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3.2.2 RS-485
The AT Communications Board (A12) 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 (2) sections describe a single connection and multi-drop
connection.
Some RS-485 networks may require terminating resistors at both ends of the serial
connections. 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 less than 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) to 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/index.cfm
2) RS-422/RS-485 Application Note
Copyright: B&B Electronics
http://www.ATSeries.net/PDFs/RS422+485AppNote.pdf
The communications board has a termination resistor built in. Jumper J22 enables or
disables the terminating resistor. If the host computer is on one (1) 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 on page
14 for configuration information for either Modbus or DNP3 protocols.
8
TB10 User Connection
Pin 1
Pin 2
(+) POS
(-) NEG
Pin 3
COM
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