CD2401 Intel, CD2401 Datasheet - Page 71

• Manufacturer Part Number: CD2401
• Description: Multi-protocol Communications Controller
• Manufacturer: Intel

6.2.2
6.2.3
Datasheet
Receiver In-Band Flow Control
The channel can request the remote to stop transmission by sending an XOFF character. Likewise,
the channel can request the remote to restart transmission by sending an XON characters. The
XON/XOFF characters are transmitted by setting the SndSpc bit (STCR[3]) to ‘1’.
The CSR contains status bits RxFloff (receive flow off) and RxFlon (receive flow on), which are
used for receiver in-band flow control.
RxFloff = 0 is normal. RxFloff = 1 indicates the channel has requested that the remote stop
transmission. This bit is reset to ‘0’ when the channel requests that the remote restart transmission.
This bit is reset to ‘0’ when the receiver is enabled or disabled, or the channel is reset.
RxFlon = 0 is normal. RxFlon = 1 indicates that the channel has requested that the remote restart
transmission. This bit is reset to ‘0’ when the next non-flow control character is received. This bit
is reset to ‘0’ when the receiver is enabled or disabled, or the channel is reset.
Out-of-Band Flow Control
Receive out-of-band flow control is enabled when the CtsAE bit (COR2[1]) is set to ‘1’. In this
mode, character transmission begins only after the CTS* pin is active (low). In asynchronous
transmission if CTS* goes inactive (high) after transmission starts, the channel stops transmission
after the current character in the Transmit Shift register and the current character in the Transmit
Holding register are transmitted. In synchronous modes, if CTS* goes inactive, the channel stops
transmission after the current frame. In either case, transmission restarts after CTS* goes active.
The CD2401 can automatically flow control the remote device by the DTR* pin. This mode is
selected by setting a non-zero DTR* threshold in COR5; when both thresholds in COR4 and COR5
are exceeded, the CD2401 sets the DTR* pin high. When the data in the FIFO falls below the
DTR* threshold, the DTR* pin is automatically driven low.
Each channel of the CD2401 has four pins that can be used either as a modem control or general-
purpose input/output pins. The modem signal names assigned to these four pins were selected to
provide an easy reference for system designers. In fact, they are all simply general-purpose inputs
and outputs (if automatic out-of-band flow-control is not used) that can be individually controlled
by the Modem Signal Value register(s). Since the pins are general-purpose, system designers can
opt to connect the pins in a manner to suit the application.
However, when the system software design employs automatic out-of-band flow control with the
pins, then the signal naming convention no longer holds true in some cases, depending on whether
the device is used as DCE or DTE. In this case, it is best to think of the pins in terms of their actual
uses within the CD2401 and connect them accordingly, without regard to their names; the RTS*
and CTS* pins are associated with transmitter, and the DTR* and DSR* pins are associated with
the receiver.
flow control is desired.
Table 12 on page 72
shows the recommended signal hookup if automatic out-of-band
Multi-Protocol Communications Controller — CD2401
71