User manual

ZEUS user manual
Issue E
50
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.
RS485
This is a half-duplex interface that provides combined TX and RX signals (TX+/RX+ and
TX-/RX-). A ground connection is also required for this interface. The maximum cable
length for this interface is the same as RS422 - 1200m (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 ZEUS 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 transmitted from the
ZEUS is automatically echoed back to the receiver. This enables the serial
communications software to recognize that all data has been sent, and disable the
transmitter when required.