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© Sealevel Systems, Inc. 3588 Manual | SL9008 9/2022
RS-485 Enable
RS-485 is optimized for party line or multi-drop applications. The output of the RS-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. Using the Sealevel SeaCOM driver, RS-485
enable is handled in hardware by the UART. This allows the RS-485 driver to be tri-stated when
inactive on a multi-drop polled network. The SIO-104-+8.485 is equipped with 16C954 UARTs that
enable the RS-485 drive automatically.
Line Termination
Typically, each end of the RS-485 bus must have line-terminating resistors. The SIO-104+8.485
features a 120-ohm resistor is across each RS-485 input and a 510-ohm pull-down resistor
combination that biases the receiver inputs. The board is not designed to have multiple adapters
on the same RS-485 network. 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 SIO-104+8.485 automatically suppresses
this ‘Echo’.
