Remote Power Control for SIMATIC Panel PC 477B with SITOP UPS and S7-300 PLC

Engineer-grade reference for implementing automatic power-up and graceful shutdown of a SIMATIC Panel PC 477B running WinCC flexible RT in generator-backed industrial systems. The architecture uses a SIMATIC S7 315-2 PN/DP controller, a 24 VDC SITOP UPS module, and a latching relay to remove the requirement of a manual power button on the panel.

1. System Overview

The reference installation has the following boundaries:

Normal operating mode is generator-online. When the generator stops, the SITOP UPS picks up the load without interruption and signals "battery mode" to the PLC. The PLC then triggers a Windows shutdown on the Panel PC, waits for a safe off-delay, and finally drops the 24 V feed to the PC so the SITOP buffer can be preserved for the PLC alone.

2. Prerequisites

• SIMATIC Panel PC 477B with 24 VDC supply input per the device manual. Refer to the SIMATIC Panel PC 477B Compact Operating Instructions (PDF) for the connector pin-out and inrush current values.
• WinCC flexible 2008 SP3 or later, installed on the Panel PC 477B with the RT runtime license activated.
• STEP 7 V5.5 (or TIA Portal with S7-300 add-on) for the S7-315-2 PN/DP program.
• SITOP UPS1600 or SITOP DC UPS with buffer module (e.g. 6EP1931-2DC42 for 24 V / 6 A, 2.5 kWs) configured for "shutdown after time" mode.
• 24 VDC latching relay or contactor with NC power contacts, rated for the Panel PC 477B inrush (peak up to 30 A for <5 ms, steady-state 4-6 A depending on display size and load).
• Free digital output on the S7-300 for relay control; free digital input for the UPS battery-mode contact.

3. Hardware Architecture

3.1 Power path

Route the 24 VDC positive conductor feeding the Panel PC 477B through the normally-closed contact of a bistable relay (K1). The relay coil is driven from a PLC digital output module (e.g. SM322, DO 24V/0.5A). When the PLC output is OFF, K1 remains closed and the PC receives 24 V. When the PLC output is ON, K1 opens and the 24 V feed is removed, which emulates a hard power-off at the device terminals.

Use a bistable (latching) relay rather than a monostable type so that a brief loss of PLC power does not inadvertently disconnect the PC. Siemens Sirius 3RH21 or 3RT201 series contactors are commonly used for this purpose. For lower current (<3 A continuous) a Phoenix Contact PLC-OSC relay socket is acceptable.

3.2 Status path

The SITOP UPS exposes a potential-free relay contact that closes (or opens, configurable) when the unit transitions to battery operation. Wire this contact to a 24 V digital input on the S7-300 SM321. This single bit is the entire control input to the shutdown sequence.

3.3 Shutdown path

The PLC writes a boolean tag to the WinCC flexible RT runtime. The HMI tag triggers a scheduled task or a value-change event that executes shutdown.exe -s -t 0 -f on the Panel PC 477B. The -f flag forces running applications to close, which is required because WinCC RT will not self-terminate.

4. SITOP UPS Configuration

The SITOP UPS must be configured to not cut 24 V to its load until the connected PC has had time to complete its Windows shutdown. Configure as follows on the SITOP UPS1600 (or equivalent DC UPS module) using the SITOP UPS Manager or the DIP-switch web interface:

If you do not use the SITOP PC software, ensure the SITOP module is set to always supply output while battery is healthy, and rely on the PLC to drop the relay K1 instead. This guarantees the SITOP module does not pre-empt the shutdown sequence with its own hard cut-off.

5. S7-300 PLC Program

The PLC executes a small state machine. The recommended representation in STEP 7 ladder logic (FBD equivalent shown):

// Tags
// I0.0   UPS_BAT_MODE      (TRUE = running on battery)
// I0.1   PC_HEARTBEAT      (WinCC RT sets a bit every 1 s, optional)
// Q0.0   PC_POWER_RELAY    (TRUE = relay energized, PC OFF)
// M0.0   STATE_MACHINE
// MW2    TIMER_RUN

NETWORK 1: Detect battery mode (rising edge)
A   I0.0          // UPS_BAT_MODE
FP  M0.1          // pos edge flag
S   M0.0          // start shutdown sequence

NETWORK 2: Delay 15 s to let WinCC close
A   M0.0
L   S5T#15S       // 15 second off-delay
SD  T1
A   T1
=   Q0.0          // energize relay -> PC loses 24 V

NETWORK 3: Stabilization delay on power restore
A   "GEN_OK"      // generator stable, configurable per site
L   S5T#30S       // wait 30 s of stable mains
SD  T2
A   T2
R   M0.0          // clear shutdown state, relay de-energizes
NOP 0

The 15-second timer is the critical parameter: it must be long enough for the Panel PC 477B to complete the Windows shutdown sequence after shutdown.exe -s -f is launched. Field measurements on a 477B with WinCC flexible RT and a typical 1-2 GB project load 6-12 seconds; a 15-second default provides a comfortable margin. On heavily loaded panels with antivirus scanning, increase to 30 s.

The 30-second power-restore timer prevents rapid on-off cycling of the Panel PC in the event of unstable generator output. The PC requires approximately 45-90 seconds to complete POST, Windows boot, and WinCC RT start before it is again responsive on PROFINET.

6. WinCC Flexible RT Shutdown Script

On the Panel PC 477B, configure a tag in the WinCC flexible project (e.g. HMI_Tag_Shutdown, type Bool) connected to the S7-300 via PROFINET. Add the following C / VBS action (WinCC flexible supports VBS actions on Windows CE / XP Embedded panels):

' Project module: shutdown_action
' Triggered on value change of HMI_Tag_Shutdown = TRUE

Dim objShell
Set objShell = CreateObject("WScript.Shell")

' Force a synchronous shutdown, close apps
objShell.Run "shutdown.exe -s -t 0 -f", 0, True

Set objShell = Nothing

Configure the action to fire exactly once on a 0->1 transition. The internal WinCC flexible scheduler must be set to a cycle of 250 ms so the value-change event is caught within the 15-second PLC window. For Panel PC 477B units running Windows XP Embedded, the shutdown.exe binary is part of the standard image; for units upgraded to Windows 7 Embedded, place shutdown.exe in the C:\Windows\System32 folder.

7. Power-On Sequence (Wake-Up)

Because the Panel PC 477B has no IPMI, no WoL, and no scheduled RTC power-on, the only reliable wake mechanism is a hard power re-application. The PLC implements this via the bistable relay K1:

1. Generator restarts, mains 24 VDC is restored to the SITOP DC UPS.
2. SITOP output is stable after < 100 ms (battery module re-engages charging).
3. PLC timer T2 (power-stable) begins counting from 0.
4. After 30 s of stable mains, PLC output Q0.0 is cleared, K1 latches to NC, 24 V is applied to the Panel PC 477B input terminals.
5. Panel PC 477B POSTs, boots Windows, loads WinCC flexible RT, and resumes the PROFINET connection to the S7-315-2 PN/DP.
6. PROFINET reconnection is observable on the PLC as a return of the HMI connection; the S7-300 Connection Status bits in the online diagnostics should be monitored in your HMI tag list.

8. Verification and Commissioning

Before site deployment, perform the following bench tests:

Verify the dirty-shutdown indicator using fsutil dirty query C: from a command prompt on the Panel PC 477B after each test cycle. A clean shutdown should report Volume - C: is NOT Dirty. If "Dirty" persists, increase the off-delay timer in STEP 7 by 5-10 s increments until it clears.

9. LED and Front-Panel Reference

The Panel PC 477B front bezel carries a multi-color status LED (L1) and a group-fault LED (SF) which can be controlled by user programs. Refer to the SIMATIC Panel PC 477B Standard Operating Instructions (PDF) for the exact LED function table. Useful mappings for this project:

• PC powered, Windows running: L1 = GREEN, SF = OFF.
• PC powered, Windows shutting down: L1 = YELLOW (configurable), SF = OFF.
• PLC fault signal to PC: SF = RED. Use a WinAC or PLC tag to drive the SF LED through the WinCC flexible area pointer or a SIMATIC KeyTools configuration.

SIMATIC KeyTools is invoked from Start > Settings > Control Panel > SIMATIC KeyTools and is used to assign the SF / L1 LED control source. The official compact manual details the procedure for selecting the controller source.

10. Edge Cases and Field Notes

10.1 PC already off when generator returns

If the PC is off (relay open or 24 V removed) and mains returns, the PC will start because Q0.0 is reset to 0 and the bistable relay latches the NC contact closed. No further action is required.

10.2 PLC program in STOP

If the S7-300 transitions to STOP while the PC is on, the digital output module behaviour depends on the SM322 type. Most SM322 DO modules default to 0 V (output off) in STOP, which means Q0.0 = 0, K1 remains closed, and the PC stays powered. This is usually the desired behaviour; if not, configure a substitute value in the hardware configuration of STEP 7 to force Q0.0 to 1 in STOP.

10.3 WinCC flexible RT crashes

If WinCC RT fails to call the shutdown action, the PC will keep running on UPS battery until the SITOP module cuts 24 V (after 120 s default). Reduce this to 30 s in the SITOP UPS Manager to bound the worst-case runtime on battery. Monitor the UPS battery health monthly; a degraded battery module will collapse early and the PC will be hard-killed, producing a dirty NTFS volume.

10.4 PROFINET name and IP after restart

The Panel PC 477B is a PROFINET IO device in some topologies and a supervisor in others. Ensure the PROFINET device name and IP are configured for static assignment rather than DHCP, or the controller will not reconnect on first boot until the WinCC RT re-assigns them. Use the SIMATIC KeyTools PROFINET IO configuration pane.

11. Alternate Controller Platforms

The same architecture ports to other Siemens controllers with no logic changes:

• S7-1200 / S7-1500: replace the S5 timers with IEC timers (TP, TON, TOF). The S7-1500 system clock provides a 1 ms tick which gives finer timing resolution.
• LOGO! 8: sufficient for sites with no HMI on the PLC. Use two on-delay blocks (15 s and 30 s) and one RS flip-flop for the state memory.
• ET 200SP CPU: identical to S7-1500 logic, useful for distributed installations where the PLC is collocated with the SITOP UPS near the panel.

For third-party (non-Siemens) controllers driving the same relay, the logic is unchanged; only the tag names and timer blocks differ. Always place the relay coil driver on a DO rated for inductive loads (a 0.5 A SM322 output requires a free-wheel diode across the relay coil; SM322 modules with 2 A output are preferred for direct relay drive).

12. Troubleshooting Matrix