Perle Systems 5500152-23 manual Parameter, Description, Tx FIFO Limit, Tx FIFO Trigger Level

In the FIFO Settings page, set the FIFO buffer levels using the parameters detailed in the table.

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Parameter	Description
Tx FIFO Limit	Sets the overall size of the Transmit FIFO buffer.You use this
	when you want to restrict the size of the buffer in order to control
	the data flow. This is useful when your application requires you
	to send small amounts of data but needs acknowledgement
	that the data has been sent.
	For example, if your application is designed to receive 4 bytes
	at a time, a larger value of say 16 would time out.
	Permitted values for an UltraPort card are 1 to 64.
Tx FIFO Trigger Level	Sets the level at which the com port Transmit FIFO buffer is
	filled with data each time a request for more data is made.
	For example, if set to 16 bytes you get 16 bytes at a time when
	driver requests data.
	Permitted values for an UltraPort card are 8, 16, 32 and 56.

UltraPort Serial Adaptors User Guide	Chapter 2 Installing drivers and host cards
Installing under Windows 2000/XP/Server 2003/Vista/Server 2008
Page 42