Rabbit OP7100 User Manual download pdf (Page 40)

34 eDisplay (OP7200)

3.5.1 RS-232

The OP7200 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

2000’s CMOS/TTL signals to RS-232 signal levels. Note that the polarity is reversed in an

RS-232 circuit so that a +5 V output becomes approximately -10 V and 0 V is output as

+10 V. The RS-232 transceiver also provides the proper line loading for reliable commu-

nication.

RS-232 can be used effectively at the OP7200’s maximum baud rate for distances of up to

15 m.

If you are planning to use any of the RS-232 serial ports and the RabbitNet port on the

OP7200, initialize the serial port(s) before you initialize the RabbitNet port. Section 4.5.4

provides some sample code to illustrates the sequence.

3.5.2 RS-485

The OP7200 has one RS-485 serial channel, which is connected to the Rabbit 2000 Serial

Port B through an RS-485 transceiver. The half-duplex communication uses an output

from the CPLD (U4) to control 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.

The OP7200 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. OP7200 Multidrop Network

RS-485

RS485+

GND

RS-485

RS485+

GND

RS-485

RS485+

GND

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Rabbit OP7100 User Manual Page 40