CD2231 Intel Corporation, CD2231 Datasheet - Page 77

• Manufacturer Part Number: CD2231
• Description: CD2231 Intelligent Two-channel Lan And Wan Communications Controller
• Manufacturer: Intel Corporation

6.5.2
Datasheet
Table 13. Signal Hookup for Flow Control
Out-of-Band Flow Control
Receive out-of-band flow control is enabled when the CtsAE (CTS Automatic Enable) 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 characters 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 CD2231 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 CD2231 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 CD2231 has four pins that can be used either as a modem control or GPIO
(general-purpose input/output) pins. The modem signal names assigned to these four pins were
chosen 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 choose to connect the pins in any way appropriate to the application.
However, when the system software design employs automatic out-of-band flow control with the
pins, 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 CD2231 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.
control is desired.
For example, if the CD2231 is designed to be DCE and automatic out-of-band flow control is
desired, connect the DTR pin to the remote CTS input. If the CD2231 is to be used as the DTE
side, then connect the CD2231 CTS output to the remote CTS input.
Note that if automatic out-of-band flow control is implemented, the activity of the DTR and DSR
pins do not implement the function assigned to those signal names by the signaling conventions of
the CCITT and other standards organizations. These names would only apply to these pins if they
are under program control and not under automatic CD2231 control. In fact, the DTR function
enables the modem to go on- and off-line, depending on the state of the pin. If automatic control is
DCE
CTS
RTS
Mode
Intelligent Two-Channel LAN and WAN Communications Controller — CD2231
CTS
DTE
RTS
Table 13
CD2231 Pin
Name
DTR
CTS
RTS
shows the recommended signal hook-up if automatic out-of-band flow
Signal remote to transmit
Not implemented in this
direction
Request remote permission
to transmit
Enable transmitter
Flow Control
Out-of-Band
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