VectorStar VNAs provide a very large input buffer that can hold up to 100 commands. Each command plus any associated data can be as large as the amount of memory available in the VNA at the time. If a programmer attempts to exceed 100 commands, the GPIB bus will go into Not Ready For Data (NRFD) Holdoff. This Holdoff condition will hold onto the controller PC until a command is executed which frees up room for another command. Then the new command will be read in. Some controller PCs can detect this Holdoff condition and programmers interpret this condition as an Error. It is not an Error. Rather, it is a function provided by any listener device to guarantee that commands and data are not lost. It might be an ATE program error, but not a VNA error.