Rabbit Fox LP3500, User Manual, Page: 32 / 143

26 Fox (LP3500)
3.4.1 RS-232
The LP3500 RS-232 serial communication is supported by an RS-232 transceiver. This trans-
ceiver 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 3000’s
logic-level signals to RS-232 signal levels. Note that the polarity is reversed in an RS-232
circuit so that a +2.8 V output becomes approximately -7 V and 0 V is output as +7 V. The
RS-232 transceiver also provides the proper line loading for reliable communication.
RS-232 can be used effectively at the LP3500’s maximum baud rate for distances of up to
15 m.
Logic-level signals are also possible on Serial Ports B, C, and E by changing the 0
Ω
surface-mounted resistor jumper settings at locations JP1–JP6.
Serial Port E can be set to “listen” and “wake up” the LP3500 when the unit is in a low-
power mode.
3.4.2 RS-485
The LP3500 has one RS-485 serial channel, which is connected to Serial Port F on the
Rabbit 3000 through an RS-485 transceiver. The half-duplex communication uses the
Rabbit 3000’s PG0 pin to control the transmit enable on the communication line.
The RS-485 transceiver used on the LP3500 is only capable of supporting a maximum
baud rate of 64,000 bits/s.
The LP3500 can be used in an RS-485 multidrop network. Connect the 485+ to 485+ and
485– to 485– using single twisted-pair wires (nonstranded, tinned) as shown in Figure 13.
Note that a common ground is recommended.