Siemens LOGO! Bluetooth Serial Connection: 9600 8E1 Setup

Overview: Why the Siemens LOGO! Bluetooth Serial Link Fails

Connecting a PC to a Siemens LOGO! logic module via a Bluetooth serial bridge is a recurring field request, especially for older installations where the LOGO! sits inside a control cabinet with no Ethernet run and the only available programming interface is the onboard RJ12 programming port. Engineers routinely attempt to bridge this port to a Bluetooth SPP (Serial Port Profile) adapter so they can drive LOGO! Soft Comfort from a laptop without opening the cabinet. The most common failure mode is the PC-to-adapter COM port being set to 115200 baud with no parity, which is the factory default of nearly every Bluetooth-to-serial module. The LOGO! never responds because the baud rate and parity are wrong. The second most common failure is the Bluetooth module operating in a non-transparent data mode that wraps the payload in framing incompatible with the LOGO! master/slave polling protocol.

This reference walks through the exact serial parameters the LOGO! programming interface expects, the Bluetooth SPP profile settings required, the AT commands used to force transparent mode on the most common Bluetooth serial modules, the step-by-step commissioning procedure, and a verification sequence using LOGO! Soft Comfort.

LOGO! Serial Interface Architecture and Protocol

The Siemens LOGO! logic module exposes its programming interface on a female RJ12 jack on the front face of the module. The pinout is a proprietary Siemens assignment, not a standard RS-232 DCE layout, and the only correct way to land a PC on it is through an official Siemens LOGO! PC Cable. The cable contains an in-cable transceiver and presents a DB-9 male RS-232 DCE port (legacy cable) or a USB-CDC COM port (USB cable) to the host.

Parameter	LOGO! 6/7 Required Value	Default of Typical Bluetooth Module
Baud rate	9600	115200 (or 9600, jumper-selectable)
Data bits	8	8
Parity	Even	None
Stop bits	1	1
Flow control	None	None or XON/XOFF (configurable)
Data mode	Transparent, asynchronous	Often RLP-framed (non-transparent)
Total frame length	11 bits (1 start + 8 data + 1 parity + 1 stop)	10 bits (no parity)

Setting 8 N 1 at 115200 baud yields a 10-bit frame at a bit period of ~8.68 microseconds, while the LOGO! expects an 11-bit frame at 9600 baud with a bit period of ~104 microseconds. The transceiver inside the LOGO! simply does not decode anything at the wrong baud rate, and the master polling never receives a valid slave response, so the host software reports "no connection" with no further diagnostics.

Required Serial Parameters: Why 9600 8E1 Is Mandatory

The LOGO! programming port uses a fixed, non-negotiable serial configuration: 9600 baud, 8 data bits, even parity, 1 stop bit (8E1). This is hard-coded in the LOGO! firmware and cannot be changed from either LOGO! Soft Comfort or the LOGO!'s onboard menu. The baud rate was chosen because the onboard UART on the LOGO! base module must coexist with the application scan, and 9600 baud is the highest rate the bootloader and runtime firmware can service reliably while still meeting the scan time SLA.

1. 9600 baud on a short, shielded RS-232 cable inside a cabinet has sufficient noise margin to be reliable without hardware flow control, which the LOGO! port does not expose.
2. Setting the host to 115200 baud is the single most common reason the PC reports "no communication" with the LOGO!. The error message is almost always generic — LOGO! Soft Comfort displays "No connection to LOGO!" or simply times out on the Tools → Transfer → PC → LOGO! operation. There is no baud-rate-mismatch error code; the protocol stack just sees garbage and discards it.

Bluetooth SPP Profile and Virtual COM Port Mapping

Bluetooth-to-serial adapters that target industrial and embedded use cases implement the Bluetooth Serial Port Profile (SPP), defined in the Bluetooth SIG specification under profile UUID 0x1101. SPP emulates a wired RS-232 connection over an L2CAP RFCOMM channel and presents the remote side as a virtual COM port on the host operating system. On Windows the COM port is enumerated as COMx (e.g., COM7), on Linux as /dev/rfcomm0, and on macOS as /dev/tty.Bluetooth-Serial-Port.

OS	Virtual COM Path	Pairing Utility
Windows 10/11	COMx (assigned at pairing)	Settings → Bluetooth & devices → Add device
Linux (BlueZ)	/dev/rfcomm0	sudo rfcomm bind 0 <BT_ADDR> 1
macOS	/dev/tty.Bluetooth-Serial-Port	System Settings → Bluetooth
Android (USB-tethered)	Not standard	Requires SPP app (not for production)

For SPP to function as a transparent serial replacement, the adapter must satisfy three conditions:

1. The local SPP service must be published with no authentication or PIN challenge, or with a known PIN documented by the adapter vendor.
2. The RFCOMM channel must pass data byte-for-byte with no escaping, no RLP framing, and no command-mode switching (no +++ guard time escape).
3. The adapter's local UART — the side that faces the LOGO! PC cable — must be configured to 9600 8E1 with no flow control, matching the LOGO! expectation exactly.

Condition (3) is where most engineers fail. The adapter ships from the factory at 115200 8N1 because that is the default firmware profile; the local UART must be reconfigured before the adapter will talk to the LOGO! cable at all.

Transparent vs Non-Transparent Data Mode

Bluetooth SPP is inherently transparent — RFCOMM is a reliable, in-order, byte-stream channel. However, many industrial Bluetooth modules (especially HC-05, HC-06, SENA Parani, and certain Laird BT800 derivatives) ship in a "command mode" where the adapter intercepts AT commands on the local UART and only forwards application data when explicitly placed into data mode. This is the source of the second failure mode.

Mode	Behavior	Compatible with LOGO! Soft Comfort
Transparent data mode	All bytes forwarded byte-for-byte between UART and RFCOMM	Yes
Command (AT) mode	Bytes interpreted as AT commands; only ATO returns to data mode	No
Non-transparent with RLP	Payload framed with Radio Link Protocol headers	No (LOGO! master polls fail CRC)
Transparent with echo	Bytes echoed back to host, doubling the data stream	No (echo corrupts polling)

To force transparent data mode on the most common Bluetooth modules, the relevant AT commands are listed in the next section.

AT Command Configuration for Common Bluetooth Serial Modules

The exact AT command set depends on the module vendor. The two most common modules in industrial cabinets are the HC-05 (CSR BlueCore) and the SENA Parani-ESD (Sena Technologies). Configuration must be performed over the local UART while the module is in command mode — typically entered by holding the KEY pin high at power-up, or by sending $$$ within the first second of boot on the Parani.

HC-05 (firmware 2.0-20100601 and later)

1. Hold KEY high at power-up to enter AT mode (LED blinks slowly, ~2 s period).
2. Set host terminal to 38400 8N1 (HC-05 command mode default) with line endings \r\n.
3. Send the following:

AT+ORGL\r\n ; restore factory defaults AT+ROLE=0\r\n ; slave role (PC will be master) AT+UART=9600,0,2\r\n ; 9600 baud, no flow ctrl, even parity AT+PSWD="1234"\r\n ; default PIN (or your chosen PIN) AT+NAME=LOGO-BT\r\n ; friendly name visible to PC

Confirm with AT+UART? which should reply +UART:9600,0,2.

SENA Parani-ESD / Parani-SD1000

1. Enter command mode by sending $$$ within 1 second of power-up. The module replies CMD>.
2. Set the local UART to match the LOGO!:

AT\N0\r\n ; disable error correction mode (transparent) AT&Q6\r\n ; asynchronous data mode, 8E1 AT+CBST=7,0,0\r\n ; bearer service: 9600 V.32 async (legacy GSM syntax) AT+ICF=2,1\r\n ; character framing: 8 data, 1 stop, even parity AT&W\r\n ; write to NVRAM ATZ\r\n ; reset so new parameters take effect

The AT\N0 command is the critical one: it disables the Radio Link Protocol (RLP) that the Parani applies by default to compensate for GSM-style lossy links. RLP wraps each payload in a sliding-window frame that the LOGO! master polling protocol cannot decode.

Generic Laird / Fanstel BT modules

For Laird BT8xx and Fanstel BT832 modules running the LBT firmware stack, the configuration is performed through AT-mode after escaping into command mode with +++ (the S2 register controls the escape guard time, default 1 second):

AT+BTNAME="LOGO-Bridge"\r\n AT+BTSPP=1\r\n ; enable SPP server on channel 1 AT+UARTCFG=9600,8,E,1,N\r\n ; explicit 9600 8E1, no flow control AT&W\r\n ; commit to profile 0 ATZ\r\n ; warm reset

Step-by-Step Bluetooth-to-LOGO! Connection Procedure

The complete commissioning procedure for a Bluetooth bridge to a Siemens LOGO! 6 or LOGO! 7 module follows. Each step has a verifiable result before moving on.

Prerequisites

• LOGO! 6 (6ED1052-xxx08-0BA0 generation) or LOGO! 7 (6ED1052-xxx08-0BA1 generation) base module.
• Siemens LOGO! PC Cable (6ED1057-1AA00-0BA0) or LOGO! USB PC Cable (6ED1057-1CA00-0BA0).
• Bluetooth SPP adapter with documented AT command set (HC-05, Parani-ESD, or equivalent).
• Windows 10/11 PC with Bluetooth radio, or external USB Bluetooth dongle (CSR8510 or Cambridge Silicon Radio based).
• LOGO! Soft Comfort V8.0 or later (compatible with LOGO! 6/7).

Procedure

1. Configure the Bluetooth adapter. Using a wired USB-to-serial converter at 38400 8N1 (HC-05) or 9600 8N1 (Parani), connect the adapter's local UART to the PC and run the AT commands from the previous section. Verify the reply to the UART query command shows 9600 8E1.
2. Wire the LOGO! PC cable to the adapter. The DB-9 male end of the LOGO! PC cable carries full RS-232 levels. Wire DB-9 pin 2 (TXD from cable) to adapter RX, pin 3 (RXD on cable) to adapter TX, and pin 5 (GND) to adapter GND. Do not connect any flow-control pins; the LOGO! cable does not use them.
3. Plug the RJ12 end into the LOGO!. Power the LOGO! on. The Bluetooth adapter should be powered from a clean 5 V supply tapped from a separate regulator rather than directly from the LOGO! 5 V rail (see field caveats).
4. Pair the PC with the adapter. In Windows Settings → Bluetooth & devices → Add device, select the adapter (named per step 1, e.g., LOGO-BT). Enter the PIN configured in step 1. After pairing, note the outgoing COM port assigned in Device Manager → Ports (COM & LPT) → Standard Serial over Bluetooth link (COMx).
5. Confirm the COM port parameters. Open Device Manager, right-click the assigned COM port → Properties → Port Settings. Set:
   • Bits per second: 9600
   • Data bits: 8
   • Parity: Even
   • Stop bits: 1
   • Flow control: None
6. Launch LOGO! Soft Comfort. Open the project, then go to Tools → Transfer → PC → LOGO!.
7. Select the Bluetooth COM port. In the transfer dialog, click Interface and select the outgoing Bluetooth COM port (e.g., COM7).
8. Click Transfer. The software polls the LOGO! at 9600 8E1 and should establish a connection within 2-3 seconds.

Verification

Check	Expected Result
LOGO! Soft Comfort transfer dialog	Reports "Connected to LOGO! 6ED1..." with serial number
LOG! status LED (top right of module)	Solid green during transfer, resumes normal run indication afterward
Bluetooth adapter LED	Solid on during RFCOMM session, blinking idle
Windows Device Manager → port properties → Events	Shows "Connected" with PC address of the adapter

LOGO! Soft Comfort Verification Procedure

Beyond the transfer dialog, run the following sanity checks to confirm the link is stable enough for production use:

1. Online test. In LOGO! Soft Comfort, select Tools → Online Test. The software continuously polls the LOGO! and displays I/O states in real time. Watch for "Communication error" pop-ups; more than one per minute indicates a marginal link (usually flow-control mismatch or RF interference).
2. Upload / Download round trip. Download the project from the PC to the LOGO!, then upload it back and compare the file hash. Any drift indicates framing corruption on the Bluetooth link.
3. Long-duration soak. Leave the online test running for at least 30 minutes. Bluetooth SPP can drop packets under Wi-Fi coexistence pressure; if the connection is lost, the RFCOMM channel closes and the COM port must be reopened.
4. Latency check. In the online test, toggle a digital output from the PC and measure the round-trip time displayed in the I/O monitor. A healthy Bluetooth bridge shows < 250 ms; anything > 1 s indicates the adapter is buffering or the local UART is being throttled.

Alternatives: LOGO! 8 Ethernet and LOGO! CMR (Teleservice)

For new deployments, Bluetooth serial bridging should be considered a legacy fallback. Two Siemens-supported alternatives offer superior reliability.

LOGO! 8 (6ED1052 generation with native Ethernet)

The LOGO! 8 generation adds a 10/100 Mbit Ethernet port on the base module. Programming is performed over TCP port 1352 using the LOGO! Soft Comfort Ethernet interface. There is no serial configuration to get wrong; you set the IP address (default 192.168.0.10), point LOGO! Soft Comfort at it, and transfer. Bluetooth is not required.

If the LOGO! 8 must be reached over a cellular or radio link, Siemens offers the LOGO! CMR (Communication Module Radio, in the 6BK1700 family) and LOGO! CMK (Communication Module variants for K-bus / KNX). The CMR provides 4G/2G fallback with an integrated VPN client (Sinema RC) and exposes the LOGO! 8 Ethernet port as a remote endpoint.

LOGO! Teleservice over GSM

For older LOGO! 6/7 sites where a wired Ethernet drop is impossible and Bluetooth is unreliable, Siemens supports Teleservice over GSM using a Siemens Modem MD720 (6NH9720-3AA00 family) connected to the LOGO! PC cable's RS-232 port. The MD720 is configured with the same AT commands referenced above (AT\N0, AT&Q6, AT+ICF=2,1) to force transparent 9600 8E1 mode. This is the documented Siemens pathway for remote service and is far more deterministic than a consumer Bluetooth SPP adapter.

Troubleshooting Matrix

Symptom	Probable Root Cause	Fix
LOGO! Soft Comfort: "No connection" after 30 s	Baud rate mismatch (115200 instead of 9600)	Set COM port to 9600
Connection establishes, but every upload corrupts	Parity mismatch (None instead of Even)	Set parity to Even
Adapter pairs, but COM port never enumerates	SPP service not advertised on the adapter	Re-check AT+BTSPP=1 or equivalent; cycle power
Transfer succeeds once, fails on retry	RFCOMM channel not closing cleanly; COM port locked	Close LOGO! Soft Comfort, disable/re-enable Bluetooth COM in Device Manager
Online test loses link every ~10 minutes	Wi-Fi coexistence deauth; Bluetooth buffer overflow	Move Wi-Fi AP away from adapter, or use a 5 GHz Wi-Fi band
Pairing fails repeatedly	PIN mismatch between PC and adapter	Verify AT+PSWD="1234" on adapter matches Windows PIN entry
Adapter LED solid, but no bytes on TX	Adapter still in AT command mode (escape sequence pending)	Send ATO to return to data mode, or power-cycle
Transfer works on bench, fails at cabinet	RF path loss; cabinet steel attenuates Bluetooth	Mount adapter on cabinet exterior, or use external antenna variant
Slow transfer (> 5 min for a 200-block program)	Stop bits wrong, retransmits at framing layer	Verify 1 stop bit on both sides

Field-Proven Caveats and Edge Cases

Several issues surface repeatedly in field deployments that are not obvious from the documentation.

Bluetooth coexistence with Wi-Fi

Bluetooth and Wi-Fi 2.4 GHz share the ISM band. If the cabinet has a Wi-Fi AP or a wireless sensor nearby, the Bluetooth SPP link will experience bursts of latency. This is not a bug; it is the Bluetooth Adaptive Frequency Hopping (AFH) algorithm reacting to Wi-Fi interference. Mitigation: enable Wi-Fi coexistence on the PC (default in Windows 10/11) or move the Bluetooth adapter away from the Wi-Fi antenna.

Adapter TX/RX swap

The DB-9 male on the LOGO! PC cable is wired as a DCE. The Bluetooth adapter is also typically a DCE on its local UART (it expects to be wired to a DTE). Connecting two DCE devices with a straight-through cable will yield TX-to-TX and RX-to-RX — neither side will see anything. Use a null-modem crossover between the cable and the adapter, or verify pin 2 (PC TX) goes to adapter RX and pin 3 (PC RX) goes to adapter TX with a multimeter.

Power supply noise

The LOGO! base module's 5 V supply (terminals U+ / U-) has significant ripple during the relay switching transitions. Tapping the Bluetooth adapter's VCC from the same 5 V rail will inject noise into the Bluetooth radio and cause intermittent link drops. Use a separate 5 V regulator (e.g., LM7805 with 100 microfarad input/output caps) fed from the LOGO!'s 24 V supply rail instead.

LOGO! Soft Comfort version mismatch

LOGO! Soft Comfort V8.0 and later dropped support for the legacy LOGO! 5 series. If the Bluetooth link is being tested against a LOGO! 5 module, LOGO! Soft Comfort V8 will not see the device even on a perfect serial link. Use an older LOGO! Soft Comfort V6.x for legacy hardware.

Even parity vs. forced mark/space

Some Bluetooth modules (notably the HC-05 with firmware older than 2.0-20100601) cannot generate even parity on the local UART — they only support none, odd, and forced-mark. If the parity query command returns an error, the module firmware is too old and must be reflashed. Modern HC-05 firmware supports even parity reliably.

Summary of Critical Parameters

Parameter	Value	Where Set
PC COM port baud	9600	Device Manager → Port Settings
PC COM port data bits	8	Device Manager → Port Settings
PC COM port parity	Even	Device Manager → Port Settings
PC COM port stop bits	1	Device Manager → Port Settings
PC COM port flow control	None	Device Manager → Port Settings
Bluetooth adapter UART	9600 8E1	AT command (e.g., AT+UART=9600,0,2)
Bluetooth SPP mode	Transparent	AT command (e.g., AT\N0, ATO)
Bluetooth PIN	Matches Windows pairing	AT+PSWD
Bluetooth role	Slave	AT+ROLE=0
LOGO! PC cable part number	6ED1057-1AA00-0BA0 (RS-232) or 6ED1057-1CA00-0BA0 (USB)	Siemens catalog

Frequently Asked Questions