<LET /D=0>: The range of disk selection is 0-3 to select one of up to four drives in a system, where disk number zero is the first disk and disk number three is the fourth disk. Disk drives are connected via two cables, the 36 conductor data/control cable, and the 3 conductor power cable. When a <LET /D=> command is sent, the SDOS sends “switching signals” to the “drive number selected by the LET/D” command”. The signal “turns on the drive power”. The disk door handle (most disk drives) is also a switch. When the door handle is open, the disk drive cannot be selected. When the door is closed, the drive may be selected. The power supply is passive, in that power is supplied to all disks, ready to be switched by SDOS.
The cables to the disk drives are very simple. Connections are parallel. Each disk drive and the SDOS controller uses the same 36 pin connector. These connectors are simply “pressed on” to the 36 conductor cable and spaced so that the connectors will reach the plugs of the disk drives. For more than one disk drive, the power is connected via “Y splitter cables”. The parts to fabricate these cables are widely available and quite inexpensive.
Most disk drive power supplys have sufficient power to operate four drives. If erratic operation is experienced while operating a multi drive system, the door latches of all drives except the one in use may be opened to relieve the power drain. Power supplies should be enclosed in a metal cabinet for shielding. The signals carried within the 36 conductor cable can be distorted by radiation of EMF from a un-shielded power supply. Also, diskettes placed within range of such fields can be made un-readable.
Although we are limited to four “addressable” disk drives in a SDOS system, more drives may be operated. At right is a “speculated” system of eight drives of different size and track formats. Such a system would allow the use of 3″, 5 1/4″ and 8″ drives in a system, and two of each type drive. When more than four drives are connected, all except the one in use should have the drive doors open so that the parallel drive is dead. Parallel drives are two or more that have the internal drive switches set to the same “drive number”. One other consideration is the “terminating resistor” should be in the active chain. I hope that someone will comment upon this, as I have operate my four drive unit with the “door open” on the drive that has the resistor, and no ill effects have been noted.
<LET /S=1> would be needed after a drive has been selected IF the disk is a single side disk (it has only one read/write head). The SDOS system will set itself for double side drive if there is no LET/S=1 command. DS/DD diskettes may be used for single side formats, but not vice versa. A diskette that is formatted with LET/S=1 can be written to or read from by a DS/DD drive, but a single side drive cannot read a disk formatted with LET/S=2.
<LET/T=40> establishes the format for 40 tracks. Normally a 40 track diskette will format to 41 tracks and give reliable read and write results. This will give 400K of disk free on a 40 track or 805K on a 80 track diskette. A diskette will accept LET/T=44, and will indicate a good format, however I have found that the capacity is really only 400K, and disk read/write errors occur if there is an attempt to store more than 400K. (LET/T=82 and 805K for 80 track drive).
I have had almost a year of trouble free disk operation with two 5 1/4″ DS/Quad DENSITY drives purchased from BG Micro (price $89.00 each). One of the original drives would not read/write upon receipt and was replaced promptly and without question. (I thought some would like to know this.) SDU will discuss more about the SDOS BASIC commands in each issue.