1 level translation – Telit Wireless Solutions GM862-GPS User Manual
Page 21
GM862-GPS Hardware User Guide
1vv0300728 Rev. 0 - 27/04/06
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Right reserved
page 21 of 55
Signal GM862-
GPS Pin
Number
Name Usage
RX_GPS
41
GPS Transmit line
Output line of GPS serial Port (
NMEA
protocol)
TX_GPS
35
GPS Receive line
Input line of GPS serial Port (
NMEA
protocol)
GND 2-4-6-8
Ground
ground
The default configuration is 4800 bps, 8, n, 1
GPS RX Lines and TX lines may need a dual supply isolation buffer like an FXLP34 to avoid CMOS
high states while in POWER SAVING.
4.1 level translation
In order to interface the Telit GM862-GPS 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 0/3V 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 for 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 driver and receiver 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 0-3V UART level to the RS232 level, while the
receiver is the translator from RS232 level to 0-3V UART.
In order to translate the whole set of control lines of the UART you will need:
- 5 driver
- 3 receiver
NOTE: The digital input lines working at 2.8VCMOS have an absolute maximum input voltage of 3,75V; therefore the
level translator IC shall not be powered by the +3.8V supply of the module. Instead it shall be powered from a +2.8V /
+3.0V (dedicated) power supply.
This is because in this way the level translator IC outputs on the module side (i.e. GM862-GPS inputs) will work at
+3.8V interface levels, stressing the module inputs at its maximum input voltage.
This can be acceptable for evaluation purposes, but not on production devices.