Measurement Computing Serial488/4 - Manual (Page 22)

Section 1 Introduction
2.10
buffer (character) space. Of the available queues, 398 are issued w ithout regard to
controlling the receipt of additional serial input data (handshaking).
When one of the last 32 queues is allocated the Serial488/4 signals the serial
devices that they should stop sending serial data. T his is accomplished by either un-
asserting RTS or sending the 'X OFF' control character depending upon how the
individual serial ports are programmed. Also the Serial488/4 sets a bit in its serial poll
status byte and optionally issues an SRQ to inform the host bus controller of the 'low
memory' condition. When more than 32 queues become available the serial devices
are signaled to allow more serial input data by asserting RTS or issuing the 'XON'
control character as appropriate. The 'low memory' bit of the serial poll status byte is
also cleared by more than 32 queues becoming
available, however if SRQs were
enabled the bit will remain set until at least one serial poll request is received after the
original 'low memory' warning.
When on of the last 16 queues is allocated the Serial488/4 w ill stop accepting
serial output data from the host bus controller. In this situation the host bus controller
should read any available serial input data or wait for the existing serial output data to
be sent to the serial devices before continuing.
2.5 Serial Port Configuration
The Serial488/4 has four separate serial ports, each of w hich can be configured
independently of the others. Complete serial port configuration is accomplished by
using both hardware jumpers and software commands. Hardware jumpers, located on
the main board behind each serial connector, are used
to to configure the port
electrically as either RS-232 or RS-422. Software configuration consists of various
commands for setting baud rate, stop bits, parity, and other asynchronous
communications characteristics.