Command Input Buffer - Racal Instruments 1256 Manual

Publication No. 980855 Rev. A
Command Input
Buffer
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The 1256 uses a command buffer to store commands sent either
through the RS-232 or GPIB interfaces. The input buffer is 1024
characters in length.
If the input buffer fills to capacity and the GPIB interface is the
active interface, the NRFD signal is asserted on the GPIB bus.
This indicates to the system controller that the 1256 is not able to
accept more data until it parses existing commands and clears
space in the input buffer.
If the input buffer fills to capacity and RS-232 is the active
interface, several possibilities exist depending on flow control
arrangements. The 1256 instrument supports either no flow
control, software flow control (XON/XOFF), or hardware flow
control (CTS/RTS). Each scenario is described in more detail
below.
If no flow control is selected, the input buffer is full, and additional
commands are sent, the buffer will accept characters as it makes
space. It is entirely possible therefore, to have only parts of a
message queuing to the input buffer. This leaves unknown
commands on the buffer that when parsed will generate errors. For
this reason, if RS-232 communications are used without any flow
control, the user has the responsibility to insure that buffer overrun
conditions do not occur.
If software flow control is selected, and the input buffer reaches
80% capacity, the 1256 will send an XOFF character to the host.
The 80% threshold provides a reasonable safety margin to allow
for host software overhead. When the input buffer reaches the
60% capacity threshold, the 1256 will send an XON character to
the host to resume data transfer.
If hardware flow control is selected, and the input buffer is full, the
RTS signal from the 1256 will assert indicating to the remote host
than no further characters will be received. As soon as room exists
in the input buffer, the RTS signal will de-assert indicating that
room exists in the input buffer.
To make a flow control selection, select:
-----SETTINGS-----
on the top line, then select:
Type: RS-232
on the second line, and then:
Option: FlowCtrl
on the third line. Then choose the appropriate setting on the
1256 User Manual
SCPI Command Basics 5-25