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Connecting an FDD for PC/AT compatible computer to PC-9821
|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.
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.
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]P[u")" or "reversed connector (Japanese notation is "½]RlN^")".
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.
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".
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|
|34-pin connector equivalent to FD1231T for 98 on PC-9821
* 3, * 4
* 5, * 6
|PC/AT compatible computer||Shugart Interface
( Drive 0 )
( Drive 1 )
|4||Head Load||Head Load||Reserved||IN USE
|6||Drive Select 3||Drive Select 3
|Drive Select 3|
|10||Drive Select 0||Drive Select 0||Motor ON 0
( Drive A )
|Drive Select 0|
|12||Drive Select 1||Drive Select 1||Drive Select 1
( Drive B )
|Drive Select 1|
|14||Drive Select 2||Drive Select 2
|Drive Select 0
( Drive A )
|Drive Select 2|
|16||Motor ON||Motor ON||Motor ON 1
( Drive B )
|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|
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
|2||Data rate Select 1
|4||Drive Type ID 1||Index|
|5||GND||Drive Select 0|
|6||+12V||Drive Select 1|
|9||Drive Type ID 0
* 2, * 3
|12||Drive Select||Write Enable|
|17||Media Type ID 1
* 2, * 3
|27||Media Type ID 0
* 2, * 3
|33||Data rate Select 0
* 2, * 3
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|
The table below shows the Media Type IDs.
|Media Type ID Pin Supplement|
|Media Type ID 1||Media Type ID 0||Media Type|
|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.
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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