Termination Resistor

Last revision of this page: March 7, 2025

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For example, if you want to buy an old floppy disk drive on ebay, always look for the termination resistor on the offer images. Often this is missing because the drive was used as the second drive; here the termination resistor must be removed! Further information can also be found at Herb Johnson: Resistor termination for floppy data cables.

Resistors with 150 or 330 Ohm value was the standard for earlier 5.25" drives in the IBM PC/XT and was descended from the 8" convention (with a cable length of a meter or two). Instead of a 150 Ohm you can also use a 180 Ohm without any problems. I bought a whole bunch on eBay for a good price.

Those of you who work a lot with old floppy disk drives will definitely know the following resistor with 330 ohms.

Terminating resistor with 330 Ohms. — Terminator resistor with 330 Ohm

The Kaypro floppy controller, however, needs a 470 Ohm resistor as standard. [1]

In some forum posts I read that sometimes there can be problems with the old 8" drives. If this happens just try a resistor with 1KΩ. So far I had no problems with 8" drives on my 286 AT and the CompatiCard I.

With my TEAC drives FD-55A and FD-55F resistors of 330 or 1K Ohm can be used depending on the cable length. 330 Ohm for cables up to 3 m and 1K for cables up to 1.5 m in length.

But this value is not critical as one might think. My practical experience: The newer the computer/controller and the shorter the cable, the higher the resistance value (up to 1K or 4.7K).

Terminator resistor and cable length — IBM floppy bus with cable twist [2]

Please pay attention, there are resistors with 2x7 = 14 pins and with 2x8 = 16 pins! In addition, there are also floppys with separate resistors per line, e.g. with 220 + 330 Ohms. In case of doubt always look into the documentation.

Terminating resistor on the Qume 142 with 150 Ohms. — Terminator resistor on the Qume 142
(150 Ohm and circuit type -3, isolated)

If you want to buy a resistor on eBay, for example, please pay attention to the correct circuit type, see next image. For almost all floppy drives, this is type -3 (isolated): Each line is isolated from all others and wired directly across.

If a disk drive suddenly behaves odd or makes strange sounds after a modification or expansion, check if the resistor is plugged in (correctly).

With two floppy disk drives, only the last drive (A:) on the standard floppy cable (with cable twister) has a termination resistor!
This also applies to older CP/M systems with Shugart bus system. Here you can connect a total of four drives.

Downloads

References

(↑) CHILTONS GUIDE TO KAYPRO REPAIR AND MAINTENANCE, page 145
(↑) minuszerodegrees: https://minuszerodegrees.net/diskette/5.25_floppy_drive_termination.jpg
(↑) TEAC FD55 Instruction Manual, page 219