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When a peripheral device transmits data to the host device through a
peripheral port, the data is received and stored in a 3740-character
receive buffer (or 7740 receive buffer for 525 units with Option 20)
which in turn retransmits it to the host device through the host port. 
After the buffer receives 3720 characters (or 7720 characters for 525
units with Option 20), the multiport will make the peripheral port CTS
(Clear-To-Send) line false (negative voltage), signaling the peripheral
device that it cannot accept more data (however, in reality it can
accept another 20 characters before overflowing the buffer).

If XON/XOFF is enabled and the peripheral device does not respond,
8 characters later the multiport will send an XOFF character, signaling
the peripheral device that it cannot accept more data.  (However, in
reality it can accept another 12 characters before overflowing the
buffer.)

When the buffer empties, the multiport will make the CTS line true
(positive voltage) and send an XON character (if XON/XOFF is
enabled), signaling the peripheral device that it can accept more data.

When the multiport is sending data to the host device through the host
port and the host device cannot receive any more data, the multiport
will expect to see a false condition on the DTR line or receive an
XOFF character.