LE940B6 HW User Guide
Rev. 2.02
Page 77 of 111
2020-01-10
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 ded1.8V power
supply.
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