ZEUS Technical Manual

Detailed hardware description

 

 

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 I2C GPIO expander (I2C 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 I2C GPIO expander (I2C 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)

0Connected

1Disconnected

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#

Slew rate

(EXP2_GPIO 6)

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.

© 2007 Eurotech Ltd Issue D

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