Data Buffering And Handshaking Schemes; Hardware Handshaking - National Instruments GPIB-232CV-A User Manual

Data Buffering and Handshaking Schemes

Hardware Handshaking

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Two protection mechanisms are used to ensure that the GPIB-232CV-A
does not lose incoming serial data: data buffering and handshaking.
The GPIB-232CV-A has an internal RAM buffer that stores incoming serial
data until it can output the data to the GPIB port. The size of this RAM
buffer determines how much serial data the GPIB-232CV-A can accept
before the buffer is completely full.
When its RAM buffer is nearly full, the GPIB-232CV-A can handshake
with the serial host to stop data transmission. When the buffer is almost
empty, the GPIB-232CV-A can again handshake with the serial host to start
data transmission. The GPIB-232CV-A is capable of using both the
XON/XOFF software handshaking and the hardware handshaking
protocols.
The hardware handshake function is always active during RS-232 transfers
and uses the Request to Send (RTS) and Clear to Send (CTS) signal lines.
When the GPIB-232CV-A is ready to accept serial data, it asserts the RTS
line. This signal remains asserted until the data buffer of the
GPIB-232CV-A is almost full. At this point, the GPIB-232CV-A unasserts
the RTS line, signaling to the serial host that the GPIB-232CV-A is no
longer ready to accept data. The serial host should monitor the RTS line and
suspend data transmission whenever the RTS line becomes unasserted. The
GPIB-232CV-A asserts RTS when it is again ready to receive serial data.
The GPIB-232CV-A is also able to suspend transmission when the serial
device is no longer ready to accept data. The GPIB-232CV-A is configured
to immediately stop transmission of serial data when CTS becomes
unasserted. The GPIB-232CV-A resumes transmission as soon as CTS is
reasserted.
Because most serial devices use the same form of hardware handshaking
as the GPIB-232CV-A, you can achieve bidirectional flow control by using
a serial cable that connects the RTS signal of the GPIB-232CV-A to the
CTS signal of the serial device. In addition, the RTS signal of the serial
device should be connected to the CTS signal of the GPIB-232CV-A. This
Chapter 3
3-11
Operation of the GPIB-232CV-A
'Draw circle
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'100 units.
GPIB-232CV-A User Manual