Quatech DS-200, Руководство пользователя

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5. Data Rate Multiplier
A standard RS-422/485 serial port operates at a clock speed of 1.8432
MHz. In order to achieve higher data rates, the DS-200/300 Enhanced
Serial Adapter can operate at two times, four times (16750 UART only), or
even eight times (16750 UART only) this standard clock speed. The user
can set this speed, known as the data rate multiplier, in two different
ways: by modifying hardware jumper settings or by writing to a
DS-200/300 register known as the Options Register.
5.1 Data Rate Multiplier Jumper Block
The data rate multiplier jumper block (J9) is used to set the data rate
multiplier if software is not available to do so. This jumper block
contains five positions (three positions with the 16550 UARTs): X8 (16750
UART only), X4 (16750 UART only), X2, SW, and SP. Placing a jumper in
the X2, X4, or X8 positions sets the data rate multiplier accordingly.
Placing a jumper in the SW position tells the board to fetch its data rate
multiplier from the Options Register. The SP jumper position determines
access to two special registers on the DS-200/300: the Interrupt Status
Register (ISR) and the Options Register (OR). Virtually no commercially
available software makes use of the Scratchpad Register (SCR) on the
UART at [base 7]. By co-locating the new DS-200/300 registers at
this address, the use of additional I/O memory locations has been
avoided. If the SP jumper is installed, the SCR of the UART will be
accessed at [base 7], as in a standard serial port. However, if the
SP jumper is removed, then either the ISR (see Section 6) or the OR will be
accessed at [base 7].
5.2 Options Register
The DS-200/300 is equipped with an Options Register which has two
functions: a two-bit rate register which allows software to set the data rate
multiplier and a two-bit ID register which allows software to identify the
DS-200/300. The register is located at [base 7], and requires that
the SP jumper be removed and the DLAB bit in the Line Control Register
(LCR) of the UART be set to 1. See Figure 9 for the structure of the OR.
The ID register provides a means for software applications to identify the
board as a DS-200/300. It does this by performing logic on the bits
written to these locations so that the values read out of these locations
will not necessarily be the values that were written to these locations.
When performing a write-read operation to the ID register, the software
DS-200/300 User's Manual 5-1