Impulse SIO-104+16.485 (3562) - Manual (Page 6)

© Sealevel Systems, Inc. - 4 - SIO-104+16.485 User Manual
IRQ Selection
The SIO-104+16.485 has an interrupt selection jumper located at (E1), which should
be set prior to use, if an interrupt is required by your application software. Consult
the user manual for the application software being used to determine the proper
setting. The SIO-104+16.485 supports the following interrupts:
3, 4, 5, 7, 9, 10, 11, 12, and 15
Clock Modes
The SIO-104+16.485 utilizes a 14.7456 MHz oscillator. This is eight times faster
than the standard COM: port oscillator, which typically is 1.8432 MHz. This allows
the adapter to achieve a maximum data rate of 921.6Kbps. The following sections
outline the baud rate calculations and instructions for achieving your desired baud
rate.
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+16.485 is equipped with four 16C954 UARTs that
enable the RS-485 driver automatically.
Line Termination
Typically, each end of the RS-485 bus must have line-terminating resistors. The
SIO-104+16.485 features a 120
Ω resistor across each RS-485 input and a 510Ω
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+16.485 automatically suppresses this ‘Echo’.