Проблему решил. Из мануала вычитал:
This parameter defines the time interval during which the NPort fetches the serial data from its internal buffer. If data is incoming through the serial port, the NPort stores the data in the internal buffer. The NPort transmits data stored in the buffer via TCP/IP, but only if the internal buffer is full or if the force transmit time interval reaches the time specified under Force Transmit timeout. Optimal force transmit timeout differs according to your application, but it must be at least larger than one character interval within the specified baudrate. For example, assume that the serial port is set to 1200 bps, 8 data bits, 1 stop bit, and no parity. In this case, the total number of bits needed to send a character is 10 bits, and the time required to transfer one character is 10 (bits) / 1200 (bits/s) * 1000 (ms/s) = 8.3 ms. Therefore, you should set Force Transmit timeout to be larger than 8.3 ms. Force Transmit timeout is specified in milliseconds and must be larger than 10 ms. If the user wants to send the series of characters in a packet, the serial device attached to the NPort should send characters without time delay larger than Force Transmit timeout between characters and the total length of data must be smaller than or equal to the NPort’s internal buffer size. The serial communication buffer size of the NPort is 1 Kbytes per port.
Получается, что для скорости 57600 Force TX нужно ставить 1.