Interesting Hardware Facts about the IBM PS/2 Modesl 25 & 30

[Great Hierophant]

From the technical reference :

Serial - 8250, not 16450/16550
Floppy - only 250K transfer rates supported (no 1.2/1.44MB), power is supplied through the drive cable (40-pin).
Hard Drive - Interface is not MFM or or RLL based but is essentially 8-bit IDE with power supplied through the drive cable (44-pin). Default drive for system from IBM is a 20MB model, type 26 in BIOS table. 2 more types added to XT/286 table.
Parallel Port - Bidirectional, but bidrectional mode is enabled by I/0 0x65, bit 7.
Serial, Diskette, Video, Parallel Port and Hard Disk interfaces can be disabled by 0x65.
512KB standard, upgradeable to 640KB by 4x64x4 & 2x64Kx1 chips.
DMA0 does not refresh DRAM but is unavailable for I/O expansion use.
8MHz 8086 & 8087, 16-bit RAM, 8-bit ROM and I/O. ROM is 64KB and includes ROM BASIC.
-5v present on non-standard 12-pin power connector
Standard keyboard assumes no LEDs and may only be a unidirectional interface.
8xIRQs, 4xDMA, Segments C, D & E available for ROMs, whole 64KB address I/O decoded.
DB-25 for serial (male) and parallel (female), 6-pin mini-DIN for keyboard and mouse
PC Speaker has earphone jack which silences it when plugged in.
System RAM can be remapped if conflict occurs or a failure exists in first 128KB.
MCGA has separate 64KB of RAM, but no ROM BIOS. Two separate fonts contained in BIOS ROM, one supporting 8x8, one supporting 8x16.
MCGA can use up to 512 character font tiles and uses IRQ for a vertical retrace interrupt. It uses a 25.175MHz crystal.
Display connector is 14-pin berg and only supports ID bits 0 & 1.

 

[DOS lives on!!]

Does this apply to the PS/2 Model 30-286 or just the model 30?

 

[IBMMuseum]

Some qualifiers - Your comments are on the 8086-based Model 25 and 30. At that level, it isn't the same planar between the two models, and there are areas that are designed differently between them. The largest difference (besides the casing) is that the Model 25 lacks a Real-Time Clock.

Floppy drives - As long as some specific models are used, each unit can run 1.44Mb drives. The Model 25 is less picky than the Model 30. There is also a controller to add a 360Kb external floppy drive (4869-001).

Memory - Your comment is correct for the Model 25 (except about the memory remapping). The Model 30 has the first 128Kb soldered in, and the two 256Kb SIMMs take the memory to 640Kb. Memory remapping comments are for the Model 30.

Hard drive - Some Model 25s were issued with an MFM controller in one slot, and a 3-1/2" MFM drive...

 

[michal]

DMA0 does not refresh DRAM but is unavailable for I/O expansion use.

Are you sure ? Does that mean that memory refresh cannot be turned off on these machines ?

 

[Great Hierophant]

I did not know that the 8086 Model 30 had a RTC, apparently it has a "coin slot" for a 3V battery cell, not a Dallas chip.

Although you may be able to use a 1.44MB drive and HD disks, you won't be able to read or write at 1.44MB on these machines, only 720K. A controller to add an external drive would probably not allow the drive to be bootable.

IBM must have had a fair number of unused MFM hard drives and ISA hard drive adapters to get rid of,
Apparently memory refresh cannot be turned off, but why would anyone want to do that?

The technical reference for the 25 discusses the memory remapping feature.

BIOS uses standard EPROMs, so a No-slot-clock can be used without difficulty.

8086 CPU is socketed on at least some planars, so replacing it with a V30 is a possible upgrade.

There is no speaker cone, only a "beeper", but the earphone jack should be able to drive something that can give better expression to PWM PC Speaker music.

Power supply is 113W for color monitor 25s, 90W for monochrome monitor 25s. Power comes from the display unit portion of the machine.

The Keyboard communication used by the machine is of the AT variety, 11-bit data stream (as opposed to the 9-bit data of the XT).

There are no jumpers on the 25, and I assume the 30 has none either. How about configuration? Well, the BIOS is written knowing what is in the system (one floppy, MCGA, one parallel and one serial, keyboard and mouse), so there is no great difficulty there. A second floppy can detected in software. Adapters made for the PS/2 would know how to disable the onboard hardware it is to replace (IBM's VGA card for example). MFM adapters have their own BIOS ROM extensions.

 

[evildragon]

You're wrong about the floppy drives.

I write and read 1.44MB disks in mine all the time with a drive IBMMuseum sent me, and it works fine!!

There is technically one jumper on the model 25 board, a key lock jumper, but you'd want a key switch for that.

 

[Great Hierophant]

The "IDE-like" interface for the hard drive only supports 1 hard drive and uses DMA3 and IRQ5. It is found at 0320-032F, like the MFM controller. There are only two bays in the system, so one internal hard drive makes sense.

On the motherboard, there should be clock crystals at 14.318MHz (OSC and Timer), 48MHz (CPU & Serial) and 25.175MHz (MCGA).

I guess I was wrong about support for HD drives, there is nothing in the technical reference that states or implies that the disk controller cannot run at 500K.

As there is no battery backed RAM, EEPROM or other type of memory to save the configuration, I am unclear how the drive parameters of the internal IDE-like drive are saved after power down. Or are they? If not, then I do not understand how you could boot off such a drive.

 

[michal]

The keyboard and mouse ports are perfectly normal PS/2 (in fact, they are _the original PS/2 ports_ that were later cloned in the mainstream PC junkyard). The 3,5" floppy drive and the 15-pin analog displays (aka VGA monitors) also pioneered on these machines and became the mainstream PC standard. Apart from the crippled 8-bit bus and in consequence the 8-bit harddrive interface, these machines were standard setters.

 

[IBMMuseum]

I did not know that the 8086 Model 30 had a RTC, apparently it has a "coin slot" for a 3V battery cell, not a Dallas chip...

Incorrect, it has a soldered lithium battery on the adapter riser. The RTC chip is like for the AST adapters, but at a proprietary I/O address. It correctly does a century byte roll-over.

...Although you may be able to use a 1.44MB drive and HD disks, you won't be able to read or write at 1.44MB on these machines, only 720K. A controller to add an external drive would probably not allow the drive to be bootable...

Incorrect, the 1.44Mb drives function fully. Normally the external 360Kb drive is cabled as 'B:', but it would be possible to modify the cabling to 'A:'. The external drive adapter has a BIOS extension, and should be able to be bootable.

...BIOS uses standard EPROMs, so a No-slot-clock can be used without difficulty...

Correct, been there, done that...

...8086 CPU is socketed on at least some planars, so replacing it with a V30 is a possible upgrade...

Correct, been there, done that...

Every 8086 I have seen in a Model 25 or 30 has been socketed...

...There are no jumpers on the 25, and I assume the 30 has none either. How about configuration? Well, the BIOS is written knowing what is in the system (one floppy, MCGA, one parallel and one serial, keyboard and mouse), so there is no great difficulty there. A second floppy can detected in software. Adapters made for the PS/2 would know how to disable the onboard hardware it is to replace (IBM's VGA card for example). MFM adapters have their own BIOS ROM extensions.

The Model 25 and Model 30 (being ISA) have a "Starter Diskette", but other than setting the date and time (which is also able to be done within DOS, as the "driver" is within ROM) there is no other configuration to speak of...

You forgot to add if an 8087 is installed, but these systems determine their installed equipment at boot time...

 

[Maverick1978]

I did not know that the 8086 Model 30 had a RTC, apparently it has a "coin slot" for a 3V battery cell, not a Dallas chip...

Incorrect, it has a soldered lithium battery on the adapter riser. The RTC chip is like for the AST adapters, but at a proprietary I/O address. It correctly does a century byte roll-over.

FWIW, I've a Model 25 color model complete, and two bottom chassis complete with motherboard, floppy, and risers. On the complete unit as well as one of the spare chassis, I've got a soldered lithium battery. On the other spare chassis, I've got no battery. I seem to remember reading that this was hit and miss with the Model 25 units on either Louis Ohland's pages, or at Peter Wendt's MCAMafia pages.

Also, on all three of my motherboards, the CPU is socketed. In regards to the RAM, I'll have to double-check, but I believe that all three boards had 256kb on-board, then two SIMM slots to add up to two more 256kb SIMMS, giving you a total of 768kb RAM. The extra 128kb was ignored by the system, and this is what gives you 640kb on the Model 25.

 

[Trixter]

There is also a controller to add a 360Kb external floppy drive (4869-001).

OT: I have one of these drives, never used. If anyone has the controller and wants the drive, PM me. If anyone has the controller and doesn't want it, PM me as well

 

[vwestlife]

Weren't these also the first PCs to ship with Microsoft Windows? Since the Models 25 and 30 couldn't run OS/2, I believe IBM let you order them with Windows 1.05 (and possibly later 2.x). They even tried to advertise the Model 25 as a Macintosh competitor, because it was an all-in-one design and you could get it with a mouse and graphical user interface.

 

[IBMMuseum]

...In regards to the RAM, I'll have to double-check, but I believe that all three boards had 256kb on-board, then two SIMM slots to add up to two more 256kb SIMMS, giving you a total of 768kb RAM. The extra 128kb was ignored by the system, and this is what gives you 640kb on the Model 25.

No, the 8086-based Model 25 used the two 256Kb SIMMs as the 512Kb base memory, then there was an optional set of DIP memory that filled the sockets to bring it the additional 128Kb. It's a mix of data and parity chips, and doesn't have any extra to "ignore". I think IBM would have added another 256Kb SIMM slot for a scheme like you mention.

 

[IBMMuseum]

OT: I have one of these drives, never used. If anyone has the controller and wants the drive, PM me. If anyone has the controller and doesn't want it, PM me as well

The controller was specific to the 8086-based Model 30, but I've also seen them issued on a few 8086-based Model 25s. Very innovative that the same proprietary floppy cable worked on a dual 720Kb Model 30 (with no space for a hard drive), or flipped the B: side to connect as a pass-through on the adapter. A BIOS extension on the adapter is hard-coded for it's address space, and set up the 360Kb external drive to the system without additional drivers or configuration.

A side project is to see if I can patch the same controller into a PC Convertible...

 

[evildragon]

No, the 8086-based Model 25 used the two 256Kb SIMMs as the 512Kb base memory, then there was an optional set of DIP memory that filled the sockets to bring it the additional 128Kb. It's a mix of data and parity chips, and doesn't have any extra to "ignore". I think IBM would have added another 256Kb SIMM slot for a scheme like you mention.

His math was wrong, he inferred 256kB x 2 = 768kB.

A little off.

 

[Maverick1978]

His math was wrong, he inferred 256kB x 2 = 768kB.

A little off.

I'm quibbling here, but my foggy memory was apparently wrong; my math wasn't. 256kb x 2 (simms)=512kb + 256kb (on-board)=768kb.

Weren't these also the first PCs to ship with Microsoft Windows? Since the Models 25 and 30 couldn't run OS/2, I believe IBM let you order them with Windows 1.05 (and possibly later 2.x). They even tried to advertise the Model 25 as a Macintosh competitor, because it was an all-in-one design and you could get it with a mouse and graphical user interface.

No, the PS/2 Model 25s did not ship with Windows 1.05, however IBM did offer an optional "Collegiate Software Kit" that consisted of 4 disks: a DOS 3.3 diskette, "Collegiate Kit Data Master," and "Collegiate Kit System Diskette 1 & 2." These 3 disks contain Windows v1.05 as well as some other software (IIRC, it was just some learning to use the computer/mouse/keyboard stuff). From some marketing materials that I read, this was offered to increase the perceived value of the Model 25 as an educational tool. I guess to a limited degree, it worked. As I've stated in other posts, the schools in the county in which I attended Jr and Sr high utilized 8525-001 systems throughout the English classrooms and typing labs.

FWIW, I have this kit - an ebay find about 6 months ago. They wanted $75 for it. I offered $15 with a note about the age of the disks and their untested nature. To my surprise, the seller accepted.

 

[vwestlife]

IBM did offer an optional "Collegiate Software Kit" that consisted of 4 disks: a DOS 3.3 diskette, "Collegiate Kit Data Master," and "Collegiate Kit System Diskette 1 & 2." These 3 disks contain Windows v1.05 as well as some other software (IIRC, it was just some learning to use the computer/mouse/keyboard stuff). From some marketing materials that I read, this was offered to increase the perceived value of the Model 25 as an educational tool. I guess to a limited degree, it worked.

Clearly that was a shot at the Macintosh, because this was when colleges and universities were buying up mass quantities of Mac Pluses, and in some cases even giving them to their students to use in their dorms. It's just too bad that Windows 1.x was such a miserable joke of a GUI; GEM or GEOS would've been a far superior choice at the time.

Nonetheless, I believe the Model 25's successors -- the 25-286, 25SX, and EduQuest -- were all sold exclusively to the educational market, which unfortunately makes them a rather rare find these days.

 

[krebizfan]

Clearly that was a shot at the Macintosh, because this was when colleges and universities were buying up mass quantities of Mac Pluses, and in some cases even giving them to their students to use in their dorms. It's just too bad that Windows 1.x was such a miserable joke of a GUI; GEM or GEOS would've been a far superior choice at the time.

Not GEOS, the first release of GeoWorks wasn't until 1990 or after the release of Windows 3. Not exactly a viable competitor to Windows 1.x because of that.

 

[IBMMuseum]

Here is a little bit more to the production of the 8086-based Model 25 and 30. The Model 30 was among the initial released models of PS/2s (with the microchannel Models 50, 60, 70, and 80). Initial models don't have as nice of sub-model encoding that came later, making it harder to readily determine the original CPU, hard drive size, and even installed OS by just looking at the system identifier.

The Model 30 was marketed as a replacement to the XT. There were migration features put in place (like the parallel port Data Migration Facility, and the external 360Kb 4869-001 floppy drive) to make the transition easier. I think the system planar became larger than what IBM originally intended, making the original Model 30 case not interchangeable with the Model 30 286 version (the power supply connections are also different).

Model 25s came after the initial PS/2 release, and seem to be more mature in respect to hard drives (the Model 30 can only run a particular 20Mb drive, where the 25 can also do a 30Mb) and floppy drives (to upgrade the Model 30 to 1.44Mb requires a single specific brand and model, where many of the 1.44Mb drives of the other PS/2s work in the 25). The Model 25 planar was also more compact, and a standard form-factor to be interchangeable with later upgrades.

When the Model 25 286 and 30 286 came out (issued with VGA, 1.44Mb floppy drives, 16450 UART serial port, and always with a hard drive), they shared the same planar (there were two different types, but they are used interchangeably between the two systems). I've even noticed that the redesigned Model 30 286 case had a headphone blockout that could be removed, so that an 8086-based Model 25 planar (the only one specifically with that jack) could be put inside. That is, with a redesign of the adapter riser to include a battery and real-time clock, the original 8086-based Model 30 could be replaced with a version that had the floppy and hard drive kinks worked out of it.

But IBM never did that, probably because the 8086-based unit was passed its prime at that point...

Only a Model 25 version was brought up to the 386SX CPU level, the "25SX" (IBM never had an ISA-based 386DX system). The same CRT was used, making much of the added resolution of the SVGA chip unobtainable. Grayscale CRTs were only used for some of the 8086-based Model 25, all 286 and 386SX units have a color CRT.

 

[michal]

Model 25s came after the initial PS/2 release, and seem to be more mature in respect to hard drives (...) The Model 25 planar was also more compact

There are two versions of the model 30 8086 planar. I think the second one might have the ability to accept 30MB drives (and is more compact, by the way).