User manual

LPC3250 Developer’s Kit v2 - User’s Guide

Page 30

4.17 SP9: RS422/485 Interface

There is a RS422/485 interface on the board. RS422 has individual transmit and receive pairs, while

RS485 share the same signal pair (and is hence half-duplex). By inserting both jumpers in JP9, the two

signal pairs are connected and a RS485 interface is created. By removing the jumpers in JP9 a RS422

interface is created. Via JP10 it is also possible to add termination resistors, if needed.

Direction is controlled via signal GPIO43 and is by default half-duplex (i.e., no simultaneous transmit

and receive). A high level enables the transmitter while a low level enables the receiver. It is possible

to remove R135 and R136 and control the transmitter and receiver individually via JP7 and JP8.

Note that on LPC3250, signal GPI_03 is connected to GPIO43 in the base board. This is an input

signal in the LPC3250, whereas an output signal is what is needed to control GPIO43. This is not

supported by the OEM base board, due to trade-offs when making different OEM boards and the base

board compatible. The RS422/485 interface is not directly supported for the LPC3250. Another

free GPO/GPIO pin has to be used for controlling GPIO43 and this requires re-wiring on the OEM base

board.

The interface component used (SN65HVD35) has built-in ESD protection.

See picture below for locating relevant components. The table below lists signal connections.

Figure 16 – RS422/484 Interface

Signals

LPC3250

GPIO25_RXD

U1_RX, UART#1 is used

GPIO24_TXD

U1_TX, UART#1 is used

GPIO43

GPI_03

RS422/485 part of Connector

J16

From top (pos 1)-to-bottom:

GND

TX-

TX+

RX-

RX+

RS422/485 Select

JP9

Termination Resistors Control

JP10

Transmit Control

JP7

Receive Control

JP8