Measurement Computing Micro 488/EX rev.2.1 - Manual (Page 173)

Section 7 Peripheral Pass-Thru Operation
7.1
Peripheral Pass-Thru Operation
7.1 Introduction
The Peripheral Pass-Thru 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 Micro488/EX is buffered and transmitted
out its serial port. Data received from the serial device is buffered by the
Micro488/EX until read by the IEEE controller. The Micro488/EX can buffer
approximately 29,000 bytes of data from both the IEEE input and the serial input. The
actual amount of memory available for buffering is dependent on the memory which
has been allocated for Macro and Log buffer storage in the Controller or Periperal
Modes. Refer to Sections 4 and 5 for more information.
The Micro488/EX 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).
7.2 Serial and IEEE Input Buffers
Memory in the Micro488/EX 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 127 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 240 available queues for a total of 29,000 bytes of
buffer (character) space. Queues are continually allocated and released as required by
the serial and IEEE input. Of the 240 available queues, 230 are issued without regard
to controlling the receipt of additional serial or IEEE input data.