User's Manual

GL865 Hardware User Guide

1vv0300910 Rev.1 – 2011-07-22

page 43 of 79

In order to avoid a back powering effect it is recommended to

avoid having any HIGH logic level signal applied to the digital

pins of the GE865 when the module is powered off or during an

ON/OFF transition.

9.2. RS232 level translation

In order to interface the GL865 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 0/2.8V 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 0-2.8V

UART to the RS232 level. The receiver is the translator from the

RS232 level to 0-2.8V 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 2.8V 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 +2.7V / +2.9V (dedicated) power

supply.

This is because in this way the level translator IC outputs on

the module side (i.e. GL865 inputs) will work at +3.8V interface

levels, damaging the module inputs.