User's Manual

CL865-DUAL Hardware User Guide
1vv0301104 Rev.1 – 2014-03-07
Reproduction forbidden without written authorization from Telit Communications S.p.A. - All Rights
Reserved. Page 42 of 70
Mod. 0805 2011-07 Rev.2
9.3. RS232 Level Translation
In order to interface the Telit CL865 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).