User manual

Detailed hardware description
Issue E 49
The control signal U3_TERM# 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 U3_TERM# signal:
U3_TERM# (EXP1_GPIO 2)
Termination resistors (120Ω)
0 Connected
1 Disconnected
U3_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.
U3_RS485_SLO# (EXP2_GPIO 5) Slew rate
0 Reduced
1 Normal
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.