Bioscrypt Inc. VSTICLASS Manual De Usuario

Document #430-90003-08

© Copyright 2003, Bioscrypt Inc.  All rights reserved.

Bioscrypt recommends that the RS-485 transmission line be terminated at both

ends.  The recommended termination at the PC end of the line is called Failsafe

termination.  This terminator (shown in figure 12a) ensures that there is a proper

bias voltage across the receiver inputs.  This, in turn, ensures that the receiver is in a

known state and puts less of a strain on the driver to provide that bias. This

termination is typically built into the RS-232/RS-485 converters and internal PC add-

on boards – you should confirm that such a termination exists, but you likely don’t

have to supply it yourself.
The termination at the opposite end of the transmission line should be parallel (or

passive) termination (see figure 12b). The value of R in figure 9b is chosen to

correspond to a proper parallel termination, R

T

, and it is chosen to be slightly

larger than the characteristic impedance of the cable, Z

O

. Over-termination tends

to be more desirable than under-termination since over-termination has been

observed to improve signal quality. R

T

 is typically chosen to be equal to Z

O

. When

over-termination is used R

T

 is typically chosen to be up to 10% larger than Z

O

. The

elimination of reflections permits higher data rates over longer cable lengths.

Power

Suppl

y

Unit

Unit

Unit

Unit

3

Data0

Data1

Wiegand

Ground

2

RS-232

/RS-485

Converter

Cat5

Twisted
Pair

Compute

r

Door

Controll

er

R

  R

R

 

+12 VDC  

Gnd 

   - 

   +  

 

R 

- (B) 

+ (A)