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Connecting an FDD for PC/AT compatible computer to PC-9821
( Overview )


The materials in the Implementation Chapter assume that the 90mm (3.5") floppy disk drive (hereafter FDD) used in PC/AT compatible computers, which is the de facto standard as of 2021, will be connected to the NEC Corporation PC-9800 series.

When applying this material to other models, it is necessary to read it as appropriate by referring to the contents described in the items of "Differences in FDD specifications" and "Signal Definition".

In Japan, due to legal regulations (Measurement Law), the yard-pound system cannot be used for transactions and certifications in principle. The metric system is generally used to describe dimensions and weight. When referring to documents, pay particular attention to the units of length and weight.

For example, magnetic media commonly referred to as 3.5-inch floppy disks are officially called "90mm flexible disk cartridges" (JIS X 6211) in the Japanese Industrial Standards. To avoid the use of inches, the word "Œ^" (read as "gata") is sometimes used. An example is "3.5Œ^FD".

What is the PC-9800 series?

The PC-9800 series are 16-bit and 32-bit personal computers manufactured and sold by NEC Corporation from October 1982 to September 2003.

The genealogy can be roughly divided into the PC-9801 series, which was introduced in 1982, and the PC-9821 series, which was introduced in 1992. Other derived products included machines with model numbers such as PC-98, PC-H98, and FC-98. All of them have their own design and are not compatible with IBM's PC/AT.

In its heyday, in 1987, it captured more than 90% of the market share of 16-bit personal computers sold in retail stores in Japan. Countless software programs existed, and operating systems such as CP/M-86, OS/2, MS-DOS, and Windows were also ported.

The PC-9800 series also had compatible machines, and one of the representative products was the "EPSON PC Series" manufactured and sold by Seiko Epson Corporation from April 1987 to July 1995.

The PC-9800 series was mainly sold in Japan, but a small number of overseas models such as the PC-9801VM2E and PC-9801FC were also exported outside Japan.

Relationship between PC-9800 series and APCIII

When NEC was selling the PC-9800 series in Japan, it was selling the APC III, which was partially compatible with the IBM PC outside of Japan.

The APC III had hardware that resembled the early models of the PC-9800 series, and in a sense could be called the PC-9800 series compatible with the IBM PC. Although it was mainly aimed at the North American market, it did not succeed in North America due to the rise of PC/AT compatible computers. Instead, it is said to have been successful in the Asia Pacific region, especially in Australia.

Adding an FDD or replacing a failed FDD

The PC-9800 series has been out of production for more than 20 years. As a result, we began to hear stories of people who couldn't get a genuine FDD when they wanted to add a FDD, or that the originally installed FDD couldn't withstand the aging process and broke down.

Of the models after PC-9821Xa7 released in May 1995, desktop machines use FDDs equivalent to FD1231T, and laptop machines use FDDs equivalent to FD1238T.

How to use the term

Note that there is a difference in the use of terms such as note PC and laptop PC between Japan and other countries.

In Japan, the term "note PC" is currently used to describe a computer that can be easily carried in the hand, and a computer weighing more than about 3 kg is sometimes called a "desk note PC". The term "laptop PC" is no longer used, but in the past, portable PCs with integrated displays were called "laptop PC".

On the other hand, in many countries outside of Japan, computers that can be easily carried in the hand are called "laptop computer", and among them, relatively lightweight computers are called "notebook computer".

Even if you want to get an FDD with the same model number as the genuine one for expansion or replacement, they are not sold at ordinary mass retailers. You can find them through internet auctions or flea market services, but the better the condition of the FDD, the higher the price will be.

It would be nice if we could leave the FDD neglected even if it breaks down, but unfortunately the PC-9800 series has many programs that require the FDD, so leaving it neglected is fatal.

In order to solve this situation, I thought it would be a good idea to modify the FDD for PC/AT compatible computers and use it as a substitute for the genuine FDD of the PC-9800 series, so I planned to modify it.

Convenience devices such as the HxC Floppy Emulator and the GoTek Floppy Drive Emulator are now widely used, but when I started planning to modify the floppy drive for PC/AT compatible computers, these convenient devices were barely known in Japan.

Differences in FDD specifications

The specifications of FDDs are different between PC-9800 series and PC/AT compatible computers. The degree of difference depends on the model and cannot be simply compared, but the main points are as follows.

  • Physical dimensions of the FDD, connector shape and orientation

In particular, the physical dimensions and connector shape of FDDs for laptops with gray enclosures are different from the common slim FDDs found in PC/AT compatible computers.

Desktop PCs and laptop PCs (laptop PCs in this case refer to portable PCs with integrated displays) are rather rare to have FDDs with special dimensions. Instead, it is not uncommon to use wiring where the connectors on both ends of the flat cable of the FDD are crimped in the opposite direction instead of the same direction.

This wiring is commonly referred to as "reversed cable (Japanese notation is "”½“]ƒP[ƒuƒ‹")" or "reversed connector (Japanese notation is "”½“]ƒRƒlƒNƒ^")".

  • Differences in signal specifications

The overall difference is that among PC/AT compatible computers, desktop machines use the Disk Change signal in principle, while the PC-9800 series uses the Ready signal for all models.

Therefore, the FDD to be modified must be able to output Ready signal or equivalent signal. If the FDD cannot output Ready signal or equivalent signal from inside, you will have to make a very troublesome modification or an incomplete modification that may cause the FDD to run out of control or freeze if the operation procedure is not correct.

Also, among the PC-9801 series, most of the models after PC-9801VM released in 1985 have a VFO circuit (data separation circuit) built into the FDD. If you want to install a PC/AT compatible computer FDD in the target model, you need to prepare the VFO circuit separately.

Some models of the EPSON PC series (PC-9800 series compatible computers) include the power supply line in the 34-pin connector.

  • Format type and disk rotation speed

Even though the format type of the floppy disk has the same name, the specifications are slightly different between the PC-9800 series and PC/AT compatible computers. For example, when comparing floppy disks called 2HD in Japan and 1.2 MB format or HD in many countries other than Japan, there are the following differences.

For PC/AT compatible computers, 130mm (5.25") floppy disk is 512 bytes/sector,15 sector/track,80 track/surface, 90mm (3.5") floppy disk is 512 bytes/sector,18 sector/track,80 track/surface.

On the other hand, for the PC-9800 series, 1024 bytes/sector,8 sector/track,77 track/surface are standard in both cases. It is said that the reason for the specification of the PC-9800 series was to make it compatible with 200mm (8") FDDs.

In addition, when comparing floppy disks called 2DD in Japan and 720 KB format or DD in many countries other than Japan, the access method is also different. For PC/AT compatible computers, the disk speed is 300 rpm and the transfer rate is 250 kbit/sec. The PC-9800 series, on the other hand, has a disk speed of 360 rpm and a transfer rate of 300 kbit/sec to avoid switching disk speeds.

Therefore, even if you modify the FDD for PC/AT compatible computers and connect it to the PC-9800 series, you cannot handle 2DD format floppy disks with some exceptions. This is because FDDs for PC/AT compatible computers often fix the disc rotation speed to 300 rpm when judged to be 2DD format.

In addition to the above, there are also differences in how to handle the head load mechanism.

If you want to know more about the history and technical aspects of FDDs, you can refer to the article "Trying to attach various 5-inch FDDs to an AT-compatible machine" by "Maybe Development Room".

Signal Definition

When applying the FDD documentation posted on Implementation to other models, the problem seems to be the difference in pin outs. Next, we present a list that contrasts the pin outs of the FDD interface part of each model. In all cases, only the most representative items are excerpted.

FDD connector pinout comparison chart Part 1
* 1
34-pin connector equivalent to FD1231T for 98 on PC-9821
* 3, * 4
* 5, * 6
PC/AT compatible computer Shugart Interface
* 17
1 GND 360/300
 ( Drive 0 ) 
2 Density Density Density
* 10
Mode Select
* 11
Disk Change
3 Window
* 2
 ( Drive 1 ) 
* 12
4 Head Load Head Load Reserved IN USE
* 18
* 12
6 Drive Select 3 Drive Select 3
* 7
ED Density
* 13
Drive Select 3
* 2
8 Index Index Index Index
10 Drive Select 0 Drive Select 0 Motor ON 0
 ( Drive A ) 
* 14
Non Connected
* 16
Drive Select 0
11 Sync
* 2
12 Drive Select 1 Drive Select 1 Drive Select 1
 ( Drive B ) 
* 15
Drive Select 1
14 Drive Select 2 Drive Select 2
* 8
Drive Select 0
 ( Drive A ) 
* 15
Non Connected
* 16
Drive Select 2
16 Motor ON Motor ON Motor ON 1
 ( Drive B ) 
* 14
Motor ON
17 Drive Selected
* 2
18 Direction Select Direction Select Direction Select Direction Select
20 Step Pulse Step Pulse Step Pulse Step Pulse
22 Write Data Write Data Write Data Write Data
24 Write Enable Write Enable Write Enable Write Enable
26 Track 00 Track 00 Track 00 Track 00
28 Write Protect Write Protect Write Protect Write Protect
30 Read Data Read Data Read Data Read Data
32 Side Select Side Select Side Select Side Select
33 GND Disk Change
* 9
34 Disk Change
Ready Disk Change Ready
  • * 1
    It is mainly installed in PC-9801 series models.
  • * 2
    Signal for VFO circuit (data separation circuit).
  • * 3
    It is mainly installed in PC-9821Xa7 and later models that were released in May 1995.
  • * 4
    The above table shows the pinout of the PC-9821 desktop machine body side.
    Because of the different terminal orientation of genuine FDDs, cables for PC/AT compatible computers cannot be used as they are, and must be connected with special wiring (this wiring is commonly referred to as "reversed cable (Japanese notation is "”½“]ƒP[ƒuƒ‹")" or "reversed connector (Japanese notation is "”½“]ƒRƒlƒNƒ^")". When replacing the wiring or FDD with a non-genuine one, care must be taken in the connection direction of the terminals and wiring to avoid misconnection. Note that wiring with special connections is not limited to FDDs, but also exists in power supply wiring for FDDs and HDDs, and in wiring for HDDs. Therefore, caution is required.
  • * 5
    According to the manufacturer's specifications, the pin number of the box-type pin header terminal is generally treated as the first pin by the triangular mark engraved on the surface having the protrusion for preventing erroneous insertion.
    In the case of this website, since the data for models other than the PC -9800 series are also shown, the terminal (However, models equipped with FD1158C are excluded.) mounted on the motherboard is used as the standard to avoid confusion. Materials posted on other websites may have been prepared based on different standards than those of this website, so you need to check the consistency of the materials before you start reading.
  • * 6
    Depending on the combination of the model number and P/N, there are two types: one for PC-9821 and the other for PC/AT compatible computers. The P/N covered by the list is "134-506790-011-0". For other P/N's, please refer to the article "PC-9821E9801EPC98-NXEPC/AT—p3.5ƒCƒ“ƒ`FDD‚ΜŒέŠ·«ˆκ——•\" in the "FDDŠΦŒW" page of HAMLIN's PAGE.
  • * 7
    In the case of a machine equipped with a 34-pin terminal equivalent to the FD1231T for PC-98, it is certain to connect to the 4th pin of the 8-pin terminal for connecting to the separately sold PC-9821-K08.
  • * 8
    In the case of a machine equipped with a 34-pin terminal equivalent to the FD1231T for PC-98, it is certain to connect to the 3rd pin of the 8-pin terminal for connecting to the separately sold PC-9821-K08.
  • * 9
    Although it is defined as a Disk Change signal, it does not seem to be actually functioning and can be treated as GND in effect. It is unclear how it came to be this way.
  • * 10
    Signals defined on the PC/AT compatible computer itself.
  • * 11
    Signals defined on the FDD side.
  • * 12
    Key pin to prevent reverse insertion of terminals. No pins are attached to the connector, and the holes are also plugged. Originally, pin 5 was used for this purpose, but gradually pin 3 was assigned for this purpose.
  • * 13
    On PC/AT compatible computers that support 2ED (2880KB) disks, it is assigned to the switching signal.
  • * 14
    10 Pin and 16 Pin are twisted between DriveA and DriveB, and the DriveB side is connected straight and the DriveA side is twisted.
  • * 15
    12 Pin and 14 Pin are twisted between DriveA and DriveB, and the DriveB side is connected straight and the DriveA side is twisted. For both FDDs, the drive number should be set to Drive Select 1.
  • * 16
    Some motherboards and FDDs for PC/AT compatible computers have the drive number fixed to Drive Select 1 on the board. In this case, the pins noted are not connected to any signals.
  • * 17
    This interface is used in Amiga and others.
  • * 18
    This signal is used to control the display of the access lamp separately from the Drive Select signal and Motor ON signal.

For reference, the pin assignments for the IBM PS/2 and Atari ST are also shown.

FDD connector pinout comparison chart Part 2
IBM PS/2 Atari ST
* 5, * 6
1 GND Read Data
2 Data rate Select 1
* 1
Side Select
3 +5V GND
4 Drive Type ID 1 Index
5 GND Drive Select 0
6 +12V Drive Select 1
8 Index Motor ON
9 Drive Type ID 0
* 2, * 3
Direction Select
10 Reserved Step Pulse
11 GND Write Data
12 Drive Select Write Enable
13 GND Track 00
14 Security Command
* 4
Write Protect
* 7
15 GND -
16 Motor ON -
17 Media Type ID 1
* 2, * 3
18 Direction Select -
19 GND -
20 Step Pulse -
21 GND -
22 Write Data -
23 GND -
24 Write Enable -
25 GND -
26 Track 00 -
27 Media Type ID 0
* 2, * 3
28 Write Protect -
29 GND -
30 Read Data -
31 GND -
32 Side Select -
33 Data rate Select 0
* 2, * 3
34 Disk Change -
  • * 1
    For models that do not support Media Sense, it is a Density signal.
  • * 2
    For models that do not support Media Sense, it is GND.
  • * 3
    A signal from the FDD to the main unit to notify the type of the FDD and the type of the magnetic media. The setting contents inferred from the specification of the uPD72070 manufactured by NEC Corporation are described in the appendix.
  • * 4
    In the case of models that do not support Media Sense, it is Reserved.
  • * 5
    This is the pin layout of the external FDD. The connector used is a 14-pin round DIN terminal.
  • * 6
    The genuine AC adapter is connected to the external FDD by a 5-pin round DIN terminal. The power output is +5V 0.55A on pin 1, GND on pins 2 and 3, +12V 0.9A on pin 4, and Non Connected on pin 5.
  • * 7
    Instead of the Disk Change signal and Ready signal, it uses the Write Protect signal to detect the disk. Therefore, if you want to use FDD for PC/AT compatible machine, you need to modify the board. For details, please refer to this article by Atari ST pages by PP.

The table below shows the Drive Type IDs.

Drive Type ID Pin Supplement
Drive Type ID 1 Drive Type ID 0 Drive Type
Low Low 90mm (3.5") FDD, 1.44MB
Low High 90mm (3.5") FDD, 2.88MB
High Low 130mm (5.25") FDD, 1.2MB
High High Reserved

The table below shows the Media Type IDs.

Media Type ID Pin Supplement
Media Type ID 1 Media Type ID 0 Media Type
Low Low Reserved
Low High 4MB ( 2ED ) 
High Low 2MB ( 2HD ) 
High High 1MB ( 2DD ) 

In case of PC-9800 series, 360/300 signal is used for disc rotation speed, and Density signal is used for switching between 2DD format and 2HD format. For this reason, the document posted at Implementation shows that you need to modify the Density (Mode Select) signal of the FDD for PC/AT compatible computers, but this modification is not necessary if you want to apply it to other models.

On the other hand, if the FDD is modified to be equivalent to the Shugart Interface, it may be necessary to modify it so that the Disk Change signal can be output from 2 pin of the 34 pin connector.

Supplement - What is a 3-mode FDD?

When 90mm (3.5") FDDs started to become popular, 90mm (3.5") FDDs for PC/AT compatible computers supported two format types: 2HD (1440KB)/2DD (720KB). On the other hand, the 90mm (3.5") FDD for the PC-9800 series supported two format types: 2HD (1232KB)/2DD (720KB).

In Japan in the latter half of the 1980s, the PC-9800 series had an overwhelming market share. For that reason, the FDDs of other computers (for example, X68000 and FM TOWNS) manufactured and sold in Japan at the same time also have specifications that are conscious of the PC-9800 series.

In the 1990s, the so-called "3-mode FDD" appeared, supporting three different format types: 2HD (1440KB)/2HD (1232KB)/2DD (720KB). Of the PC-9800 series, the PC-9801 series has been installed since January 1993, and the PC-9821 series has been installed since the first model released in November 1992.

It is not clear when other models, including PC/AT compatible computers, were first equipped with "3-mode FDD". However, there is information that Toshiba's J-3100 series and IBM's PS/55Z series were equipped with it from the models introduced around 1990.

In terms of hardware, "3-mode FDD" switches between 2HD (1440KB)/2HD (1232KB) operation using Density (Mode Select) signal for PC/AT compatible computers and 360/300 signal for PC-9800 series.

From a software perspective, it is necessary to check whether the operating system of the PC-9800 series supports "3-mode FDD". For PC/AT compatible computers, in addition to the operating system, various preparations are required, such as checking the motherboard's compatibility, and if it is compatible, BIOS settings and device drivers.

Note that the "3-mode FDD" that supports the 2HD (1232KB) format type is basically only used in Japan. The meaning of the term itself is not likely to be understood outside of Japan.

Specific methods are described in the "Implementation" section, please see there.

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