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Proto Amiga - SCSI Device Emulator RaSCSI (AMSCSI)

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RaSCSI is a type of emulator that virtually reproduces SCSI devices (hard disk, MO, CD-ROM) that have been converted into past relics. It was developed using SCSI control of XM6 Type G. By introducing it to Raspberry Pi (hereinafter referred to as RPI), RPI behaves like a SCSI device. Use 18 RPI GPIOs + 1 GND instead of SCSI connector. In order to connect RPI to X68000 (SUPER or later SCSI machine), you need to make your own conversion cable. By the way, RaSCSI performs various functions expansion by combining with X68000, but I think that it can be used with other retro PC adopting SCSI such as FM TOWNS when using it as a simple SCSI hard disk.

Characteristic

    
Virtual disk device (HDD, MO, CD)
    
Emulate SCSI connected hard disk, magneto-optical (MO) disk, CD-ROM. On the RPI side, the disk image file that can be created with XM 6 is recognized as a virtual SCSI disk device, but on the X68000 side, a physical device is recognized as being present. I also officially supported SASI from version 1.24.
    
Ethernet communication
    
By installing the Ethernet driver provided by RaSCSI on the X68000 side, communication using the TAP device (virtual network I / F) can be performed through the RPI side bridge device. The X68000 side driver can be replaced with the Neptune - X driver.
    
Remote drive
    
By installing the remote file system driver provided by RaSCSI on the X68000 side, you can let RPI file system be recognized as a drive through RPI side bridge device as it is. It provides the same function as Windmill's XM6 Windrv XM on the actual machine.
    
Initiator mode
    
You can use the initiator mode to control SCSI devices (host side). This allows applications such as dumping and restoring physically connected SCSI hard disks etc. However, it is necessary to use a conversion board which can be used directly or initiator mode explained later.

Connection method (conversion board edition)

    
Recently, there are people who mainly produce conversion board for RaSCSI through Twitter neighborhood. Since it is consigned to sell in Akihabara somehow please get it. Please refer to the connection method below (direct connection) if you are directly connected at risk prejudice.


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Circuit design of conversion board


For amateurs who want to create their own conversion board, it is amateurs but I will write reference information. At the TTL level, 5 V is divided at 220 Ω and 330 Ω at the TTL level (in the case of passive terminator), so that the state where the voltage slightly less than 3 V is applied to each signal line is in a steady state (signal negatively). When trying to assert the signal (= 0 V) ​​by the initiator side or the target side, the total current of 5000 ÷ 220 × 2 = 45 mA will flow from the terminators at both ends (the data sheet of MB 88352 which is the SCSI controller of X68000 It is Iol 48 mA as a sink current when looking). RPI GPIO can not absorb such large sink current. In order to make an electrically safe connection, it is necessary to make conversion board with general purpose logic IC etc. For general-purpose logic ICs, we use high power type open collector such as 74LS06 or 07 which can withstand 48mA sink current. The authors confirmed that it operates without any problems by performing basic SCSI direction control with 74 HC 541 × 3, 74 HC 126 × 1,74 HC 0 4 × 1 and further combining circuits that drive the bus using 74 LS 07 × 3. In addition, if you use 74LS641-1 which is a derivative version of 74LS641, the circuit can be constructed simply. Unlike normal products, it is not necessary to use 74LS07 because the sink current is 48mA compatible. But availability is not that good.

This is a circuit diagram example of a conversion board that supports target mode (HDD, MO, CD, etc.). There are many conversion boards developed based on this circuit diagram (GAMERnium.com version etc.).

This is a circuit diagram example of a conversion board that supports the initiator mode (host function) in addition to the target mode (HDD, MO, CD etc.). We will use one more GPIO to control the initiator mode. Also, 74LS86 is required to invert the direction control. There is no conversion board currently based on this circuit diagram.

This is a circuit diagram example of a conversion board that supports target modes (HDD, MO, CD etc.), initiator mode (host function), monitor mode (SCSI signal monitoring). We use three GPIOs. The direction control of all 74LS641-1 is controlled from the RaSCSI side. There is no conversion board currently based on this circuit diagram. The Aibomu version using a dedicated driver IC is close to this.

Warning!
Due to the difference in electrical specifications between GPIO and SCSI, there is risk involved in direct connection. Conversion boards must be used to make an electrically safe connection. People who can not understand this risk should give up. Your X68000 is probably okay but Raspberry Pi has an unexpected load. The unexpected load is the sink current explained at the conversion board. Since there is no terminator in direct connection, a sink current of about 22 mA will be generated but since it exceeds the maximum drive capability of GPIO, it will not be able to drive up to near 0 V with low sink current. Incidentally, RaSCSI is running for several months, but it has not yet become a failure. Even if it breaks down, it is not an expensive item, so please consider it as a consumable item.


 

Cable connection diagram
Raspberry Pi (GPIO) <=> Connection (conversion) cable <=> (external SCSI connector) X68000

It seems to be stable when pulling out the GND on the SCSI connector side as much as possible (preferably 8 wires) and connecting it to the RPI side GND.

Cable production example

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