360/361 in that case.
Configuration Verification Checklist
| Check | Method | Expected result |
|---|---|---|
| IM 360 SF LED | Visual, after STOP → RUN | OFF |
| IM 361 SF LED | Visual, per expansion rack | OFF |
| CPU BF / SF LEDs | Visual | OFF |
| Diagnostic buffer | STEP 7 / TIA Portal → Online → Diagnostics | No rack configuration or address errors |
| I/O access test | Monitor table with VAT, force a digital output on the new SM | Output toggles; PIW updates with sensor feedback |
| Address map | HW Config → Print module list | Continuous address range, no overlaps |
| 5 V budget | Sum the 5 V draw of all modules in the expansion rack, compare with PS 307 rating (5 V / 5 A or 10 A) | Sum ≤ 80% of PS rating |
Troubleshooting Matrix
| Symptom | Likely cause | Corrective action |
|---|---|---|
| CPU goes to STOP with "module configuration error" after inserting IM 360 | CPU 312/312C in the rack — these do not support rack expansion | Replace CPU with 313C V2.6+ (or higher family). Re-evaluate whether distributed I/O via CP 342-5 is acceptable. |
| IM 361 SF LED stays ON, IM 360 SF LED OFF | IM 361 in wrong slot (not slot 3) or cable reversed | Move IM 361 to slot 3 of the expansion rack. Reverse the cable (both ends are keyed, so it is keyed not reversed — inspect for bent pins). |
| IM 360 and IM 361 both SF ON | No PS 307 in the expansion rack, or 24 V supply to the IM chain is missing | Install PS 307 in slot 1 of the expansion rack. Verify 24 V at the IM 361 power terminals. |
| Modules in the expansion rack are not reached; CPU SF lights up during STOP→RUN | Wrong cable length, or the chain is broken (Rack 1 disconnected breaks Rack 2 and Rack 3 downstream) | Re-seat cable connectors. If using IM 365, note the fixed 1 m limitation. If chaining three racks, replace any 1 m cable exceeding the path length. |
| Address gaps / overlap between Rack 0 and Rack 1 | Manual address override in HW Config that conflicts with auto-addressing | Reset the Rack 1 modules to "System default" addresses; recompile and download. |
| Existing program suddenly uses different I/O addresses after adding the expansion rack | Auto-addressing inserted the new modules in front of the old ones | Manually set the I/Q start addresses of the new modules to the next free byte above the original central rack. |
| Expansion rack modules lose process image update during a hot event | Hot-swap attempted on a non-IM 153 station — S7-300 central racks are not hot-swappable | Power down the expansion rack, swap the module, power it up. The CPU will report rack return (SF briefly ON, then OFF). |
Edge Cases and Field-Proven Caveats
- CPU 313C V2.6 boundary. Earlier 313C firmware (V1.x) does not support rack expansion. Always verify the firmware with "Online → Accessible Nodes" or by reading the operator panel "Module Information" before assuming rack expansion works on a 313C.
- IM 360 in slot 3 of Rack 0 uses a small amount of I/O address space for internal diagnostics (one input byte and one output byte in STEP 7 V5.x; TIA Portal auto-hides this). Do not place user-addressable modules at those addresses.
- CP 342-5 is a master-capable PROFIBUS-DP interface. Using it on Rack 1 (expansion) is not allowed by the S7-300 design rules — only the central rack can hold an active bus master. If you need both rack expansion and DP, the central rack must host CP 342-5, and a second CP 342-5 cannot be added in the expansion rack for further DP. The expansion rack's SM/FM count is bounded at 8.
- Earth bonding. Each rack's PS 307 ground stud must be bonded to the cabinet PE bus. A floating 24 V return between racks is the leading cause of "phantom" SF errors on the IM 361 during welding or large VFD switching transients in the cabinet.
- EMC separation. If an expansion rack is in a different cabinet, use the 2.4 m cable only and route it through a dedicated EMC cable duct. Avoid bundling the IM cable with 400 V motor conductors.
- Spare-parts symmetry. The IM 360 and IM 361 have separate MLFB numbers. Do not order them as a pair — you need one IM 360 and one IM 361 per chain (plus an additional IM 361 for each further rack).
Summary: Choosing the Right Path
For a project that started on a CPU 312 with nine modules, the S7-300 hardware rules force a clear engineering decision:
- If the CPU can be replaced with a 313C V2.6 (or higher) and the second rack sits in the same cabinet within 1 m, use an IM 365 pair — lowest cost, simplest wiring.
- If the second rack is in a different cabinet or further than 1 m, use an IM 360/361 pair with a 2.4 m cable.
- If the CPU must remain a 312/312C, abandon central rack expansion and use a CP 342-5 master with an ET 200M PROFIBUS-DP slave — the new modules are no longer central but the project remains a single CPU with distributed I/O.
All three paths are supported by STEP 7 and TIA Portal through the central-rack configuration workflow, and the S7-300 Module Data manual (chapter "Interface Modules") plus Siemens FAQ 4068543 confirm the CPU and IM compatibility rules.
Can a CPU 312 support a second S7-300 rack?
No. The CPU 312 and CPU 312C support only one central rack. Central rack expansion requires a CPU 313C (firmware V2.6 or higher), 313C-2, 314, 314C, 315, 315-2 DP/PN, 316, 317, or 319.
What is the difference between IM 365 and IM 360/361?
IM 365 is a fixed pair with a 1 m integral cable and supports exactly one expansion rack. IM 360 (sender, slot 3 of the central rack) plus IM 361 (receiver, slot 3 of each expansion rack) supports up to three expansion racks with user-selectable 1 m or 2.4 m cables.
What is the minimum CPU for S7-300 rack expansion?
The CPU 313C is the lowest member of the family that supports central rack expansion, and only with firmware V2.6 or higher. Earlier 313C firmware revisions (V1.x) do not allow IM 360/361 chains.
Can I use an ET 200M as a substitute for a second central rack?
Yes. Add a CP 342-5 PROFIBUS-DP master in the central rack and connect an ET 200M with an IM 153-1 or IM 153-2. The I/O is no longer central and is exchanged through DP_SEND / DP_RECV, but the CPU stays a CPU 312 if required by the project.
How many modules fit in an S7-300 expansion rack?
Eight modules can be placed in slots 4…11 of the expansion rack (plus the IM 361 in slot 3 and a PS 307 in slot 1). Modules are addressed automatically after the last byte of the central rack; in STEP 7 you can override the I/Q start address manually.