If the device connected to the BL-521 or BL-819/830 does not support RTS/CTS handshaking then problems can occur which are usually seen as the data only being communicated in one direction.
Therefore, in the case of the BL-521 when connecting them to a device that does not support RTS/CTS, a physical change is required in the male to female grey connector that came with the BL-521. Later versions of the BL-521 (post 3.01) can change the RTS/CTS handshaking mode using an application called “BL521 Firmware Upgrade Utility”. Please see the BL-521 webpage for more information and downloads: BL-521 Information Note: You can determine the firmware version of your BL-521 by accessing the configuration menu with Hyperterminal and making a note of the version displayed on the Main Menu screen.
For the BL-819/830 the RTS/CTS handshaking mode can be changed using the BL819 configuration utility. Please see the BL819-830 webpage for more information and downloads: BL-819/830 Information