what is the difference between a ql microdrive and the spectrum version ?

[techy]

what is the difference between a ql microdrive and the spectrum version ?

are they the same or ....... ( i know the cables are different ) .

who can shine a light on this question ?

[martyn_hill]

Hi techy!

There is in fact very little (substantive) difference indeed.

Here are the main points that I can think of:

A. The drive electronics of the ZX unit are all incorporated in to the enclosure, with the equivalent of the power circuitry that drives the motor and Erase line on a fixed, but still separated board. The QL includes the power drive cct instead on the motherboard with a detachable connection.

B. There are a couple of small refinements to the passive components used on the head board in the QL. One curiousity is that the ZX head board uses different bias resistors (as suggested in both the ZX and QL Service manuals) for positive versus negative flux transitions on Write (the reason cited being that the Erase current will have already biased heavily in one direction). The QL units appear instead to have these resistors equal in value.

C. The most significant difference is how the Write Protect signal is used and here is probably the one difference that you need to be careful of when attempting to use one unit in place of another: on the ZX unit, the WP signal is simply read by the Int-1 ULA and it's up to the software to detect amd respond to any attempt to write to a protected cartridge. On the QL the WP isn't even connected to the IPC/ULA and instead the WP signal is directly used to override the RW signal feeding the MDV ULA so that even though the QL is trying to write (it doesn't know it shouldn't), the MDV ULA will see it as a Read anyway if the cartridge is WP. There is a clever bit of signal routing used on the MDV headboard to make this happen on the QL, as well as using both NC and NO connections on the WP switch (the ZX unit only uses the NO connection.)

D. On the QL a delay is added to the Erase signal on the QL motherboard by way of a capacitor in the base cct of the Erase transistor. There is also a slightly higher drive current delivered to the QL MDV Erase cct.

E. On the QL the two Data lines are biased against -12v (via 33k ohm) versus the ZX biassing them just to Gnd (via 10k ohm.) The ULAs being hybrid digital/analogue cope with the negative biassing - any pure digital replacement FPGA/CPLD would pop with the QL arrangement. The biassing (to Gnd/-ive) is important to overcome the inherent bus-capacitance in what is effectively an Open Collector (+ive drive) pair of signals - otherwise you end up with lengthy falling edges that can upset the decoding of the Differential Machester encoding signal.

The QL units overall seem slightly more reliable than the ZX units, though I suspect the biggest improvement was in the MDV driver software and high-level format used by QDOS (including provision of a MAP and a Directory file.) The QL ULA is also timed at 10us per bit-cell versus the 12us of the Int-1 ULA.

I also have the feeling that the two Data lines/Tracks) are switched - or at least mis labeled - on the Int-1, but the respective ULAs manage that internally/transparently to the software and I get a headache trying to think about it, so can't be sure.

The OPD units are even better designed with the 5v regulated power line derived 'off-board', though the passive component values appear to match the QL, whilst the WP handling matches the ZX.

Hope that helps.

Out of interest, what has prompted your question?

[bwinkel67]

The net effect for me is that the single ZX Microdrive I have has trouble reading QL formatted cartridges. It formats and reads fine on my Spectrum/IF1 after I refurbished it a bit. I think on a few occasions I got a directory listing connected to the QL, but could never get it to read anything.

Here's a video of my experience:

https://www.youtube.com/watch?v=-UZa81FZG2I

[techy]

this made me interested in the differences .

[Derek_Stewart]

Hi,

Does the design of the external microdrive enclosed not clash with the new case design?

[techy]

Hi Derek ,


the external mdv is for the classic ql , not the ql+

[Derek_Stewart]

Hi Derek ,


the external mdv is for the classic ql , not the ql+

Can you think about a QL+ version, might be a nice enhancement for Microdrive, vDrive, Oqtadrive.

[techy]

unfortunately the ql+ with wafer-drive has not made it to production , nice idea tough.

looking for prototypes is FUN !!

A big thumbs UP for the people who make the 3d designs so more people can enjoy the ideas of sir clive.

[Derek_Stewart]

Hi,

Maybe the Wafer technlogy could compared to the MSATA that could be put into a Qubide in the QL.

[martyn_hill]

Hi again techy!

Ok, so you want to attach a ZX MDV unit within the nice QL style external drive casing (I think that was originally designed for the vDrive.)

Makes sense.

The other point that becomes relevant is that the orientation of the MDV expansion bus is reversed between the QL and Interface-1, meaning that you would need to arrange for a longish MDV cable to feed out from the RHS of the QL, under/past the external drive and back in from the RHS of the external drive.

I've done something like this before (so that you can have the MDV slot actually facing you), but can't recall if you also need to swap top/bottom connections of the plug as well - I'll take another look and confirm later.

As for the other differences already described between the (internal QL and external ZX) units, the only one that needs care is the WP handling - basically, with a ZX unit attached, the QL will:

A. Never know whether the inserted cartridge is Write Protected and

B. Continue to overwrite a WP'd cartridge as the ZX unit doesn't include the 'RW over-ride' circuit that the internal QL drives include.

Otherwise, a good ZX unit should function fairly well attached to the QL.

Good luck!