User guide

UC864-E/G/WD/WDU Hardware User Guide

1vv0300766a Rev.4

- 03/02/09

8.2. RS232 Level Translation

In order to interface the Telit UC864-E/G/WD/WDU with a PC com port or a RS232

(EIA/TIA-232) application a level translator is required. This level translator must:

• invert the electrical signal in both directions;

• change the level from 0/2.6V to +15/-15V .

Actually, the RS232 UART 16450, 16550, 16650 & 16750 chipsets accept signals

with lower levels on the RS232 side (EIA/TIA-562), allowing a lower voltage-

multiplying ratio on the level translator. Note that the negative signal voltage must be

less than 0V and hence some sort of level translation is always required.

The simplest way to translate the levels and invert the signal is by using a single chip

level translator. There are a multitude of them, differing in the number of drivers and

receivers and in the levels (be sure to get a true RS232 level translator not a RS485

or other standards).

By convention the driver is the level translator from the 0-2.6V UART to the RS232

level. The receiver is the translator from the RS232 level to 0-2.6V UART.

In order to translate the whole set of control lines of the UART you will need:

• 5 drivers

• 3 receivers

NOTE:

The digital input lines working at 2.6V CMOS have an absolute maximum input

voltage of 3.0V; therefore the level translator IC shall not be powered by the +3.8V

supply of the module. Instead, it must be powered from a +2.6V / +2.9V (dedicated)

power supply.

This is because in this way the level translator IC outputs on the module side (i.e.

UC864-E/G/WD/WDU inputs) will work at +3.8V interface levels, damaging the

module inputs.