User`s manual

58 www.rabbit.com More Information on RabbitFLEX BL300F
5.2 Serial Communication
Dynamic C provides drivers for RS-232 and RS-485. For a full discussion of these drivers, see the Techni-
cal Note, TN213: “Rabbit Serial Port Software.”
5.2.1 Comparison of RS-232 and RS-485
Both RS-232 and RS-485 are popular and well-established serial communication protocols that are used
by many different devices. There are a number of considerations when choosing which serial protocols are
right for a particular application. The following table looks at both protocols and compares these consider-
ations.
RS-485 was developed to overcome some of the limitations of RS-232, such as the distance between trans-
mitter and receiver. RS-485 also has a more open topology, allowing the RabbitFLEX BL300F to be part
of a multidrop network of serially connected devices.
Figure 5.1 Multidrop Network Wiring
5.3.1 Termination Resistors
You can design the RS-485 circuit to include termination resistors or not. A third option is to have select-
able termination. For best performance in a multidrop network, termination resistors are enabled only on
the end nodes and are disabled on intervening nodes.
At the time of this writing, more information on the termination of transmission lines can be found at:
www.bb-elec.com/tech_articles/rs422_485_app_note
Table 5-3. Serial Communication Parameters
Parameter RS-232 RS-485
Max number of drivers 1 32
Mode of operation Full-duplex Half-duplex
Topology End-to-end
Multidrop, with node-to-node
communication
Max distance 15 m 1200 m
Max slew rate 30 V/µs n/a
485+
GND
485-
485+
GND
485-
485+
GND
485-