Information Technology Reference
In-Depth Information
This is typically achieved with an opto-isolator which converts between the RS-232 interface
and the RS-485 interface, as shown in Figure 14.5. Data is transmitted from the RS-232 port
to the RS-485 line only if the RS-485 driver is in active mode. This active mode is controlled
by the RTS signal from the RS-232 port (or by detecting transitions on the transmit data line,
TD).
T+
T-
TD
RD
RTS
GND
RS-232
interface
Opto-
isolator
RS-485
interface
RS-232
interface
Opto-
isolator
RS-485
interface
R+
R -
COM
Figure 14.5
Isolation between RS-232 and RS-485
14.5 Exercises
14.5.1
Which device provides isolation between RS-232 and RS-485:
(a)
Capacitor
(b)
Opto-isolator
(c)
Integrator
(d)
Resistor
14.5.2
What is the maximum range for RS-422:
(a)
500 m
(b)
1.2 km
(c)
5 km
(d)
10 km
14.5.3
Outline the advantages of balanced lines as opposed to unbalanced lines.
14.6 Note from the author
The RS-422/RS-485 standard really does enhance the basic RS-232 standard and allows for
standard RS-232 connections to be increased to over 1.2 km (although at low bit rates and
unnoisy environments allows for even greater distances). A surprising thought in these days
of cheap electronics, and PC motherboards that cost less than $100, is that RS-232 to RS-
485 convertors are relatively expensive. In fact, it is possible to get a complete PC mother-
board for the same price as an RS-232/RS485 convertor (which contains just a few internal
components), but as the convertor saves lots of time and energy, they are probably worth the
high costs.
