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Update on Mac Classic communications


Veteran Member
Apr 24, 2009
Canberra, Australia
Further fumbling with a degree of success.

Through online auctions I got hold of a DSAS-3720 700Mb Apple SCSI hard drive ($17) and also a ZIP 100Mb SCSI drive ($20).

The Zip was without cable or power supply, but I already had a salvaged ZIP SCSI DB25 cable and no problem providing 5Vdc power supply, for benchtop use at least.

After buying the ZIP, I realised that it would have been smarter to buy the ZIP PLUS, which is a unique model that allows switchable connection to either parallel or SCSI ports, so it is ideal for moving between old Apple SCSI and any other machine with standard parallel port. Hindsight...

Hardware Connections

Next move was to get the new HD connected. It is SCSI-1, standard molex power and 50-pin IDC SCSI header. I made 2 adapters. Easiest was a 50-way strap with 3 IDC inline sockets on it, to replace the 2-socket 50-way strap that normally connects the Classic's internal HD to the motherboard. Pressing the sockets into place with a bench vise is not too difficult. Only issue is to take care with orientation of the plugs - copy an original exactly!

2nd adapter is needed to connect 2nd HD, with Mac case closed, to the external 25DB SCSI port, or to connect the HD to the SCSI daisy-chain port on the ZIP. Pinout can be found on Apple.com, but Google locates it fastest.

To minimize soldering, I took care of the 50-pin side by pushing a 50-pin right-angle IDC header into a piece of project board, oriented so that the back row of pins is nearest to the edge. All the pins in that row except 25 are GND and can be solder-bridged to make a convenient GND rail. A standard 50-pin strap goes into that and the other end into the HD.

For the 25-pin side I cut the end off a salvaged D25-25-pin strap and separated the strands back about 6cm and stripped them 20mm. A plastic cable-tie was enough to fix the strap to the back of the board ready for soldering according to the correct pinout.


It's important to remember that the pin numbering of a DB25 goes across top row, then across bottom row,

1 2 3 4 5 6 7 8 9 10 11 12 13
14 15 16 17 18 119 20 21 22 23 24 25

but the pin numbering of an IDC header alternates top and bottom rows

1 3 5 7 9 11 ........
2 4 6 8 10 12 .....

I found errors in my first couple of goes at this, due to losing track of pin locations.

[By the way, this adapter is useful for attaching any loose 50-pin SCSI disk to your DB25 SCSI chain. I have made my own shoestring "external enclosure" using this adapter, a molex drive power plug extender, an old VHS Cassette box and a commodity LED as an activity indicator. More details on request!]

Mac Classic has a single molex power plug and no room inside for a 2nd HD. With case open and using the straight 50-pin strap adapter to both original and 2nd HD, I powered the second HD from a spare AT power supply that I use for bench power. ( I run one of the AT 12v rails through a box to give me 12v, 5v, a variable 0-11vdc, and a variable resistance ground through a 10K potentiometer - useful for testing LEDs or other things that need a limited current).

Both the 40Mb (SCSI:0) and the 700Mb (SCSI:6) disks appeared on boot, and it turned out the 700Mb HD contained a whole heap of software that I wanted to keep, so I didn't want to format it without saving it somehow. I also had to get System 7.5.3 installed, then update to 7.5.5. There was going to be a lot of data shunting among the drives, and more to load through the only external vector - the serial-to-serial link to my iMac G3.

Software shuffle

Here was the sequence to get through -

1. Move 22Mb System 7.5.3 sources from 40Mb HD to 700Mb HD.
This left enough space on the original HD to do a proper upgrade to 7.5.3.
2. Upgrade 40Mb HD to 7.5.3 from sources on 700Mb HD.
3. Connect ZIP as SCSI ID:5 on external 25DB port.
4. Download and install Iomega ZIP driver and tools and SCSI tools package.
5. Rename System Folder on 700Mb to preserve it as backup.
6. Copy 250Mb of software from 700Mb HD to ZIP disks
7. Format/initialize 700Mb disk
8. Install 7.5.3 on 700Mb HD from sources on ZIP and copy from 40Mb the ClarisWorks serial communications application plus ZIP driver and tools.
9. Disconnect original 40Mb HD, re-number 700Mb HD as SCSI ID 0, boot from 700Mb HD.
10. Download and install System 7.5.5 upgrade (maximum version available for Mac Classic)
11. Copy back everything backed up on on Zips.

Serial link

The fastest sustained communication rate I have been able to achieve on direct serial link to the iMac G3 is about 8Kbps, reported as "20% efficiency".

The main limitation has got to be that Mac Classic has only a 8MHz CPU. Speed increased slightly with the new HD, which must have a better internal transfer rate. Best protocol setting on the Classic is for hardware handshaking, DTR only. If both DTR and CTS are selected, there is about a 10% slowdown as the protocol is checking both signals.

HD SCSI bus problem

The IBM DSAS-3720 seems to communicate less reliably with the SCSI bus on the Classic, resulting in a consistent error message at Restart. My thread on that is in the Technical Support/Vintage Hardware section of the Forum. There are some useful comments about jumpering etc (though no actual cure for the original symptom - yet)


Experienced Member
Dec 29, 2004
Lenexa, KS
You're probably better off with the SCSI Zip drive. The Zip Plus had a reputation for being rather flaky. The one I own only works on parallel. I've never gotten it to work on a SCSI port.


Veteran Member
Apr 24, 2009
Canberra, Australia
That's interesting. Does the Plus use a different adapter cable for SCSI? I've wondered how it would switch modes with just one switch, because the cable pinouts are quite different, including some power and ground signals.