User's Manual

LE940B6 Hardware User Guide
1VV0301331 Rev. 1.8 - 2017-03-15
Reproduction forbidden without written authorization by Telit Communications S.p.A. - All Rights Reserved
Telit Confidential Information, provided under NDA Page
74 of 104
8.2.4. RS232 Level Translation
To interface the LE940B6 with a PC COM port or an RS232 (EIA/TIA-232) application, a level
translator is required. This level translator must perform the following actions:
• Invert the electrical signal in both directions
• Change the level from 0/1.8V to +15/-15V
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 0-1.8V UART to the RS232 level. The
receiver is the translator from the RS232 level to 0-1.8V UART. To translate the whole set of
control lines of the UART, the following is required:
• 2 drivers
• 2 receivers
Warning:
The digital input lines, operating at 1.8V CMOS levels, have absolute maximum input voltage of
2.0V. The level translator IC outputs on the module side (i.e. LE940B6 inputs) will cause
damage to the module inputs if the level translator is powered with +3.8V power.
So the level translator IC must be powered from a dedicated +1.8V power supply.