Quatech RS-422 - Manual (Page 18)

4.5.2 Clock Rate Multiplier
A standard serial port operates at a clock speed of 1.8432 MHz. In order to
achieve higher data rates, Quatech Enhanced Serial Adapters can operate at two times
or four times this standard clock speed. This is controlled by the clock rate multiplier
bits in the Options Register.
Software can determine the UART clock frequency by reading the clock rate
multiplier bits RR1 and RR0 in the Options Register as shown in Figure 12. RR1 and
RR0 can be set by writing to the Options Register if the X2 and X4 jumpers (J4-J5) are
all removed. If one of these jumpers is applied, the RR1 and RR0 bits are forced to the
appropriate value. Reading the Options Register will always return the clock rate
multiplier at which the board is operating.
460.8 kbaud 7.3728 MHz X4 0 1
230.4 kbaud 3.6864 MHz X2 1 0
115.2 kbaud 1.8432 MHz
X1
(default)
0 0
Maximum Data
Rate
UART Clock
Frequency
Clock Rate
Multiplier
RR0 RR1
Figure 12 --- Rate Register bit definition
At powerup and reset, the Options Register is initialized to 0. The
DSCLP-200/300 will thus powerup in the x1 mode with software control of the clock
rate multiplier enabled as long as the X2 and X4 jumpers are not installed.
Software can control high baud rates through a combination of changing the
clock rate multiplier and the UART baud rate divisor. For example, a baud rate of
230.4 kbps could be achieved by setting the clock rate multiplier to X2 mode (or by
applying the X2 jumper) and setting a software application for 115.2 kbps.
The X4 clock multiplier settings
require the presence of 16750 UARTs.
Operation at X4 cannot
be guaranteed with 16550 UARTs.
DSCLP-200/300 User's Manual 13