NE866B1_Hardware_Design_Guide
1VV0301354 Rev. 6
Page
40
of
70
2018-02-28
5.5.1.2.
MODEM SERIAL PORT 2
The secondary serial port on the NB866B1 is a CMOS1.8V with only TX signal which is
used for logging.
The signals of the NB866B1 serial port are:
PAD
Signal
I/O
Function
Type
NOTE
C1
TX_AUX
O
Auxiliary UART (TX Data
to DTE)
CMOS
1.8V
Main Debug & logging
port
NOTE:
In order to avoid a back powering effect it is recommended to avoid
having any HIGH logic level signal applied to the digital pins of the
NB866B1 when the module is not supplied or during a
powerup/reboot transition.
Using bidirectional level translators which do not support High Z
mode during power off is not recommended.
NOTE:
TX_AUX is shared with SPI functionality
SPI and AUX UART cannot work simultaniously
5.5.1.3.
RS232 LEVEL TRANSLATION
In order to interface the NB866B1 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 +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).