Telit Wireless Solutions CE910-DC, Hardware User'S Manual

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In order to interface the Telit CE910 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 0/1.8V to +/-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 is 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.
In order to translate the whole set of control lines of the UART you will need:

5 drivers

3 receivers
An example of RS232 level adaption circuitry could be accomplished using a MAXIM transceiver
(MAX218). In this case the chipset is capable of translating directly from 1.8V to the RS232 levels
(Example on 4 signals only).
The RS232 serial port lines are usually connected to a DB9 connector with the following layout: