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CHAPTER 5: Peripheral Operation
5.1 Peripheral-Mode Operation
This mode of operation is useful in interfacing a serial device, such as a serial
printer, plotter or instrument, to an IEEE controller. Data which is sent by the
IEEE controller to the interface converter is buffered and transmitted out its
serial port. Data received from the serial device is buffered by the interface
converter until read by the IEEE controller. The interface converter can
buffer approximately 32,000 bytes of data from both the IEEE input and the
serial input.
The interface converter will refuse to accept more data from the IEEE
controller when its buffer memory is full. It does this by preventing
completion of the bus handshaking sequences. It will also request that
additional serial data not be sent by negating its Request To Send (RTS)
output or by transmitting the X-OFF ASCII character. The serial handshake
used depends on the handshake selection (see Chapter 3).
5.2 Serial and IEEE Input Buffers
Memory in the interface converter is dynamically allocated for the serial input
and IEEE input buffers. This allows for the most efficient partitioning of
memory for any given application.
At power on, or device clear, each buffer is allocated a 128-byte mini-buffer
or queue. When the serial input (or IEEE input) requires more buffer space,
additional queues are allocated. When a queue is empty, it is released from
the input buffers so that it may be re-allocated wherever it is required.
There are approximately 250 available queues for a total of 32,000 bytes of
buffer (character) space. Queues are continually allocated and released as
required by the serial and IEEE input. Of the 250 available queues, 240 are
issued without regard to controlling the receipt of additional serial or IEEE
input data.
When the serial input buffer requests one of the last 10 queues (in other
words, when there are 1280 character locations left), it signals the serial host
that it should stop sending data. This is accomplished by either unasserting
RTS or issuing “X-OFF,” depending on which serial handshake control has
been switch selected.
5. Peripheral Operation