Serial Ports
22
– Reference
VSBC-8 Reference Manual
Serial Ports
The VSBC-8 features four on-board 16550 based serial channels located at standard PC I/O
addresses. COM1 and COM2 are RS-232 (115.2K baud) serial ports. IRQ lines are chosen in
CMOS Setup, and can be mapped to any IRQ line.
COM3 and COM4 can be operated in RS-232, RS-422, or RS-485 modes. Additional non-
standard baud rates are also available (programmable in the normal baud registers) of up to
1.5M. baud. IRQ lines are chosen in CMOS Setup, and can be mapped to any IRQ line.
Each COM port can be independently enabled or disabled in the CMOS Setup screen.
COM P
ORT
C
ONFIGURATION
There are no configuration jumpers for COM1 and COM2 since they only operate in RS-232
mode.
Jumper V3 is used to configure COM3 for RS-232/422/485 operation. Jumper V2 is used to
configure COM4. See page 14 for jumper configuration details.
COM3 / COM4 RS-485 M
ODE
L
INE
D
RIVER
C
ONTROL
The TxD+/TxD– differential line driver can be turned on and off by manipulating the DTR
handshaking line.
The following code example shows how to turn the line driver for COM3 or COM4 on and off:
mov dx,03ECh ; Point to COM3 Modem Control register
mov dx,02ECh ; or COM4 if desired
in al,dx ; Fetch existing value
or
al,01h ; Set bit D0
out
dx,al ; Turn DTR on (enables line driver)
in al,dx ; Fetch existing value
and al,0FEh ; Clear bit D0
out dx,al ; Turn DTR off (disables line driver)
