idea for a universal disk interface

[Tom Gardner]

Interesting idea, there are three broad classes of HDD interfaces:

1.       Dumb, that is serial data and parallel control

2.       Intelligent parallel

3.       Intelligent serial

IMO if you can do dumb interraces then the others follow and given today’s technology I suspect it is feasible

Within the dumb group there are several “standards” with very similar controls but with different data rates and track formats

*         2311/2314 including PCM versions

*         DEC RL0x which is pretty much the same as 2311/2314 except interface voltages and goes up to 200 MB disk pack drives, maybe higher

*         Diablo which is a simplified control version of RL0x

*         SMD which is an enhanced control version of RL0x

*         ST502/412/412 RLL which is a simplified control version of RL0x

*         ESDI which unfortunately serialized some of the control over the parallel interface otherwise similar to RL0x

There are a few others like ANSI and CalComp but they are probably not worth investigating.

 

 

I don’t know who invented the 1311 interface but IMHO he spawned an industry :)

 

I think the maximum data rate is something on the order of 15 MHz so one ought to be able to read in an entire track at a sufficiently high data rate so as to be able to decode the data  using an appropriately programmed DSP.  Essentially all the hardware used for serializing/deserializing, formatting/deformatting and ECC in a traditional controller reduced to firmware

 

Likewise there is a maximum number of control pins and only two voltage signaling levels (IBMs and DTL) so a combination of a programmable transceiver with a personality module ought to allow connection to and signaling with any physical device.  Communicating control and status then is also a programming exercise

 

Writing all the firmware would be a challenge but in the end you would be dealing with an array of blocks of good data, which would then be the starting point for tackling  the intelligent interfaces which purport to start and stop at the same array of blocks of good data.  I think u might be able to make this work with all the flavors of SCSI (with maybe a DSP on the personality module to handle the bus states) but good luck with intelligent serial interfaces.

 

Just my 2 cents :)

 

tom

 

 

 

-----Original Message-----
From: shadoooo [mailto:shadoooo at gmail.com] 
Sent: Tuesday, April 12, 2022 10:36 PM
To: cctech at classiccmp.org <mailto:cctech at classiccmp.org> 
Subject: idea for a universal disk interface

 

Hello,

I'm a decent collector of big iron, aka mini computers, mainly DEC and DG.

I'm often facing common problems with storage devices, magnetic discs and tapes are a little prone to give headaches after years, and replacement drives/media in case of a severe failure are unobtainable.

In some cases, the ability to make a dump of the media, also without a running computer is very important.

 

Whence the idea: realize an universal device, with several input/output interfaces, which could be used both as storage emulator, to run a computer without real storage, and as controller emulator, to read/write a media without a running computer.

To reduce costs as much as possible, and to allow the better compatibility, the main board shall host enough electrical interfaces to support a large number of disc standard interfaces, ideally by exchanging only a personality adapter for each specific interface, i.e. connectors and few components.

 

There are several orders of problems:

- electrical signals, number and type (most disk employ 5V TTL or 3.3V TTL, some interfaces use differential mode for some faster signals?)

- logical implementation: several electrical signals are used for a specific interface. These must be handled with correct timings

- software implementation: the universal device shall be able to switch between interface modes and be controlled by a remote PC

 

I suppose the only way to obtain this is to employ an FPGA for logic implementation of the interface, and a microprocessor running Linux to handle software management, data interchange to external (via Ethernet). This means a Xilinx Zynq module for instance.

I know there are several ready devices based on cheaper microcontrollers, but I'm sure these can't support fast and tight timing required by hard disk interfaces (SMD-E runs at 24MHz).

 

The main board should include a large enough array of bidirectional transceivers, possibly with variable voltage, to support as much interfaces as possible, namely at least Shugart floppy, ST506 MFM/RLL, ESDI, SMD, IDE, SCSI1, DEC DSSI, DEC RX01/02, DG6030, and so on, to give a starting point.

The common factor determining what kind of disc interface can be support on hardware side is obviously the type of transceiver employed, for instance a SATA would require a differential serial channel, which could not be available.

But most old electronic is based on TTL/CMOS 5V logic, so a large variety of computer generations should be doable.

 

For the first phase, I would ask you to contribute with a list of interfaces which could be interesting to emulate, specially if these are similar to one from my list.

I please submitters to send me by email or by web link when possible, detailed documentation about the interface they propose, so I can check if it could be doable and what kind of electrical signals are needed.

Also detailed information about interfaced I listed is appreciated, as could give some detail I'm missing.

 

Thanks

Andrea