Quatech RS-485 user manual Clock Rate and Optional Registers, Enable Scratchpad Register SPAD, J2

2.3 Clock Rate and Optional Registers

Figure 3 shows the jumper configuration as shipped from the factory, with two spare jumpers applied in neutral positions. Remove one or both and apply as shown in following subsections to set optional features.

SPAD J2 J3

J4

J5

X8

X4

X2

Figure 3 --- Factory default clock rate and options settings

2.3.1 Enable Scratchpad Register (SPAD, J2)

In the default configuration, an Interrupt Status Register and an Options Register (see page 15) replace the scratchpad (base address + 7) of each UART. If the SPAD jumper is applied as in Figure 4, the UART scratchpad registers are enabled, and the Interrupt Status Register and the Options Register are not available.

SPAD	J2 J3	X8
	J4	X4
	J5	X2

Figure 4 --- Enable scratchpad registers

2.3.2 Force High-Speed UART Clock (X2, X4, or X8, J3-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 section 4.5).

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, four, or eight times the value for which the UART itself is programmed.

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