The concept of RS-485

[labmaster]

Dear all,

I am a physicist.
Recently, I was told RS-485 could be used as the multi-communication method like GPIB.
Like RS-232, can the desktop (or notebook) computer supports RS-485 port?
or should I buy a special interface card for the port?

How about the compatiblity between RS-232 and RS-485?
Can I use RS-485 method to RS-232 devices? or Is there a device for RS-485?

Please let me know some speciality of RS-485 such as the connection cable.

Thank you.

Lee

[JohnM 64]

Hi labmaster,

The RS-232 and RS-485 protocols are not compatible. They use the 9 pins of the serial port differently, and you therefore cannot use a 232 port with a 485 device or vice-versa. However, National Instruments does sell several products that would provide you with a 485 port from your laptop, such as this USB to 485 device:

NI USB-485

Hope this helps!

john m
Applications Engineer

[Ben]

Hi labmaster,

I am a physicist as well.

RS-232 and RS-485 are definitions of the electrical interface.

The data encoding in both schemes is the same, what differs the hardware.

RS-232 is way old and originally used a signal line and a return line. The signals used to be 0 and 12 volts but as time went on the 12V eventually became anything over 1.5 volts for a mark. RS-232 was intended for a single driver and single receiver. The original spec also limited it to only 50 feet.

RS-485 went to a differential scheme. This let them drive the line faster, longer, and picked up the option of multiple nodes. In the multi-drop scenario RS-485 comes in two flavors, 2-wire and 4-wire. In two wire the same pair of wires is used to transmit and recieve. In 4-wire the transmit and receive piars are distict. In two wire the interface has to switch from tranmit to recieve.

Companies like NI and Black Box make converters that will let you adapt from one scheme to the other.

Ben