Rockwell Automation 1400-SP PowerMonitor Software - Manual (Page 70)

Appendix B
Communications Application Note
B–2
RS–485, like RS–232C, is also an electrical standard used to provide serial
communications between electronic devices.
IBM PC–AT/286/386 or compatible computers can also communicate via
RS–485. This requires the installation of an RS–485 communications card or
“PORT” into one of the internal PC expansion slots. Alternatively, if the
computer is already equipped with an RS–232C serial port, an external
RS–232C to RS–485 converter device can be used to convert the existing
RS–232C port to RS–485.
When two electronic devices are equipped to communicate via RS–485, they
can be connected together via an RS–485 cable over which information is
transferred. This RS–485 cable consists of a 22 AWG shielded two
conductor twisted pair cable.
One of the advantages of RS–485 over RS–232C is that RS–485 can
communicate over much greater cable distances. The maximum length of an
RS–485 cable used to connect electronic devices is 4000 feet.
The most important feature of RS–485 communications is that it allows up
to 32 devices to be connected anywhere along the length of the two
conductor cable. This feature has important benefits for customers who wish
to connect a large number of Powermonitors to a single computer and to
display the data from any of the devices. RS–232C does not have this ability
as it allows only the connection of one device to each RS–232C serial port.
Advantages Of RS–232C:
1. Very easy to install.
2. Allows single cable connection from the computer to the Powermonitor.
Disadvantages Of RS–232C
1. Only one device can be connected to each computer RS–232C port.
2. Maximum cable length limitation = 50 feet.
3. RS–232C is more susceptible to electrical noise than RS–485.
Advantages Of RS–485
1. Very easy to install.
2. Up to 32 devices can be connected to a single computer serial port.
3. RS–485 can communicate over very large distances, up to 4000 feet.
4. RS–485 has greater immunity to electrical noise than RS–232C.
Disadvantages Of RS–485
1. Requires an RS–485 communications card to be installed into an empty
slot in the computer or an external RS–232C to RS–485 converter device.
What Is RS–485?
Summary Of Advantages And
Disadvantages Of RS–232C vs
RS–485