Rabbit Coyote BL2500, User Manual

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User’s Manual 29
3.5.2 RS-485
The Coyote has one RS-485 serial channel, which is connected to the Rabbit 3000 Serial
Port D through an RS-485 transceiver. The half-duplex communication uses PA4 to con-
trol the transmit enable on the communication line. Using this scheme a strict master/slave
relationship must exist between devices to insure that no two devices attempt to drive the
bus simultaneously.
Serial Port D is configured in software for RS-485 as follows.
#define ser485open serDopen
#define ser485close serDclose
#define ser485wrFlush serDwrFlush
#define ser485rdFlush serDrdFlush
#define ser485putc serDputc
#define ser485getc serDgetc
#define DINBUFSIZE 15
#define DOUTBUFSIZE 15
#ifndef _485BAUD
#define _485BAUD 115200
#endif
The configuration shown above is based on circular buffers. RS-485 configuration may
also be done using functions from the
PACKET.LIB
library.
The Coyote can be used in an RS-485 multidrop network spanning up to 1200 m (4000 ft),
and there can be as many as 32 attached devices. Connect the 485+ to 485+ and 485– to
485– using single twisted-pair wires as shown in Figure 15. Note that a common ground is
recommended.
Figure 15. Coyote Multidrop Network
RS-485–
RS485+
GND
RS-485–
RS485+
GND
RS-485–
RS485+
GND