BL4S200 User’s Manual
36
3.3 Serial Communication
The BL4S200 has up to three serial communication ports, one RS-485 channel, and either
one RS-232 serial channel (with RTS/CTS) or two RS-232 (3-wire) channels. Table 7
summarizes the serial ports.
Two RabbitNet™ expansion ports are multiplexed from Serial Port D. The BL4S200 also
has one CMOS serial channel that serves as the programming port.
All three serial ports operate in an asynchronous mode. An asynchronous port can handle
7 or 8 data bits. A 9th bit address scheme, where an additional bit is sent to mark the first
byte of a message, is also supported. Serial Port A, the programming port, can be operated
alternately in the clocked serial mode. In this mode, a clock line synchronously clocks the
data in or out. Either of the two communicating devices can supply the clock. The BL4S200
boards supports standard asynchronous baud rates from 3.7 Mbps to 9.2 Mbps, depending
on the frequency the Rabbit microprocessor on a particular model is operating at.
3.3.1 RS-232
The BL4S200 RS-232 serial communication is supported by an RS-232 transceiver. This
transceiver provides the voltage output, slew rate, and input voltage immunity required to
meet the RS-232 serial communication protocol. Basically, the chip translates the Rabbit
microprocessor’s CMOS signals to RS-232 signal levels. Note that the polarity is reversed
in an RS-232 circuit so that a +3.3 V output becomes approximately -10 V and 0 V is out-
put as +10 V. The RS-232 transceiver also provides the proper line loading for reliable
communication.
RS-232 can be used effectively at the BL4S200’s maximum baud rate for distances of up
to 15 m.
3.3.2 RS-485
The BL4S200 has one two-wire RS-485 serial channel, which is connected to Serial Port C
through an RS-485 transceiver. This port operates in a half-duplex communication mode,
which requires directional control on the communication line.
Table 7. Serial Communication Configurations
BL4S200
Model
Serial Port
B
C
E
F
BL4S200
—
RS-485
RS-232
(PD6/PD7)
RS-232
(PD2/PD3)
BL4S210
RS-232
RS-485
—
—
BL5S220
—
RS-485
RS-232
(PD6/PD7)
RS-232
(PD2/PD3)
BL4S230
—
RS-485
RS-232
(PD6/PD7)
RS-232
(PD2/PD3)