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3.3.1. Register x90h: Serial port
Used by the BIOS during the POST only.
Serial port 2 configuration/use
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Bit
7
6
Reset
Read
Write
Enable RTS2 to be used as 485TX enable when in 485 mode
ST1
(1: enable, 0: disable).
Enable RS-232 mode for serial port 2 (1: enable, 0: disable).
RS232
Enable RS-422/RS-485 mode for serial port 2 (1: enable, 0: disable).
RS485
NOTE
The RS232 and RS485 bits are initialized by the BIOS during POST
(Power-On Self Test). If a modification of these bits is required, be aware that
there is a hardware protection so that RS232 and RS485 buffers cannot be
activated at the same time. This protection is provided at the register level. If
you write to x90h register with bits 3 and 4 set, you will actually write 0 in both
of these bits. This condition can be read back.
5
4
3
2
0
0
0
RS485
RS232
ST1
RS485
RS232
ST1
3-5
Software Setups
1
0
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