Impulse SIO4-104.2+2 (3544) - Manual (Page 7)

© Sealevel Systems, Inc. - 5 - SIO4-104.2+2 User Manual
Electrical Interface Selection
Each of the RS-422/485 ports on the SIO4-104.2+2 can be individually configured as RS-422, or
as a two wire RS-485 interface. This is a software selectable feature and is found at Base+15. To
enable a port for RS-422, simply write a ‘0’ to Base+15 (this is the power up default). To enable a
port for two-wire RS-485 simple write a ‘1’ to Base+15. If the Sealevel Software Windows
drivers are used, this is accomplished as a function of the Device Manager.
RS-485 Enable
RS-485 is backwardly compatible with RS-422; however it is optimized for party line or multi-
drop applications. The output of the RS-422/485 driver is capable of being active (enabled) or tri-
state (disabled). This capability allows multiple PCs to be connected in a multi-drop bus and
selectively polled. Half-duplex two-wire operation is also possible by connecting TX+ to RX+
and TX- to RX- in your cable hood. The enable to the driver is connected to the UART Request
to Send (RTS) line for RS-485 communications. This allows the RS-485 driver to be tri-stated
when inactive on a multi-drop polled network. The software that you are using must "know how"
to enable the driver when it is answering a poll. Failure to correctly utilize the enable can cause
transmitter contention problems preventing operation by any node on the network. If the Sealevel
Systems Windows Software driver is used, a ‘Radio’ style button on the ‘Advanced’ property
page under the Device manager can be selected that will automatically toggle RTS for use as an
enable.
If the SIO4-104.2+2 is equipped with the optional 16C954, (Item Number 3544-SN) a hardware
mode can be selected in the UART that will also enable the RS-485 drive automatically.
Line Termination
Typically, each end of the RS-485 bus must have line-terminating resistors (RS-422 terminates at
the receive end only). Typically a 120-ohm resistor is across each RS-422/485 input in addition to
a 1K-ohm pull-up/pull-down combination that biases the receiver inputs. If multiple
SIO4-104.2+2 adapters are configured in an RS-485 network, only the boards on each end should
have the termination in place. Contact Sealevel Technical Support if you have questions on how
to remove this termination.
RS-485 ‘Echo’
The RS-485 ‘Echo’ is the result of connecting the receiver inputs to the transmitter outputs. Every
time a character is transmitted; it is also received. The SIO4-104.2+2 automatically suppresses
this ‘Echo’.