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© Sealevel Systems, Inc. 12000 Manual | SL9245 10/2021
Subsystems of COM Express™
Connector Rows A&B
High Performance Serial Communication Ports
The 12000 provides three high speed serial communication ports supporting data rates up to 921.6kbps.
Ports 1 and 2 are RS-232 serial ports. Port 3 is an RS-485 port for communication with equipment up to
4000 ft. away from the computer or in noisy environments. The RS-485 2-
wire mode is optimized for ‘Multi
-
Drop’ or ‘Party
-
line’ operations selecting data from multiple peripherals (as many as 31 devices can be
connected on an RS-485 bus).
In RS-485 mode, our special auto-enable feature allows the RS485 ports to be viewed by the operating
system as a COM: port. This allows the software application to utilize the serial port for RS485
communication without the need to control the direction of data between the master and slave device. Our
on-board hardware automatically handles the RS-485 driver enabling.
Features of the serial ports include:
•
16C954 buffered UARTs with 128-byte FIFOs
•
Software configurable clock prescaler and divisor support wide range of baud rates
•
Supports 9-bit protocol framing
•
Each port supports data rates to 921.6K bps
•
All modem control signals implemented on RS-232 ports
•
RS-485 line termination, pull-up and pull-down resistors are selectable via dipswitch on Port 3
•
Automatic RS-485 enable/disable in hardware on Port 3
•
Uses Sealevel’s SeaCOM enhanced serial driver
Serial Port Setup
The 12000 Serial Ports are assigned I/O addresses and IRQs by the COM Express module BIOS or by a
‘Plug
-n-
Play’ Operating System.
Clock Modes
The 12000 Serial ports are implemented on a PCIe x1 lane and derive a 62.5 MHz clock from the PCI express
link. The 62.5 MHz clock is divided by an 8 bit clock prescalar and a 16 bit clock divisor to provide a wide
range of possible baud rates. Note that there are many combinations that can give the same result (e.g.,
Prescalar=1 and Divisor = 8, Prescalar =2 and Divisor = 4, or Prescalar=8 and Divisor = 1). As long as the
calculated data rate is /- 2% you should communicate fine.
