Detailed hardware description
Issue G 107
PCI dual UART
An Exar XR17D152 PCI based dual UART is provided on the APOLLO board, this
supports COM3 and COM4. COM3 is configured as a standard RS232 port, whilst COM4
is configured as a software selectable RS232, RS485 or RS422 port.
RS232 interfaces
The RS232 signals associated with COM3 and COM4 are routed to a 20-way 2.54mm
(0.1
") boxed header J22, which has been designed to provide a direct ribbon cable
connection to 9-way D-type plugs. For connector details, see page XX. (Reference
connector J22)
RS485/422 interfaces
The COM4 serial interface supports RS232, RS422 and RS485 interfaces.
The RS422 interface provides full duplex communication, in point to point or point to
multi-point configurations. The signals available are TXA, TXB, RXA, RXB and Ground.
The maximum cable length for an RS422 system is 4000ft and support is provided for up
to 10 receivers in a system.
RS485 is a half-duplex interface that provides combined TX and RX signals. The
maximum cable length for the RS485 interface is the same as for RS422 (4000ft), but
RS485 supports up to 32 transmitters and receivers on a single network. Only one
transmitter should be switched on at a time to prevent bus contention.
The PCI UART includes circuitry for half duplex flow control also referred to as Auto RTS
flow control, eliminating the requirement for software flow control. Auto RTS flow control
uses the serial port RTS control signal to enable and disable the RS485/RS422
transmitter. This can be configured in the BIOS (see page 65
).
Port RS232 IrDA RS422/485 Max Baud Rate
COM3 J22 N/A N/A 921.6K
COM4 J22 N/A J25 921.6K
When using connector J25 to interface to RS485 signals only the TXA/TXB signals
should be used.
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