Siemens SIMATIC TIWAY 1 - Manual (Page 65)

CONFIGURING THE HOST COMPUTER PORT
UNILINK HOST ADAPTER 3–7
USER MANUAL
3.2.4 XON/XOFF Flow Control
XON/XOFF flow control is provided at Port 3 to accommodate host computers
with small input buffers. This is achieved with the XON and XOFF flow control
characters sent from the host computer. Transmission will stop within four
characters after XOFF is received from the host. For BDLC, the XON and XOFF
characters must be sent as the 8-bit hexadecimal characters 11 and 13,
respectively, with the eighth bit set to zero. For NITP, the XON and XOFF
characters must be sent as 7-bit ASCII characters 11 and 13 with the parity
matching the host computer parity. This flow control will operate with either
the NITP or BDLC protocols, but the adapter must be set for Full Duplex
communications by setting Position 3 of Dipswitch 2 up.
The adapter will recover from host computer failures by resetting the flow
control when the starting delimiter of the next frame is received. However, that
frame will be discarded and must be retransmitted by the host computer.
If you are using a Network Interface Unit (NIU); i.e., Ethernet, Sytek, etc., or a
host device that implements XON/XOFF flow control with the BDLC protocol,
the BDLC data values 11 and 13 hex may inadvertently trigger the flow control.
If possible, you should deselect the XON/XOFF flow control on the NIU or host
device, or select the NITP protocol on the adapter. Flow control is only provided
as a convenience for the host devices; the adapter will not try to stop a host
transmission with XOFF.