VIPER
Detailed hardware description
COM3 – RS232 interface
Uses the Standard UART in the PXA255 (STUART). The port is buffered to RS232
levels by a 5V transceiver with ±15kV ESD protection, but does not support any
handshaking (RX / TX only). Maximum baud rate on this channel is 230kb/s (this is
limited by the transceiver performance).
COM4 – RS232 interface
Supported on Channel 0 of an external Exar XR16C2852 with 128bytes of Tx and Rx
FIFOs, and buffered to RS232 levels by a 5V transceiver with ±15kV ESD protection.
The maximum baud rate on this channel is 115kb/s.
COM5 – RS422/485 interface
Supported on Channel 1 of an external Exar XR16C2852 with 128bytes of Tx and Rx
FIFOs, and buffered to RS422/485 levels by a 5V transceiver with ±15kV ESD
protection, to provide support for RS422 (default) and RS485 (jumper selectable)
interfaces. The maximum baud rate on this channel is 115kb/s.
RS422
The RS422 interface provides full-duplex communication. The signals available are
TXA, TXB, RXA, RXB and Ground. The maximum cable length for an RS422 system is
4000ft (1200m) and supports 1 transmitter and up to 10 receivers.
To enable RS422 operation,
should be in position ‘B’. LK4 and LK5 should
be made if the board is at the end of the network. See
RS485/422 configuration – LK4,
, page
, for details.
RS485
This is a half-duplex interface that provides combined TX and RX signals. PL4 pin 5
provides TXB/RXB and pin 6 provides TXA/RXA. A ground connection is also required
for this interface. The maximum cable length for this interface is the same as 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.
The VIPER uses the RTS signal to control transmission. When this signal is at logic ‘1’
the driver is switched off and data can be received from other devices. When the RTS
line is at logic ‘0’ the driver is on. Any data that is transmitted from the VIPER is
automatically echoed back to the receiver. This enables the serial communications
software to detect that all data has been sent and disable the transmitter when required.
should be in position ‘A’ to enable the RS485 interface. The UART used
on the VIPER for COM5 has extended features including auto-RTS control for RS485.
This forces the RTS signal to change state (and therefore the direction of the RS485
transceivers) when the last bit of a character has been sent onto the wire. Please refer
to the XR16C2852 datasheet on the Development Kit CD.
provide parallel line termination resistors and should be made if the
VIPER is at the end of the network.
© 2004 Arcom Issue H
46
