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ZEUS Technical Manual Detailed hardware description
© 2007 Eurotech Ltd Issue D 55
COM4 – RS422/485 interface
Supported on Channel D of the external Quad UART, and buffered to RS422/485
levels with ±15kV ESD protection, this channel provides support for RS422 or RS485
(software selectable) interfaces. The maximum baud rate on this channel is 921.6
Kbaud. COM4 port is connected to the connector J19.
The GPIO15 pin of the I
2
C GPIO expander (I
2
C address 0x21) is used to select RS422
or RS485 interface mode as shown in the following table:
U4_RS485EN (EXP1_GPIO 15) Selected COM4 function
0 RS422 Full duplex
1 RS485 Half duplex
U4_RS485EN is at logic ‘1’ upon reset.
The GPIO16 pin of the I
2
C GPIO expander (I
2
C address 0x21) is used to
enable/disable the RS485/422 line termination and must be enabled if the ZEUS board
is at the end of the network. The following table shows the usage of U4_TERM# signal:
U4_TERM# (EXP1_GPIO 16)
Termination resistors (120Ω)
0 Connected
1 Disconnected
U4_TERM# is at logic ‘1’ upon reset.
The RS485/422 driver (LTC2859) on ZEUS features a logic-selectable reduced slew
mode that softens the driver output edges to control the high frequency EMI emissions
from equipment and data cables. The reduced slew rate mode is entered by taking the
SLO# pin low, where the data rate is limited to about 250kbps. Slew limiting also
mitigates the adverse effects of imperfect transmission line termination caused by
stubs or mismatched cables.
U4_RS485_SLO#
(EXP2_GPIO 6)
Slew rate
0 Reduced
1 Normal
Further details about the RS422 and RS485 interfaces are provided below.
RS422
The RS422 interface provides full-duplex communication. The signals available are
TX+, TX-, RX+, RX- and ground. The maximum cable length for an RS422 system is
1200m (4000ft). It supports one transmitter and up to ten receivers.