![]() This let us know that the problem was not with the signals from the Alto but something inside the drive. It didn't even hold the head in place over a track. The drive didn't return the head to track 0 when the disk was unloaded. The drive did not seek when requested by the Alto or Carl's FPGA-based disk controller. Getting back to our disk drive that had problems seeking, we did some testing and determined that seeking had totally failed. Thus, the transducer outputs show the direction of head motion, the number of tracks moved, and alignment over the track. When the head is aligned over the track, the dot is at the top of the screen. The direction of rotation indicates which way the head is moving. The video below shows the two transducer outputs displayed in X-Y mode on an oscilloscope.Īs the head is (manually) moved, the dot rotates 360° on the screen for each track. The disk drive counts these pulses to determine the current track number.īy looking at the phase of the two signals, the drive can determine the direction of head movement. ![]() The transducer generates two " quadrature" signals 90° apart, The pointer and dial indicate what track the heads are on. The green head positioner transducer provides feedback to the head servo mechanism. The yellow pointer and the scale on the transducer show the track number visually. The head position transducer, the green disk in the photo below, provides electronic feedback on the head position. ![]() The head positioner motor (hidden underneath) moves the heads in and out to the appropriate track. The heads are in the foreground, two barely-visible white ceramic circles on flat metal arms. The photo below shows the disk heads and the head position transducer, a key component of the seek circuitry. In a modern system, the seek logic could be compactly implemented with a microcontroller.īut in the 1970s, controlling the heads took three boards full of integrated circuits. The next board computes the difference between the drive's current track and the desired track and determines how fast to move the head.įinally, the rightmost board is the analog board that drives the head positioner motor as well as processing head position signals from the transducer. The innermost board receives the desired track number from the Alto. These boards mostly use simple DTL (Diode Transistor Logic) gates, integrated circuits from the 1960s that predated TTL. ![]() The seek logic is implemented by the three circuit boards on the right. The drive has three circuit boards on the left and three on the right. The Diablo drive's circuitry pulls up for repair. Once the head reaches the desired track, the servo mechanism constantly adjusts the head positioner motor to keep the head centered over the track. To make seeks faster, the motor runs at four different speeds, accelerating quickly and then slowing as it approaches the desired track. Instead of a stepper motor, the drive moves the heads with a DC motor controlled as a servo. (Keep in mind that a track is only 0.007 inches (.18mm) wide.) Then, the heads must be held perfectly steady over the track. 3Īt this point, I should explain a bit about the Diablo drive and the complex mechanism it uses for seeking.įirst, when the Alto wants to read from a particular track, the drive must seek, moving the heads to the desired track as fast as possible. This was rather distressing since now we couldn't use our Alto. We suspected an electrical problem with the cursed drive had damaged the Alto's interface board or the good drive's circuitry. To make things worse, our previously-working drive started seeking erratically and then stopped seeking entirely. 2īoot almost worked, except any disk in the cursed drive got hopelessly corrupted. After installing another set of heads and fixing various other issues the cursed drive finally seemed to work, so we connected it to our Alto. I'll describe the full saga of the cursed drive in another post, but to make a long story short we installed new heads that immediately crashed so badly that the head arms were physically bent. Replacing the heads in the cursed drive should have taken an hour or so, but became much more complex. (Because of the problems this drive has caused, it will be called the cursed drive, although diabolical fits too.) Our disastrous adventure started when we tried to help out another Alto owner whose disk drive suffered a head crash. The disk isn't scratched it's just the lighting. Opening a disk cartridge reveals the single hard disk platter.
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