RS-485 signaling relies upon balanced and differential signaling scheme, and has many advantages over unbalanced signaling such as RS-232, such as strong noise immunity and multi drop configuration capability. These are the most frequently-asked questions on using RS-485 signaling systems, which are worth reading before designing a RS-485 BioStar network system.

The RS-485 transmission system requires twisted-pair cables for best results, to maximize its noise rejection characteristics. The characteristic impedance of the cable is recommended to be 120 ohms.

As the cable gets longer, it acts as a transmission line, and the signals tend to reflect at the end of the line. To suppress this reflection, it is recommended to add termination resistors between signal pairs at both ends of the cable. The BioStar devices have on board termination resistors that can be enabled through slide switches or software configuration, so you may enable it if a device is placed at one of the ends of the cable.

• Use proper resistor for termination. The resistance should match the characteristic impedance of the cable. As the RS-485 system recommends 120 ohm cables, the BioStar devices have 120 ohm resistors on board. The BioStar devices are designed for use with RS-485 cables that have a characteristic impedance of 120 ohms.
• Place at the right place. The resistors should be placed at both ends of the cable to suppress line reflection. DO NOT PLACE more than two termination resistors within a cable.

Use daisy chain.

Avoid using long stubs for each node. Long stubs act as transmission lines and should be kept as short as possible if unavoidable.

Do not use star-shaped wiring. This makes transmission line system complicated and hard to suppress line reflections if not impossible.

Ground
A common misunderstanding in an RS-485 network is that it only requires two-wire cables. This is wrong, and it requires a ground wire to equalize node voltage differences. Ground potential differences are harmful to signal quality and signal transceiver reliability, so they should be kept as low as possible within a network. A simple approach is to use ground wire at the cost of ground loop current.