Although *CHAPTER 2 didn't do anything when it was first issued, it has been accepted by the player when it was followed by the *FRAME command. This brings me on to the method for selecting SCSI communications mode. The SCSI mode change command is in fact a SCSI Start Unit command. All SCSI devices accept this command, which is one of the standard SCSI commands dating right back to the development of the first SCSI command set in about 1984. Whilst in serial mode, the LV-ROM player will ignore any transmissions received into its SCSI interface except a Start Unit command, which will be acted upon and will then select SCSI mode.

The difficulty in showing this in pictures is that certain of the VFS playback commands issue a Start Unit command whereas others don't. So, it looks here as if *FRAME issues a Start Unit command and then attempts to search for the frame requested. From the Start Unit command onwards, the player is then in SCSI mode and so the LV-ROM player issues a negative acknowledgement, because it's a CLV disc and there aren't any frame numbers, which is received by VFS and interpreted into its "Bad number" error. The next command goes through because we're now in SCSI mode, and so chapter 2 starts to play as requested by *CHAPTER 2.

There are two other points about mode changes. The first is that it's best not to assume that the player is already in the mode you want to use if you are planning on doing any programming with the VP415. If you want to write a utility for serial communications, make sure you start off by issuing the RS232 mode change command. Similarly, if you are going to communicate by SCSI, begin by issuing the SCSI Start Unit command. It doesn't matter if the player is already in the correct mode, but it does matter if it's not in the right mode and you don't send a mode change because the rest of the utility won't work!

The second and final point is that the default communications mode of the player on power-up is serial mode. It's a good job, too, since I wouldn't have had much hope of working with the player in serial mode if I'd had to work out the serial mode change command first!

Click here to return to Domesday 2010 Communications Modes