Serial Ports (J18); Com Port Configuration; Com3 / Com4 Rs-485 Mode Line Driver Control - VersaLogic VL-EBX-37 Reference Manual

Serial Ports (J18)

The VL-EBX-37 features four on-board 16550-based serial channels located at standard PC I/O
addresses. COM1 and COM2 are RS-232 (115.2 Kbps) serial ports. IRQ lines are chosen in
CMOS Setup. COM ports normally cannot share interrupts with other COM ports or with other
devices.
COM3 and COM4 can be operated in RS-232 4-wire, RS-422 or RS-485 modes. Additional non-
standard baud rates are also available (programmable in the normal baud registers) of up to 921
Kbps. IRQ lines are chosen in CMOS Setup.
Each COM port can be independently enabled, disabled, or assigned a different I/O base address
in CMOS Setup.
COM P C
ORT ONFIGURATION
There are no configuration jumpers for COM1 and COM2 since they only operate in RS-232
mode. Use CMOS Setup to select between RS-232 4-wire, RS-422, and RS485 operating modes
for COM3 and COM4.
Jumper block V3 is used to enable the RS-422/485 termination resistor for COM3 and COM4.
Jumper V3[1-2] enables the RS-422/485 termination resistor for COM3, and jumper V3[3-4] for
COM4. The termination resistor should be enabled for RS-422 and the RS-485 endpoint station.
It should be disabled for RS-232 and the RS-485 intermediate station.
If RS-485 mode is used, the half-duplex differential twisted pair (TxD+/RxD+ and TxD-/RxD-)
is formed by connecting both transmit and receive pairs together. For example, on VL-CBR-5009
connectors J6 and J5, the TxD+/RxD+ signal is formed by connecting pins 3 and 5, and the TxD-
/RxD- signal is formed by connecting pins 2 and 4.
COM3 / COM4 RS-485 M
The VL-EBX-37 features automatic RS-485 direction control for COM3 and COM4. The
purpose of this function is to save the effort of RS-485 direction control in software. The
direction control signal RTS is used to tri-state the transmitter when no other data is available, so
that other nodes can use the shared lines.
RS-485 direction control is set using the Serial Port 3 > Mode and Serial Port 4 > Mode
parameters in CMOS Setup. To enable manual direction control, set the COM port mode to
RS485 ManuFC; to enable auto direction control, set the parameter to RS485 AutoFC. Manual
direction control is configured by asserting the RTS handshake line. Asserting the RTS
handshake line puts the RS-485 port in transmit mode; de-asserting the line puts it in receive
mode.
EBX-37 Reference Manual
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ODE INE RIVER ONTROL
Interfaces and Connectors
41