Technology and retro computer blog

Serial cable file transfer, pc to Macintosh 128k/512k

Getting software onto floppy disks for old Macintosh computers, such as the original 128k and 512k can be difficult.  The problem is the floppy disk standard they used is no longer supported on any modern machine.  These early Macs used standard double density 3.5" disks, but they were formatted with a 400k and later 800k floppy format.  This standard is no longer used by modern computers.  The old Macs are also difficult to connect to networks, especially pre-SCSI Macs like the 128k and 512k machines.  They had serial ports but a problem with serial transfer is that they used the RS422 standard, not the widely used RS232 standard that most computers, like the Apple II, use.


Many enthusiasts of these old Macs use a bridge computer to transfer software to disk.  There were several Macintosh computers produced during the late 1980's and 1990's that supported both the old 400k and 800k formats and also the more widely used PC, 1.44mb high density format.  The drive in these machines is called the Superdrive.  Some models that have the Superdrive are the Macintosh SE/FDHD, the SE/30 and the Powerbook 520c and 540c laptops.  There are also others.  A bridge machine offers a great solution especially if that machine has ethernet capabilities.  While a bridge machine is great, if you don't have one, the simple solution is to use a serial cable.


USB to serial adapter cables are cheap and readily available and they will add a serial port to your modern computer, but they are wired for RS232 standard serial communications not RS422 like the old Macs.  The solution is to create an RS422 to RS232 adapter cable.  Then you simply connect this cable between the Mac and the USB to serial cable.  See description below for how to create this cable.


Another issue with old Macintosh computers is that they're unusable without a boot disk.  Unlike the Apple II which has inbuilt Basic in ROM.  So unfortunately if you purchase one without any disks you can't create the disk to use it, like you can with the Apple II using something like ADT Pro.  You need to get your hands on a bootable 400k floppy disk.  What's more to transfer software to the old Mac via serial you need a couple of important utilities on that boot disk.  The required software is described below.


Macterminal transfer
Xmodem transfer from Windows 7 pc running TeraTerm to Mac 512k running MacTerminal 1.1


Creating the serial cable

The following table shows the pin assignment of the Mac 128k/512k de-9 serial connector (RS422 standard).  The second table shows the 5 pin adapter cable required to convert RS422 to RS232.


Macintosh 128k/512k RS422 serial connector
Pin Description
1 gnd
2 +5v
3 SG
4 TxD+
5 TxD-
6 +12v(HSKo)
8 RxD+
9 RxD-


The table below describes the connections required to create an RS422 to RS232 adapter cable.  The connector at the back of the Mac 128k/512k is a de-9 female connector.  And the connector on most USB to serial adapters is usually a de-9 male connector.  Therefore the cable below has a male de-9 connector at the Mac end and a female de-9 connector at the other end.


Macintosh 128k/512k RS422 to RS232 serial adapter cable
Mac de-9 Male PC RS232 de-9 Female PC Signal Name
7 7 CTS
6 8 RTS
9 3 TXD
1 5 GND
5 2 RXD



Required software

In order to transfer files to the Mac, you need a few utilities: MacTerminal 1.1, BixHex 5 and/or Packit 1.0.  The terminal program is required to do the file transfer and BinHex or Packit is required to preserve the resource fork on the old Macintosh files. BinHex 5 will only archive a single file, whereas Packit allows you to put multiple files into an archive.  But the great thing about these utilities is that both will open any file once transferred, they don't rely on the file type.  The following table shows the software on a 400k boot disk that will allow you to boot the  Mac and do file transfers.  Even with all required software this disk still has around 180k free, so you can transfer files directly to it.  The system components for this disk have been stripped right down of unnecessary fonts and desk accessories to maximize free space on the disk.


Macintosh 400k boot disk for serial transfers
Utility size
System 2.0 50k
Finder 4.1 47K
MacTerminal 1.1 98k
BixHex 5 8k
Packit 1.0 9k


Download the disk image for this disk here: (176.29 kb)



By booting an old Mac with this disk, you can transfer files in a BixHex or Packit archive across the serial link using XModem in MacTerminal.  If the files will fit, you can transfer them directly to the boot disk.  If the files are too large for the boot disk, MacTerminal allows you to eject the boot disk prior to receiving a file, so you can write the receive file to a blank disk.  This gives you the ability to transfer larger files.  This process is made much easier if you have and external floppy drive, as you can extract the archived files to the external drive.  If you have a single drive you can still do it but you the process will require disk swaps.

This is a complete solution for transferring files from an internet computer directly to a Mac 128k/512k, without using a bridge machine.

See also my Macintosh 128k/512k disk image archive

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