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Document Revision
Document number: 10551448
Release: May 21, 2015 15:13
Document version: 14


Bluetooth Serial Port Adapter™ firmware, cB-2254, with Wireless Multidrop™ and Extended Data Mode support (up to 7 simultaneous Bluetooth connections).

The firmware is valid for the following Serial Port Adapter™ models:

Products with Bluetooth low energy support

  • RBS421

OBS421 dual mode (Rev B):

  • OBS421i-24
  • OBS421x-24
  • OBS421i-26
  • OBS421x-26
  • OBS421i-C? (where ? is any digit)
  • OBS421x-C? (where ? is any digit)

OBS421 dual mode (Rev A):

  • OBS421i-14
  • OBS421j-14
  • OBS421x-14
  • OBS421i-16
  • OBS421j-16
  • OBS421x-16
  • OBS421i-B? (where ? is any digit)
  • OBS421j-B? (where ? is any digit)
  • OBS421x-B? (where ? is any digit)

This firmware is valid for the module versions listed above only. Firmware for other OBS421 module versions (modules without Bluetooth low energy support) is available in document cB-OBS421 Firmware

If you experience problems with your Serial Port Adapter™ please fill out an anomaly report and email it to

Version 5.3.2

Firmware Binary




5.3.2 [00:28:06,Apr 29 2015]

  • Fixed test command for production

Remember to also update to the latest version of the Bluetooth Serial Port Adapter AT Commands and Serial Port Adapter Toolbox when updating the firmware.

Known Limitations and Notes

Remote name does not work for peripherals with random address

The work-around is to use GATT AT-commands for this instead, see example below.


Read remote device name:


Re-bond with resolvable address does not work

A work-around is to first delete the bonded device and then do an inquiry to get the current address

Windows 7 Authentication Work-Around

The way that Microsoft Windows 7 handles Bluetooth connections, may in some use-cases, cause issues during the authentication phase between the device and the Bluetooth module. To solve these potential issues, connectBlue has implemented a Windows 7 authentication work-around in the Bluetooth module.

To enable the work-around, set the Windows 7 Work-Around feature bit using the feature bit mask register command.


The command needs to be called only once and the module must be restarted for the command to take affect.

Automatic Packet Control

An automatic packet control algorithm was implemented to improve link robustness for long range or if there are obstacles. It is enabled only for link policy 0 (see AT*AMLP). The algorithm decreases the packet size if the link is poor, to improve robustness. It prioritizes link robustness at the cost of keeping the best performance for as long as possible. If you find it too defensive and prefer to keep the bigger size packets for better throughput the automatic packet control algorithm can be disabled by setting link policy (0,1) or any other link policy than the default one (0).

  • Use link policy 0,0 for BR packets with the connectBlue packet control algorithm.
  • Use link policy 0,3 for EDR packets with the connectBlue packet control algorithm (faster for short range).
  • Use link policy 13,0 (or 0,1) for BR packets only without the connectBlue packet control algorithm.
  • Use link policy 2,0 for EDR packets without the connectBlue packet control algorithm.
Exit Sniff on Data Activity

When the module is configured with a link policy for sniff mode with exit sniff mode on data activity (Link policy (4,1), (5,1), (6,1), (7,1), (8,1)), there is a risk of data loss.

Removing of stored link keys

When upgrading from previous versions the stored link keys will be removed because of a new link key format.

Advertising and scan response data

For the v5.1.3 release the connectBlue Serial Service service UUID was added to the scan response data. It was added to the scan response data instead of the advertising data for backwards compatibility with earlier firmware versions. However, when a central device is using passive scan instead of active scan the central device will only see the advertising data and not the scan response data.

For example Apple recommends filtering on UUIDs when doing a scan in iOS apps and may use passive scan when the app is not active. This may require that the UUID is part of the advertising data instead of the scan response data.

Use the commands below to set the connectBlue Serial Service service UUID in the advertising data and set the device name in the scan response data:

AT commandDescription
AT*AGAD=020a04110701d7e9014ff344e7838fe226b9e15624,1Set advertising data to:
Tx power: 4dBm [020a04]
128-bit Service UUIDs, complete list: connectBlue Serial Service UUID[110701d7e9014ff344e7838fe226b9e15624]
AT*AGSD=07094f4253343231,1Set scan response data to:
Complete Local Name: OBS421 [07094f4253343231] where 
07 is the length of the data
09 is the AD type complete local name
4f4253343231 is the name data
to set a different name change the length and name data fields



Firmware Binary



cB-2254-11(fw_obs421_rd_v5.3.1).bin5.3.1 [23:47:57,Mar 31 2015]
  • Fixed problem when connecting to a shut-down previously bonded resolvable peripheral
  • Connection problem to privacy enabled low energy peripherals fixed.
    Read the chapter Bluetooth Serial Port Adapter AT Commands#Privacy thoroughly if low energy should be used.
  • iOS 8.2 connection problem on Bluetooth low energy fixed
  • Maximum number of links is increased to 7 for rev B and 6 for rev A.
    In order to connect more than 3 links the connection interval might be necessary to increase. Note that the rev A might need a higher connection interval for a specific number of links compared to rev B. See Bluetooth Serial Port Adapter AT Commands#WirelessMultidropConfiguration(AT*ADWM)
  • Serial port service for low energy connections without credit based flow control is now supported
  • Updated radio patch
  • Minor bug fixes
cB-2254-09(fw_obs421_rd_v5.2.1).bin5.2.1 [11:50:43,Jun 12 2014]
  • Bug fix for stability problems
  • Updated radio patch
cB-2254-08(fw_obs421_rd_v5.2.0).bin5.2.0 [00:55:10,Feb 27 2014]
  • Support for Rev B
  • Support for EN 300 328 v1.8.1 for Rev B products (product name ends with "-24", "-26", or "-C?" where ? is any digit)
cB-2254-07(fw_obs421_rd_v5.1.4).bin5.1.4 [20:18:39,Dec 12 2013]
  • Production related updates
  • Bug fixes for advertisements
cB-2254-06(fw_obs421_rd_v5.1.3).bin5.1.3 [16:31:02,Oct 24 2013]
  • Read bonded device reported wrong device when using mixed Classic and Low Energy bonds. This has been fixed now.
  • Firmware for the radio has been updated to the latest version which fixes some connect/disconnect issues.
  • connectBlue Serial Port service UUID added to scan response data. This makes it possible to filter on the Serial Port Service UUID when doing a scan in for example iOS or Android apps. Please see the notes section below for further information.
cB-2254-04(fw_obs421_rd_v5.1.1).bin5.1.1 [20:00:29,Jun 19 2013]
  • Bug fix for credit based flow control for connectBlue Low Energy Serial Port Service when operating in central mode with multiple connections.
cB-2254-03(fw_obs421_rd_v5.1.0).bin5.1.0 [17:01:45,Jun 11 2013]
  • Modified Serial Port Service protocol. This version of cB-OBS421 includes changes in the Serial Port Service protocol and is NOT backwards compatible. For communication with cB-OLS425/426, version 1.1.0 or later is required.
  • Faster connections.
  • Added new feature mask bits, including Power Reduction feature, see AT*AMWFM bit 14 in Bluetooth Serial Port Adapter AT Commands.
  • Support for custom advertise data, see AT*AGAD and AT*AGSD in Bluetooth Serial Port Adapter AT Commands.
  • Two new modes for Low Energy inquiry which shows all advertising data.
  • Changed default link loss timeout for Low Energy from 5 sec to 2 sec.
  • Changed default max connection interval from 6 to 8 for Low Energy.
cB-2254-02(fw_obs421_rd_v5.0.2).bin5.0.2 [15:49:16,Feb  1 2013]
  • The meaning of AT*ALE is slightly modified and is replaced by AT*AGLE.
    role_and_profile value 14 has been slightly modified and value 15 has been removed from AT*ADCP, AT*ADDCP and AT*ADDSP.