I need to connect a MezIO-D220 card to an external device (i.e. U2901A Terminator board), but I'm not sure about the compatibility of SCSI connections. The card has a SCSI-II HD68P female port, but I cannot find any SCSI-II HD68P M/M cable to connect it to the external device (which has a SCSI-II HD68P female port). Since it's easier to find a SCSI-III HD68P M/M cable, I would understand more about the SCSI compatibility. In general, SCSI supports backward compatibility, but I don't understand if the connectors are compatible when the number of pins is equal.

In specific, could I use a SCSI-III HD68P M/M cable to connect two SCSI-II F/F ports? In other words, are SCSI-II and III compatible? Are there any electrical issues that could damage the system?

Images: (MezIO port, Terminator Board port)


Be careful! Your question is (understandably) talking about SCSI, due to the connectors, but the MezIO-D220 card is not a SCSI HBA (it is a digital I/O module) and so any decisions made on the basis of SCSI compatibility are not applicable here :-(

I also notice that you don't mention exactly what device you want to connect to your MezIO-D220 card. How have you confirmed that the (proprietary) pin-out of the MezIO-D220 card is compatible with whatever device you plan to connect to it? Or do you have complete control over the device which you are attaching, such that you can modify it to match the pin-out and electrical characteristics of that MezIO-D220 card?

Your situation isn't helped by the fact that terms like "SCSI-2 connector" or "SCSI-3 connector" are not well-defined. Instead, you will have fewer opportunities for misunderstanding by using the actual connector names, when searching for physically compatible cables.

I could go back to my SCSI documents for the various "Alternative-n" numbers for the different connectors, but since almost no-one uses them, I have given the more common names below. Although, as I mention above, there is no guarantee of electrical signal compatibility, without more information than has been supplied so far.

The MezIO-D220 card has what are commonly known as HD-68 connectors, according to the image on its datasheet which you kindly linked. Those are one of the connectors commonly referred to as SCSI-2 connectors, although some SCSI-3 devices also use them.

When companies mention a "SCSI-3" cable, they will often (though not always) mean that there are VHDCI connectors at each end - and those are not physically compatible with your MezIO-D220 card.

So in short: For physical compatibility with the connectors shown on the datasheet of the MezIO-D220 card which you linked, you need an HD-68 connector (probably with thumbscrews and not latches, based on the datasheet photo - the presence of latches are one variation of the HD-68 connector). That is what some companies may call a SCSI-2 wide connector - but you should check the specific connector type to be sure.

Regarding the device you are connecting to, which you say "has a SCSI-II 68 pins female port" - that is also probably an HD-68 connector, based on your description, and not the VHDCI style of connector. But as I said, the pin-out may not be electrically compatible with your MezIO-D220 card, and without a photo we can't see whether it is designed for the latch or thumbscrew variant of the HD-68 connector.

To help you see the different SCSI connector types, so that you can compare HD-68 and VHDCI connectors, there are many websites with photos. Some do not differentiate between the latching and thumbscrew variants of HD-68 (which will usually connect even if they are of those two different types, but will have no way to stop them being pulled apart accidently). For example, here is one such website:


If you are at all unsure of the compatibility of cable connectors between the MezIO-D220 card and whatever device you intend to connect to it, I recommend talking to the manufacturer of that card - their datasheet for it includes details of the various cables which they apparently sell.

So to answer your 3 questions:

could I use a SCSI-III 68 pins M/M cable to connect two SCSI-II F/F ports?

That depends on what specific connector types are involved, as I explain above. The term "SCSI-III" does not allow a yes or no answer, although it is likely to be a "no" if by "SCSI-III" they really mean "VHDCI".

In other words, are SCSI-II and III compatible?

As I have explained, that question is not applicable here - the MezIO-D220 card does not use SCSI, so SCSI compatibility answers do not apply.

Are there any electrical issues that could damage the system?

Potentially yes, plenty. That MezIO-D220 card uses a connector which was commonly used for SCSI, but it is not using it for SCSI signals. The pin-out on the card will not follow any standard like SCSI and so you will need to follow the detailed documentation from its manufacturer. You will need to ensure that whatever device you attach is compatible with the signals from that card and that the pin-out of the card and attached device are compatible.

  • Thank you Sam. Yes, I have total control on the system, so I'm free to choose the external device, that could be a U2901A terminal board. I need to be sure about the physical compatibility: according to your link, the ports are both HD68 (not VHDCI). The only difference between them is that the MezIO card has the holes (female port) slightly smaller than the board; despite this difference, the cable (SCSI-II HD68P M) apparently fits it perfectly. May 9 '16 at 12:54
  • @Marco : "that could be a U2901A terminal board" - that seems similar to the description by the card's manufacturer of the "TB-10" mentioned on their datasheet. However, until you see the actual wiring diagram (pin-out) of both the card's connector and any terminal block, you can't be 100% sure how the signals from the card are allocated to specific pin numbers on the terminal block. "according to your link, the ports are both HD68 (not VHDCI)" - based on the photos, I agree. "the MezIO card has the holes (female port) slightly smaller than the board" - sorry, I don't understand :-(
    – SamGibson
    May 9 '16 at 13:14
  • In addition to the previous comment, both the MezIO card and the terminator board use thumbscrews. Also, I should clarify that I need to use the MezIO card to send a little amount of data (in specific, a trigger signal). Since I'm very new to this field, I really don't know what SCSI HBA means and so the difference between SCSI and I/O module (I've never developed on this kind of systems). May 9 '16 at 13:19
  • @Marco : "I really don't know what SCSI HBA means " - SCSI Host Bus Adapter (HBA); it's the "thing" in a host ("initiator" in SCSI terminology) which connects to a SCSI device ("target"). Since this question got migrated (and at least one answer) on the belief that this was a SCSI question (it isn't), I just wanted to make it clear that, despite having a "SCSI-type" connector, that card isn't a SCSI HBA, which some people interpret it being. "difference between SCSI and I/O module" - massive differences; much too big for a comment. I think I can open a chat room here, if you like?
    – SamGibson
    May 9 '16 at 13:31
  • Sorry, I added two images to better understand the female port type. May 9 '16 at 13:31

The connectors are compatible and parallel SCSI is usually backward compatible with everything except High Voltage Differential (or HVD). If both devices are SE or LVD, then they should be compatible. The external 68pin connectors (the bigger ones, not VHDCI) are the same as internal 68pin connectors - you can usually use an internal cable externally

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