CD2481 Datasheet, PDF (85/222 Pages) Intel Corporation – Programmable Four-Channel Communications Controller

Programmable Four-Channel Communications Controller — CD2481
7.5.3
to the threshold being exceeded. For this reason, the user should not use the Send Special Character
command in the STCR (Special Transmit Command register) to send XON/XOFF characters
because the CD2481 does not keep track of flow control characters that it did not send
automatically. The result could cause confusion on the other end of the connection due to
conflicting flow control commands.
Automatic in-band flow control is functional only in standard Async and Async-HDLC/PPP
modes; SLIP and MNP 4 expressly forbid in-band flow control. See the COR5 description for
programming details.
Out-of-Band Flow Control
Receive out-of-band flow control is enabled when the CTS Automatic Enable (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 has started,
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 will stop transmission after the current frame. In either case,
transmission will restart after CTS* becomes active again.
The CD2481 can automatically flow control the remote device via the DTR* pin. This mode is
selected by setting a non-zero DTR* threshold in COR5 (COR5[3:0]; when both thresholds in
COR4[3:0] and the threshold in COR5 are exceeded, the CD2481 will set 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 CD2481 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 have been 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 or examined via the modem signal value register(s). Since the pins are general-purpose,
system designers may choose to connect the pins in a manner that suits 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 CD2481 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. The table below shows Intel recommended signal hook-up if automatic, out-of-band flow
control is desired.
Mode
DCE
CTS
RTS
DTE
RTS
CTS
CD2481
Pin Name
DTR
RTS
CTS
Out-of-Band
Flow Control
Signal remote to transmit
Not implemented in this direction
Request remote permission to transmit
Enable transmitter
For example, if the CD2481 is designed to be DCE and automatic out-of-band flow control is
desired, the pin DTR should be connected to remote CTS input. If the CD2481 is to be used as the
DTE side, then the CD2481 CTS output would be connected to the remote CTS input.
Datasheet
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