User's Manual

COMPONENT INSTALLATION

Table 1: RS-485 / RS-232 Communications Comparison

7.2.1 Basics of RS-485 networks

RS-485 is a networking specification similar to Ethernet, which is used for computer networks but RS-485 is not

compatible with modern computer networks.

RS-485 has two distinct advantages over the more common RS-232. First, it allows you to connect up to 31

PIV-Station readers to a PC using an external RS-232 to RS-485 converter available from Bioscrypt. Second,

the RS-485 specification allows for cable run lengths up to 4000 feet (1200 meters) at modest baud rates.

An RS-485 network is required instead of RS-232 if:

• There are multiple readers that must be connected together so that templates can be distributed among

the readers

• There is only a single reader, but it is over 150 feet (45 meters) from the host PC.

7.3 RS-485 Cable Specification

The PIV-Station readers provide a 2-wire, half-duplex RS-485 interface. The main run cable should be low

capacitance, twisted pair cable, with approximately 120-ohm characteristic impedance. Category 5 rated

communications cable is used in RS-485 networks and is defined in Table 2. This is the recommended cabling

for RS-485 communications. The cable connection includes a differential line (+ and -) and a GND connection.

Spec Recommendation

Capacitance (conductor to conductor) <20 pF/ft.

Characteristic Impedance 100 – 120 ohms

Nominal DC resistance <100 ohms/1000 ft.

Wire gauge 24 AWG stranded

Conductors/Shielding >2 pair (shielded optional)

Table 2: Cat 5 Cable Characteristics

In certain electrically noisy environments, a shielded cable may be required. It is often hard to determine if

shielded cable is required in an application or not but since the cost difference between shielded and unshielded

cables is very small, it’s worth using the shielded type.

Spec RS-485 RS-232

Mode of Operation: Differential DC Coupled Single-ended DC Coupled

DC Isolation: No No

Max. Distance: 4000 ft.* 150 ft.*

Number of Readers on one line: 31 1

Max. Data Rate 56 kbps 56 kbps*

*Communications distances are dependent on baud rate (bps). For example, with RS-232 at 9600 baud, a

distance of 150 feet is possible with shielded cable, but at 56 k-baud (kbps) a maximum of 20 feet is

recommended.