5.1. Message Flow Control

It is important to remember that the user program is in charge of instrument communication at all times, independent of the communications bus used to control the instrument. The instrument cannot initiate communication, determine which device should be transmitting at a given time, or guarantee timing between messages. All of this is the responsibility of the user program.

Most commands sent to the Model 346 execute within 100 ms of the last character being transmitted. However, the Model 346 command protocol does not return an acknowledge message when a given command is finished executing. Because of this, a user program could potentially flood and overload the command buffer.

It is highly recommended to append the operation complete query (*OPC?) to the end of any given command string. The operation complete query is a mandated IEEE 488.2 query that will cause the instrument to send a “1” back to the user program when all commands have been processed. This scheme closes the timing loop and provides the proper balance for a user program to maximize command throughput yet not risk overrunning the instrument command buffer. *OPC? can be appended onto a single command or a chained message.