Siemens SIMATIC S5: CPUs, memory modules and power supplies

SIMATIC S5 PLCs are the generation of Siemens controllers that preceded S7. They have been discontinued for years, but still control many machines and lines, often with programs that were never migrated. When an S5 CPU or power supply fails, the goal is to get back up and running quickly with the original hardware.

In the E-Repair catalogue the S5 range counts about 1,000 part numbers. This guide covers the "heart" of the PLC: CPUs, memory modules and power supplies.

⚙️ The S5 families

• Compact S5-90U, S5-95U and S5-100U series: controllers for machines of low and medium complexity. In the catalogue: S5-95U CPUs (root 6ES509) and CPU 100, 102 and 103 for S5-100U (root 6ES510).
• S5-115U: the most widespread modular series, with CPU 941 – 945 (root 6ES594).
• S5-135U and S5-155U: systems for large and multiprocessor plants, with CPUs such as the 948 for S5-155U and PS 955 power supplies.

🧠 Memory modules

In S5 PLCs the program is often stored on removable memory modules, EPROM, EEPROM or flash (for example the 375 modules, root 6ES537, with 39 part numbers in the catalogue).

A faulty or erased memory module stops the machine even with a perfectly working CPU: keeping a copy of the program and a spare module is the simplest protection.

🔌 Power supplies

S5 rack power supplies, such as the PS 951 for S5-115U and the PS 955 for S5-135U/155U (root 6ES595, more than 50 part numbers in the catalogue), feed the CPU and modules and often house the backup battery. After decades of service they are among the components most prone to failure, mainly due to capacitor ageing.

🔍 Examples from the catalogue

👉 6ES5095-8FA01 – S5-95U CPU

👉 6ES5944-7UA11 – CPU 944 for S5-115U

👉 6ES5948-3UA11 – CPU 948 for S5-155U

👉 6ES5375-0LA11 – 375 EEPROM memory module

👉 6ES5955-3LC13 – 24 V DC power supply

👉 Search the catalogue: 6ES594 · 6ES595 · 6ES537

⚠️ Typical faults

• exhausted backup battery, with loss of the program in RAM
• power supplies with degraded capacitors, shutdowns or unstable voltages
• unreadable or erased memory modules
• CPU that does not go into RUN or stops intermittently
• oxidised contacts and connectors after many years in industrial environments

🧩 Other S5 components in the catalogue

• battery modules, for example nickel-cadmium for CPU 900, and housings (roots 6ES5980 and 6ES5981, about 30 part numbers)
• interface modules, for example V.24/RS232 for CPU 945, and SPM modules (roots 6ES5752 and 6ES5753, about 28 part numbers)
• S5-210 microcomputer systems (root 6ES5921)
• CPUs for S5-010 (root 6ES5900) and PG 605U handheld programmers (root 6ES5605)
• module holders and accessories (roots 6ES5731 and 6ES5741)
• terminal blocks, adapters and mounting accessories (roots 6ES5700, 6ES5703, 6ES5722)

🛠️ E-Repair services for SIMATIC S5

As Approved Partner Siemens – Industry Services, E-Repair supports the continuity of plants running S5 PLCs:

🔧 Component-level repair of CPUs, power supplies and modules in E-Repair's certified in-house laboratory, where technically feasible.

♻️ Refurbishment with replacement of components subject to ageing and functional testing according to the agreed scope of work.

🚚 Advance replacement with a refurbished product, if available in stock and after a compatibility check.

💾 Support in checking and copying the contents of memory modules.

✅ Standard 12-month warranty on the services for which it applies, extendable up to 24 months under the applicable conditions.

❓ Frequently asked questions

✔️ Can an S5 CPU still be repaired?

Yes, in most cases. E-Repair carries out a diagnosis and assesses a component-level repair; the proposal depends on the fault and on the condition of the unit.

✔️ Do I lose the program if I change the CPU?

It depends on where it is stored. If the program is on a removable memory module it can be reused; if it is only in RAM, a backup copy is needed.

✔️ Is it worth migrating to S7?

Migration requires rewriting the program and new machine checks. It is often better to keep the S5 running and plan the migration around production schedules.

📚 Further reading

👉 SIMATIC S5 · S5 I/O modules and communication processors · S5 HMI Legacy

ℹ️ The information provided is general and indicative. Before purchase, installation or commissioning, check the data against the official Siemens documentation or contact E-Repair.