CD1865 datasheet(72/150 Pages) INTEL

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CD1865 — Intelligent Eight-Channel Communications Controller

Datasheet

7.3

Flow Control

Variations in response times and system data transfer rates between systems communicating across

asynchronous interfaces give rise to a need to control the flow of data between them. Systems

typically are implemented with a receive buffer for temporary storage of data. When this buffer is

nearly full, the receiving computer ‘flow-controls’ the remote transmitter. When, after processing

the existing data, more buffer space is available for the receive process, the receiving computer

signals the remote to resume transmission.

Flow control is implemented in one of two ways — ‘out-of-band’ or ‘in-band’ signaling. Out-of-

band signaling is a hardware-based mechanism, performed by extra wires such as the RTS/CTS

and DSR/DTR pairs. It has the advantage of complete independence from the data stream.

However, it is not always possible to provide all of the wires necessary to support Out-of-Band

Flow Control. Also standards for implementing Out-of-Band Flow Control vary widely.

In-Band Flow Control works by inserting special flow-control characters into the stream of data

being sent. It has the advantage that only the data circuit is required, thus only two wires are

needed. The disadvantage of In-Band Flow Control is that the two communicating computers must

perform additional functions, specifically, they must monitor the data stream for flow-control

characters and take the appropriate action. This can be quite burdensome because the host

computer that receives a flow-control command must recognize this event quickly and respond in a

timely manner to avoid overrun at the remote receiver.

Although there are advantages and disadvantages to each system, in general the trend is toward In-

Band Flow Control. This is because it is more useful than Out-of-Band Flow Control over a wider

range of applications, such as communication by modems.

The CD1865 provides significant performance advantages over conventional solutions during both

the receive processing of and the transmission of flow-control characters. It does this by handling

almost all flow control automatically, without host intervention. It also provides tools to make host

intervention, when required, much easier. Because the CD1865 performs flow-control functions

automatically, before the data is passed to the host, the response time required of the host is

substantially reduced. The possibility of data overrun is also reduced. Additionally, the flow-

control characters themselves can be stripped from the data stream, relieving the host from

processing them. The flow-control status of the transmitter is always available to the Host as a bit

in the Channel Control Status register (CCSR).

7.3.1

Receiver Flow Control

The CD1865 provides both In-Band (Xon/Xoff) and Out-of-Band Flow Control functions for

ensuring that the receiver does not overflow. In-Band Flow Control is semi-automatic and helps the

host manage its buffer size. Out-of-Band Flow Control is fully automatic and can be used to

prevent the CD1865 Receive FIFO from overflowing. Figure 25 on page 73 diagrams the receiver

flow-control logic.

00 81h

Send one-character time of line break

00 82h xxh

Delay for ‘xx’ prescaler time ticks (for example,

Transmit Timer Value is ‘xx’)

00 83h

Stop break

Char. Sequence

Effect

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