Microdrive Mysteries - unanswered questions...
Posted: Tue Feb 06, 2018 1:19 pm
Hi everyone
Prompted by recent testing of a batch of MDV units and inspired by Dave's recent announcement of a potential replacement MDV board being made available some time soon, I thought to gather together some of my own observations and unanswered questions that have accrued over time around the operation and circuitry of our (least?) favourite storage technology...
To be up-front - I still hold a flame to this Sinclair technology of the 80's - beyond all reason and after many hours of frustration and lost files over the years. What I learn about their design and operation has only served to fan the flame of my fascination for the microdrive!
All/any comments, additional questions - and answers - most welcome
+++
Much like the humble bumble-bee, which physics suggests simply shouldn't be able to fly and yet it does - the design of the microdrive - both cassette and circuitry shouldn't on the face of it ever work at all.
1. A continuous spool of tape has a finite difference in radius around the spool from the point where it enters the spool to where it exists. This must create a difference in linear speed at entry/exit, albeit small, yet the tape is pulled at a single linear speed (barring fluctuations). Tape stretch might account for this - but only if it both stretches AND contracts as it passes round the loop. Can any one explain this to me?
2. The ERASE function in the QL MDV design (which differs from the original ZX SPectrum approach) is rather funky in that, with the WP tab open, Vin (c. 9V) is routed not to the Erase coil, but instead to the READ/WRITE input of the ULA - effectively switching ULA mode from WR to RD. Atleast the 9v is divided across those 2x 2k2 resistors that are placed in the MDRDWL line, thus supplying c. 4.5V to the MDV ULA pin. Clever, except that now both MDV ULA _and_ the ZX8302 are both driving the RAW1/2 lines simultaneously with different signal levels. Why does this not hurt each ULA?
3. Perhaps related to the above, but just how is tieing RAW1/2 down to -12V via that pair of 33k Ohm resistors (as recommended in the mandatory modification list) supposed to improve MDV reliability? Is this something to do with ULAs that 'latch-up' their outputs (and that the Service Manual suggests - 'never left the factory'...)?
4. The Service Manual provides a very simple SuperBasic routine to effectively generate a constant 100 KHz square-wave on the tape. I know it does this successfully because I've measured the output with an oscilloscope - on both tracks. However, after enabling the selected MDV unit in WR mode, it simply writes a single 0x01 to the ULA Transmit register (and then pauses for about 8 seconds to cover the entire tape-loop.) I can understand that, if you only write once, the data for the second track is effectively taken from the same single ULA register (normal writing would write two bytes to this register spaced slightly apart in time), so both tracks get to see the same data - but then why does a 0x01 pattern translate after DME (differential Manchester encoding) into a constant 100 KHz waveform?
5. Does the video circuitry (PAL encoder) affect MDV reliability, or the other way around? One can definitely see the display distort slightly (wavey edges) whenever the MDV is in use - especially during FORMATing. Or are both negatively impacted?
M.
Prompted by recent testing of a batch of MDV units and inspired by Dave's recent announcement of a potential replacement MDV board being made available some time soon, I thought to gather together some of my own observations and unanswered questions that have accrued over time around the operation and circuitry of our (least?) favourite storage technology...
To be up-front - I still hold a flame to this Sinclair technology of the 80's - beyond all reason and after many hours of frustration and lost files over the years. What I learn about their design and operation has only served to fan the flame of my fascination for the microdrive!
All/any comments, additional questions - and answers - most welcome
+++
Much like the humble bumble-bee, which physics suggests simply shouldn't be able to fly and yet it does - the design of the microdrive - both cassette and circuitry shouldn't on the face of it ever work at all.
1. A continuous spool of tape has a finite difference in radius around the spool from the point where it enters the spool to where it exists. This must create a difference in linear speed at entry/exit, albeit small, yet the tape is pulled at a single linear speed (barring fluctuations). Tape stretch might account for this - but only if it both stretches AND contracts as it passes round the loop. Can any one explain this to me?
2. The ERASE function in the QL MDV design (which differs from the original ZX SPectrum approach) is rather funky in that, with the WP tab open, Vin (c. 9V) is routed not to the Erase coil, but instead to the READ/WRITE input of the ULA - effectively switching ULA mode from WR to RD. Atleast the 9v is divided across those 2x 2k2 resistors that are placed in the MDRDWL line, thus supplying c. 4.5V to the MDV ULA pin. Clever, except that now both MDV ULA _and_ the ZX8302 are both driving the RAW1/2 lines simultaneously with different signal levels. Why does this not hurt each ULA?
3. Perhaps related to the above, but just how is tieing RAW1/2 down to -12V via that pair of 33k Ohm resistors (as recommended in the mandatory modification list) supposed to improve MDV reliability? Is this something to do with ULAs that 'latch-up' their outputs (and that the Service Manual suggests - 'never left the factory'...)?
4. The Service Manual provides a very simple SuperBasic routine to effectively generate a constant 100 KHz square-wave on the tape. I know it does this successfully because I've measured the output with an oscilloscope - on both tracks. However, after enabling the selected MDV unit in WR mode, it simply writes a single 0x01 to the ULA Transmit register (and then pauses for about 8 seconds to cover the entire tape-loop.) I can understand that, if you only write once, the data for the second track is effectively taken from the same single ULA register (normal writing would write two bytes to this register spaced slightly apart in time), so both tracks get to see the same data - but then why does a 0x01 pattern translate after DME (differential Manchester encoding) into a constant 100 KHz waveform?
5. Does the video circuitry (PAL encoder) affect MDV reliability, or the other way around? One can definitely see the display distort slightly (wavey edges) whenever the MDV is in use - especially during FORMATing. Or are both negatively impacted?
M.