Measurement Computing Micro 488/EX rev.2.1 - Manual (Page 178)

Section 7 Peripheral Pass-Thru Operation
7.6
DIO4 IEEE Input Buffer FULL
When this bit is set, it indicates that the Micro488/EX may hold
off the controller on subsequent data transfers. The interface may
continue to accept an additional 512 characters but this is
dependent on the serial input buffer size.
DIO3 Received Last Serial Terminator
If the Peripheral SRQ feature is enabled, the Micro488/EX will
issue a request for service by asserting the SRQ line and setting
this bit along with the rsv bit [DIO7]. It is cleared, along with rsv,
when serial polled by the controller. If this feature is not enabled,
this bit is always "0".
DIO2 Serial Handshake
This bit indicates the present state of the serial handshake. If it is
set to "1", the serial device connected to the Micro488/EX is
capable of accepting serial data. If "0", the RTS line is unasserted,
if configured for hardware handshake, or the XOFF character has
been received, if configured for XON/XOFF software handshake.
DIO1 Not Used - Always "0"
7.5 Use of Serial and Bus Terminators
The Micro488/EX can be configured to provide serial to IEEE 488 and IEEE 488
to serial terminator substitution. This is useful when interfacing a serial device, which
only issues carriage return [CR] as an output terminator, to an IEEE controller, which
expects a carriage return followed by a line feed [CR-LF].
In this previous case, the serial terminator should be selected for CR Only while
the IEEE terminator is set to CR-LF. When a serial CR character is received it is
discarded and substituted with an IEEE CR followed by an IEEE LF. In the IEEE to
serial direction, the IEEE CR is unconditionally discarded. Upon receipt of the IEEE
LF
a serial CR is substituted.