Siemens SIMATIC S5-90U/S5-95U Compact PLC

The official system manual for SIMATIC S5-90U/S5-95U compact PLC released by Siemens comprehensively covers product positioning, hardware design, installation and wiring, startup and debugging, STEP 5 programming, diagnosis and troubleshooting, I/O addressing, and communication networking processes. It specifies that S5-90U is an economical basic model and S5-95U is a high-performance enhanced model, supporting S5-100U module expansion, STEP 5 language programming, SINEC L1/L2 network communication, providing complete hardware specifications, wiring specifications, fault diagnosis methods, and module selection lists. It is a comprehensive guidance document for both PLCs from design to operation and maintenance.

Product positioning and hardware differences

Model Core Positioning Board I/O Memory Timer/Counter Features

S5-90U economical basic PLC 10 in 6 out 4KB 32/32 basic logic, 1-channel counter/interrupt

S5-95U high-performance enhanced PLC 16 in 16 out+8 analog inputs+1 analog output 16KB 128/128 PID, real-time clock, dual counter, time control program

Scalability:

S5-90U: IM90 interface module is required, up to 6 S5-100U modules can be connected

S5-95U: Direct expansion, up to 32 S5-100U modules can be connected

power supply

S5-90U: 115/230V AC direct power supply

S5-95U: 24V DC power supply, can be equipped with PS931 power module

Installation and wiring specifications

Installation method

Standard: 35mm DIN rail installation, supports wall mounting

Expansion: Up to 4 layers of rack, connected with IM315/IM316

Spacing: Vertical installation spacing ≥ 210mm, ambient temperature ≤ 40 ℃

Wiring requirements

Terminal: screw terminal, SIGUT double terminal, crimping terminal

Wiring: Separate wiring into A/B/C groups, simulate and shield communication lines

Grounding: equipotential connection, copper wire cross-sectional area ≥ 16mm ²

Analog/high-speed signals: Shielded cables must be used, with a single end grounded

Operation mode and startup process

working mode

STOP: Program stops, output disabled, data preserved

RUN: Loop program execution, running normally

REST: Restart, load DB1 parameters, execute OB21/OB22

Startup steps

Power off inspection of machinery and wiring

Connect the power supply and switch to STOP

Insert the battery and perform a power on self-test

Loading program (EEPROM automatic/manual)

Switch to RUN, test I/O and logic

Total reset (clearing program)

S5-90U: Power off → Remove battery → Wait for 15 seconds → Power on → Install battery

S5-95U: STOP → Remove battery → Turn off power → Turn on power → Install battery

STEP 5 Programming Core

Programming method: STEP 5, supports statement tables, ladder diagrams, and functional diagrams

Program Structure: Organizational Block OB, Program Block PB, Function Block FB, Data Block DB

core functionality

Interrupt input: 90U (1 point), 95U (4 points), OB3 processing

Counter: 90U (1-channel 1kHz), 95U (2-channel, up to 5kHz, supports 32-bit cascading)

Analog quantity: 95U comes with 8 inputs and 1 output, processed by FB250/FB251

Advanced features: 95U built-in PID algorithm (OB251), real-time clock

Diagnosis and troubleshooting

status indicator

RUN (green): Running

STOP (red): stop/malfunction

BF (yellow, only 95U): Battery failure

diagnostic tool

Diagnostic byte IB35: Monitoring interrupts, count overflow, power, battery status

ISTACK: Interrupt stack, check the cause of the fault (power, program, I/O errors)

BSTACK: Block Stack, Tracking Program Call Flow

Common Faults

Unable to switch RUN: Program error, I/O not ready, power exception

Program loss: battery failure, not backed up to EEPROM

I/O exception: wiring error, address error, module not encoded

Communication and Expansion Module

communication

Programming port: 15 pin 20mA TTY, connected to programmer/OP

Network: SINEC L1 (90U/95U), SINEC L2/DP (95U optional)

Protocol: 3964 (R), ASCII, PLC Link

Module Type

Digital quantity: input, output, hybrid module

Analog quantity: Analog input/output module

Functional modules: high-speed counting, positioning, closed-loop control, communication processor

Key questions and answers

Question 1: What are the differences in core hardware and functionality between S5-90U and S5-95U?

answer:

S5-90U: Economy entry-level model, 115/230V AC power supply, onboard 10 in 6 out, 4KB memory, 32 timers/counters, only supports basic logic and 1 counter/interrupt, requires IM90 to expand module.

S5-95U: High performance enhanced version, powered by 24V DC, onboard 16 in 16 out+8 analog in+1 analog out, 16KB memory, 128 timers/counters, built-in PID, real-time clock, dual high-speed counters, can directly expand up to 32 modules.

Question 2: How to backup and restore the programs of S5-90U/S5-95U?

answer:

Backup: Save the program to the EEPROM submodule. For S5-90U, use the COMPRESS programmer. For S5-95U, press and hold the COPY button for 3 seconds.

Recovery: Power off and insert EEPROM → Power on, automatic loading; S5-95U can be manually forced to load by pressing and holding the COPY button to power on.

Power outage maintenance: Relying on lithium batteries, RAM data is lost after power outage when there is no battery.

Question 3: Common reasons and troubleshooting methods for PLC unable to switch to RUN mode?

answer:

Common reasons

Program error or no valid program

External I/O module not ready or faulty

Abnormal power supply, battery failure

Scan time timeout, block stack overflow

troubleshooting method

Check the flashing/steady state of the red LED

Using a programmer to read ISTACK and locate the fault type

Reload the program after executing the overall reset

Check the coding, wiring, and power supply of the I/O module