Rs-485 - HindlePower AT Series Operating Instructions Manual

HARDWARE

3.2.2 RS-485

The communications board supports half-duplex RS-485 communications. Set configuration
switch S1-5 to ON to select RS-485. TB10 terminal block is for the RS-485 connections.
The next two sections describe a single connection and multi-drop connection.
Some RS-485 networks may require terminating resistors at both ends of the serial network.
The decision of whether or not to use termination resistors should be based on the BAUD
rate, the cable distance, and the type of cable being used to build the network. In most cases
for BAUD rates under 19.2K, terminating resistors are not required. If termination resistors
are used, the network must be designed with the appropriate biasing resistors to ensure
reliable communications.
The biasing resistors are responsible for ensuring that the network remains in the idle state
when all drivers are tri-stated. In order to guarantee that the receivers remain in a known
state, +/- 200mV must always be maintained across the (+) and (-) or (A) and (B) RS-485
inputs. The use of termination resistors requires a significantly lower value of biasing
resistors which results in greater dc loading of the network.
Network design and biasing resistor calculations depend on the number of nodes on the
network, the type of drivers and receivers on the network, and any biasing already designed
into other devices sharing the network. As a result, whether or not use termination resistors
and biasing resistor calculation is beyond the scope of this manual. For more information on
biasing and termination details see the following references:
1) EIA/TIA-485 standard
Telecommunication Industry Association
http://www.tiaonline.org/standards/catalog/
2) RS422/485 Application Note
Copyright: B&B Electronics
http://www.bb-elec.com/tech_articles/rs422_485_app_note/table_of_contents.asp
The communications board has a termination resistor built in. Jumper J22 enables or
disables the terminating resistor. If the host computer is on one of the ends and does not
have a built in terminating resistor, add the resistor at the connection across the plus (+) and
minus (-). Each device needs to have a unique address number. Refer to Section 4 for
configuration information for either Modbus or DNP protocols.
8
TB10 User Connection
Pin 1 Pin 2
(+) POS (-) NEG
Pin 3
COM