Section Four GPIB-PC Functions — Overview©National Instruments Corp. 4-11 GPIB-PC User ManualCount VariableThe IBCNT variable is updated after each read, write, or commandfunction call with the number of bytes actually transferred by theoperation.Read and Write TerminationThe IEEE-488 specification defines two methods of identifying the lastbyte of device-dependent (data) messages. The two methods permit aTalker to send data messages of any length without the Listener(s)knowing in advance the number of bytes in the transmission. The twomethods are as follows:• END message. In this method, the Talker asserts the EOI (EndOr Identify) signal simultaneously with transmission of the lastdata byte. By design, the Listener stops reading when it detectsa data message accompanied by EOI, regardless of the value ofthe byte.• End-of-string (EOS) character. In this method, the Talker usesa special character at the end of its data string. By priorarrangement, the Listener stops receiving data when it detectsthat character. Either a 7-bit ASCII character or a full 8-bitbinary byte may be used.The methods can be used individually or in combination. However, it isimportant that the Listener be properly configured to unambiguouslydetect the end of a transmission.The GPIB-PC always terminates IBRD operations on the END message.Using the configuration program, you can accommodate all permissibleforms of read and write termination. The default configuration settingsfor read and write termination can be changed at run time using theIBEOS and IBEOT functions, if necessary.
