Take a peek at the shipping example VISA Serial Read -Write. With RS-232 devices there is usually some way to determine when a "packet" is completed. There are several popular methods:
Hardware Handshaking: There are a few flavors but each uses signal lines that are not the ones that carry data to establish when its ok to send data and when data is available. This used to be used more frequently than it is today and is more usefull in schemes where one sender has data once in a while and the reciever does not want to poll the device unless it sees a "Request to send." Modern computors and modern communication schemes are obsoleting the need for this.
XON/XOFF: The Reciever sends an XOFF charater to the transmitter when its recieve buffer is nearly full and the transmitter shuts up while the reciever processes its buffer. Again, modern computors and modern communication schemes are obsoleting the need for this.
Termination characters: The sending device sends a special charater at the end of every message (often a linefeed ASCII 0x0A) To implement this you need to make sure that your message does not contain that letter- You would be surprised how often you want to transfer raw ADC counts 0x00-0xFF rather than scaled readings (10.234e+5) takes 9 times as long to send.
VISA handles each of these and can automatically detect a read "End" and return your data right away. Otherwise, VISA read will return when the number of characters you ask for is read OR when the timeout expires. The default timeout is 10 seconds you never read 1024 characters.
Message Based flow is also popular and works by the sender always sending the packet length at the start of a message. There is no standard on this so VISA cannot implement it. Digi-International uses this a lot.
"Should be" isn't "Is" -Jay
