Quatech RS-422 - Manual (Page 11)

SPAD
X4
X2
J2
J4
J5
Figure 4 --- Enable scratchpad registers
2.4.2 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, four, or eight. 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 page
10).
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.
Figure 5 --- Clock multiplier jumper options
SPAD
X4
X2
J2
J4
J5
factory default
6 DSCLP/SSCLP-200/300 User's Manual