Measurement Computing Serial488A - Manual (Page 50)

4.1
Peripheral Operation
4.1 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 Serial488A is buffered and transmitted out its serial port. Data
received from the serial device is buffered by the Serial488A until read by the IEEE
controller. The Serial488A and the Serial488A/OEM can buffer approximately
32,000 bytes of data from both the IEEE input and the serial input. The
Serial488/512K can buffer 512,000 bytes of data from both the IEEE input and the
serial input.
The Serial488A 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 Xoff ASCII
character. The serial handshake used is dependent on the handshake selection (Refer
to Section 2).
4.2 Serial and IEEE Input Buffers
Memory in the Serial488A 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 when, and where, required.
There are approximately 250 available queues in the Serial488A and the
Serial488A/OEM 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.