Quatech DSCLP-100 - Manual (Page 8)

2.3 Force High-Speed UART Clock (X2 or X4, J4-J5)
These jumpers force an increase of the UART input clock frequency by a
factor of two or four. This feature can allow legacy software to use baud rates
above 115,200 bits per second. It is also useful if the serial port device driver
does not directly support setting the higher baud rates through the Options
Register (see section 4.4).
If one of these jumpers is applied, it overrides any value written to the
Options Register to set the clock multiplier by software. The effective baud rate
will be either two or four times the value for which the UART itself is
programmed.
The factory default is none of these jumpers applied, which allows for
software control of the clock multiplier via the Options Register. The Options
Register powerup default is for a standard times-1 clock of 1.8432 MHz for
compatibility with standard serial ports.
SPAD
X2
X4
J2
J4
J5
Force times-two clock
Baud rates up to 230.4 kbps
SPAD
X2
X4
J2
J4
J5
Factory default
software control
SPAD
X2
X4
J2
J4
J5
Force times-four clock
Baud rates up to 460.8 kbps
Figure 4 --- Clock multiplier jumper options
8 DSCLP/SSCLP-100 User's Manual