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

The official system manual for Siemens SIMATIC S5-90U/S5-95U compact PLC covers the entire process of hardware structure, installation and wiring, power on startup, STEP 5 programming, I/O addressing, interrupt/counter/analog processing, real-time clock (only 95U), SINEC L1 communication, module expansion, and fault diagnosis for small automation scenarios. The S5-90U is a basic economy model, and the S5-95U is a high-performance enhanced model that supports S5-100U series module expansion. It adopts STEP 5 instruction table/ladder diagram programming and has three execution modes of cycle/interrupt/time control. It provides three-level troubleshooting methods including LED status, diagnostic byte, ISTACK/BSTACK. It is part of this series. The authoritative basis for PLC installation, debugging, programming, and maintenance.

Product model and core specifications

The positioning of the two models is different, and the S5-95U has more comprehensive functions.

Parameter S5-90U S5-95U

Onboard I/O 10 in/6 out 16 in/16 out

Analog I/O without 8 inputs/1 output

Interrupt input 1 channel and 4 channels

Counter 1 channel (1kHz) 2 channels (2kHz/5kHz)

User memory 4KB 16KB

Real time clock not available

Communication SINEC L1 (Slave) SINEC L1/L2 (Master/Slave)

Power supply 115/230V AC 24V DC

Hardware structure and electrical configuration

core unit

CPU、RAM、ROM、 Process Image Area (PII/PIQ), Battery Backup Unit

Programming interface: 15 pin D-sub, supports programmer/upper computer/SINEC L1

Electrical isolation

Onboard I/O adopts optocoupler isolation

Analog quantities, counters, interrupts, and control circuits are grounded together and only support non floating grounding

Installation and wiring specifications

Installation requirements

Method: 35mm rail or wall mounted (only 90U)

Spacing: Up and down ≥ 10cm, left and right ≥ 5cm

Environment: -10~55 ℃, no condensation, no dust

Key points of wiring

Digital I/O: 24V DC, grouped power supply, common M terminal

Analog quantity (95U): shielded wire, single ended grounded, length ≤ 50m

Interrupt/Counter: High speed signal, must be shielded, length ≤ 50m

Grounding: 400V system<10 Ω, 200V system<100 Ω

Operation mode and startup process

Three operating modes

STOP: Program stops, output is reset to zero

RUN: Loop program execution

Restart: Load DB1 parameters and execute OB21/OB22

Total cleaning operation (factory reset)

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

S5-95U: STOP → Remove battery → OFF → ON → Install battery

program loading

Auto: Power on and read from EEPROM

Manual (95U): Press and hold the COPY button to power on

STEP 5 Programming System

programming method

Instruction table STL, ladder diagram LAD, function block diagram CSF

Program block type

OB organizational blocks: OB1 (main loop), OB3 (interrupt), OB13 (timed)

PB program block: user-defined

FB Function Block: FB250 (Read Analog), FB251 (Write Analog)

DB data block: DB1 (system parameter)

SB sequential block (only 95U)

core instruction

Logic: A, AN, O, ON, X, XN

Timer: S5 timer, 10ms~9990s

Count: add count, 0~999

Transmission, comparison, arithmetic operation

Special functions (mainly exclusive to 95U)

Interrupt handling

4-channel interrupt input, fixed priority

Jump to OB3 after triggering, fast response

High-speed counter

2 independent counters, up to 5kHz

Support cascading into 32-bit counters

Analog signal processing

8 inputs, 1 output

Call FB250/FB251 read/write and calibration

real-time clock

Equipped with battery backup, supporting time comparison and running hour counting

communication

SINEC L1 bus, up to 31 stations

95U supports L2/DP and can be used as the main station

Diagnosis and troubleshooting

LED status

Green RUN: Running

Red STOP: Stop/Fault

Yellow BF (95U): Low battery

diagnostic tool

IB35 diagnostic byte: interrupt, count overflow, power, battery status

ISTACK: Interrupt stack, investigate the cause of the fault

BSTACK: Block Stack, Tracking Program Calls

Typical faults

Unable to enter RUN: Program error, DB1 error, I/O failure

Red light flashing: Program loading/saving failed

Key questions and answers

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

answer:

I/O and Resources: 95U with 8 analog channels, 3 interrupts, 1 high-speed counter, and memory ranging from 4KB to 16KB.

Advanced features: 95U exclusive real-time clock, time control execution PID、 Dual serial port, SINEC L2 master station.

Expansion: The 95U can directly mount 16 bus units; 90U requires IM90, up to 3.

Question 2: How to quickly troubleshoot the fault of PLC being unable to switch to RUN mode (red light constantly on)?

answer:

Use a programmer to check ISTACK and locate the cause of the malfunction.

Check if the DB1 parameters are configured incorrectly.

Check the wiring and bus connection of the external I/O module.

Reinstall the program after executing the overall cleaning.

Check if the 24V/power supply is functioning properly and if the battery is installed.

Question 3: How to use the analog function of S5-95U? What functional blocks must be called?

answer:

Hardware: 8 analog inputs (IW40~IW54), 1 analog output (QW40), using shielded twisted pair cables.

Software: Read using FB250, output using FB251, complete engineering quantity calibration.

Restrictions: The analog port is not isolated and must be grounded together, with a cable length of ≤ 50m.