In an RS-485 daisy chain connection it is crucial that the resistors are properly installed and the cable is an AWG 24 Twisted pair:
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 an 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.
Please Do Not Use Ethernet cable but only below recommended cable
|Belden 9841||24 AWG Str (0.20 mm²)||1||120 ohm|
|Belden 9842||24 AWG Str (0.20 mm²)||2||120 ohm|
|Belden 82841||24 AWG Str (0.20 mm²)||1||120 ohm|
|Belden 82842||24 AWG Str (0.20 mm²)||2||120 ohm|
|Belden 89841||24 AWG Str (0.20 mm²)||1||120 ohm|
|Belden 89842||24 AWG Str (0.20 mm²)||2||120 ohm|
|Alpha Wire 6412||24 AWG Str (0.20 mm²)||1||120 ohm|
|Alpha Wire 6413||24 AWG Str (0.20 mm²)||2||120 ohm|
|General Cable C0841A||24 AWG Str (0.20 mm²)||1||120 ohm|
|General Cable C0842A||24 AWG Str (0.20 mm²)||2||120 ohm|
|Connect-Air W241P2050FRIB||24 AWG Str (0.20 mm²)||1||120 ohm|
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.
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.
For BioStar 2 refer to the below article as well:
Maximum Slave Devices
One master device can connect up to 31 slave devices, but up to 8 fingerprint devices are able to be connectable through RS485.
We recommend a minimum number of RS485 connected fingerprint devices considering the number of users and connected devices.
This is because it can lead to slow matching speed depending on the device which is used for the master device.
However, CoreStation has a different specification. Corestation supports 5 RS-485 ports and up to 31 devices can be supported per port and a maximum of 64 slave devices can be supported by CoreStation in total.
Sample CoreStation Configuration 1
Port 1: 31 slave devices
Port 2: 31 slave devices
Port 3: 2 slave devices
Sample CoreStation Configuration 1
Port 1: 10 slave devices
Port 2: 10 slave devices
Port 3: 10 slave devices
Port 4: 10 slave devices
Port 5: 24 slave devices
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 the transmission line system complicated and hard to suppress line reflections if not impossible.
CoreStation Sample Configuration:
- Install the termination resistor on each ends of the daisy chain (red box)
- CoreStation has a internal termination resistor so turn the DIP switches on
A common misunderstanding in RS-485 network that it only requires two-wire cables is wrong, and it requires a ground wire to equalize node voltage differences. Ground potential differences are harmful in signal quality and signal transceiver’s reliability, so it 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.