RAM arrived. The sticks are not identical. System works fine with either stick installed by itself (32MB) and with a 4MB stick in the other slot, but not together. I know that 32MB is lots, really, for what I'm using it for, but was still hoping to "max it out" just for the sake of doing so really.

I have a few IBM PS/1 computers.
It was a Marketing Decision on Which PS/1 configuration you purchased back in 1993
It all depended on Which Store you purchased your PS/1 from
Different Stores carried different Variant's of the PS/1

The model 2155 is one of my Favorites
These are my Favorite 486 computers from 1993/94
They support no proprietary IBM components unlike the PS/2 or AT IBM computers
They Support Standard ISA cards
Support up to 64mb of RAM
You can add a Video card if you change the motherboard jumper
What really set IBM apart from all tghe other computer manufactures was the IBM Windows installations
Which contained Easy use Menus and Batch files for administrations and maintaining your computer
I would Download the PS/1 Service Manual and Original hard drive image which you can find online

My PS/1 Upgrade recommendations:
1) CPU ( Intel Over drive 50, 75, or 100mhz or Pentium 83mhz Over drive CPU )
2) RAM 16mb
3) Hard drive replace with Compact Flash card
4) Sound Blaster Pro. The ones I have came with a CT-2950
5 ) 3com 3c509 Network card
6) CDROM faster is Better 8x or Faster like 52x

Its just a 486 so don’t expect to much
Don’t worry about motherboard cache. 256kb of motherboard cache will only improve performance by about 3% at best.
Better to spend your money on a Faster CPU.

1mb Video card is Good enough for Most DOS games
I use with an old DELL LCD display monitor

See my Posts:
IBM PS/1 Multimedia ( max restore ).

IBM PS/1 Multimedia Consultant with Sound blaster ( 1993 )

Intel486dx33 wrote on 2025-02-02, 16:39:

I have a few IBM PS/1 computers. It was a Marketing Decision on Which PS/1 configuration you purchased back in 1993 It all depen […]
Show full quote

I have a few IBM PS/1 computers.
It was a Marketing Decision on Which PS/1 configuration you purchased back in 1993
It all depended on Which Store you purchased your PS/1 from
Different Stores carried different Variant's of the PS/1

The model 2155 is one of my Favorites
These are my Favorite 486 computers from 1993/94
They support no proprietary IBM components unlike the PS/2 or AT IBM computers
They Support Standard ISA cards
Support up to 64mb of RAM
You can add a Video card if you change the motherboard jumper
What really set IBM apart from all tghe other computer manufactures was the IBM Windows installations
Which contained Easy use Menus and Batch files for administrations and maintaining your computer
I would Download the PS/1 Manual and Original hard drive image which you can find online

My PS/1 Upgrade recommendations
1) CPU ( Intel Over drive 50, 75, or 100mhz or Pentium 83mhz Over drive CPU
2) RAM 16mb
3) Hard drive replace with Compact Flash card
4) Sound Blaster Pro. The ones I have came with a CT-2950
5 ) 3com 3c509 Network card
6) CDROM faster is Better 8x or Faster like 52x

Its just a 486 so don’t expect to much
Don’t worry about motherboard cache. 256kb of motherboard cache will only improve performance by about 3% at best
Better to spend your money on a Faster CPU.

1mb Video card is Good enough for Most DOS games
I use with an old DELL LCD display monitor

See my Posts:
IBM PS/1 Multimedia ( max restore ).

IBM PS/1 Multimedia Consultant with Sound blaster ( 1993 )

Thanks! Yes, I have a 486DX4100 Overdrive on the way for it.

So you are saying I shouldn't worry about trying to get up to 64MB? I am wondering if it's just because the sticks aren't identical.

I've got a CF flash adapter coming for it as well as the upgrade that should bump me up to 1MB of Video RAM with the onboard video. I'm using an SB AWE32 for sound, as that's what I had kicking around and it works perfectly with all the DOS games I've tried so far. It has a Plextor CD-R (52x or 48x, can't recall) and the 3C509-TCO, which is how I transfer most stuff to it, via SMB.

Video RAM arrived today! Install a success! I can now run High Colour (and True Colour, but I have it on High Colour) in Windows 95 at 800x600.

I'm not that familiar with the PS/1 but know they are famous for been picky on RAM.
You shouldn't need matching pairs. I'd put it down to the sticks not likening each other. You do run the risk though if you buy another mismatched stick of having the same compatibility issue.
If it was me, I'd play it safe and get one that matches something you already have.

Not that you need 64MB, as you said you don't even need 32MB, but I understand the desire to max the thing out and why my 3 486's also have 64MB.

With my 486 with only 1 IDE channel on the motherboard I had spinning rust as the Primary for authentic boot sounds and 8GB or so CF card for data, games, etc (Use drive overlay software if needed)
As long as you have enough disk space and networking you don't really miss the lack of CD drive, but that's just personal choice

chinny22 wrote on 2025-02-05, 22:36:

I'm not that familiar with the PS/1 but know they are famous for been picky on RAM. You shouldn't need matching pairs. I'd put i […]
Show full quote

I'm not that familiar with the PS/1 but know they are famous for been picky on RAM.
You shouldn't need matching pairs. I'd put it down to the sticks not likening each other. You do run the risk though if you buy another mismatched stick of having the same compatibility issue.
If it was me, I'd play it safe and get one that matches something you already have.

Not that you need 64MB, as you said you don't even need 32MB, but I understand the desire to max the thing out and why my 3 486's also have 64MB.

With my 486 with only 1 IDE channel on the motherboard I had spinning rust as the Primary for authentic boot sounds and 8GB or so CF card for data, games, etc (Use drive overlay software if needed)
As long as you have enough disk space and networking you don't really miss the lack of CD drive, but that's just personal choice

Thanks! Yeah, I'll reach out to the seller and see if he's got one that matches the ones I have, glad to see somebody can relate on this, 🤣.

I ordered a Startech CF to IDE adapter for it too, similar plan as you, also, easy to swap OS's if I wanted, could put NT4 on one, 95 on another, 98 on another, Linux on another...etc.

Overdrive arrived! Of course I have the same problem as epictronics, the cache is completely disabled with the ODP DX4. I downloaded the TOM BIOS that he's using and I believe I have a flash chip here that will be appropriate to flash with that image. It's an Intel 1Mbit (128K x 😎 P28F001BX-T150 that I had in my parts drawer. It seems pin-identical with the ST M27C1001 OE IBM flash chip (128K x 😎, so I've ordered a TL866-3G programmer from Amazon, which will be here tomorrow, that I'm assuming, based on what I'm seeing here, to flash the TOM 1.0 BIOS to to the Intel chip and replace my BIOS chip with that one.

If somebody sees something wrong with this plan that I've missed, please let me know.

It seems the BIOS has not enabled internal L1 cache.
It is possible when it does not know the CPU type.
May you check whether there is an option to enable cache in BIOS?

Note there is no L2 cache too on your board.

Disruptor wrote on 2025-02-07, 05:09:

It seems the BIOS has not enabled internal L1 cache. It is possible when it does not know the CPU type. May you check whether th […]
Show full quote

It seems the BIOS has not enabled internal L1 cache.
It is possible when it does not know the CPU type.
May you check whether there is an option to enable cache in BIOS?

Note there is no L2 cache too on your board.

Yes, this is a known issue, epictronics had the same problem. There's an updated BIOS that solves it, you can see his BIOS chip has "TOM 1.0" on it in this thread:
IBM PS/1 2155-593 Restoration

I have a copy of the ROM, but need to flash it to a chip, so I had to order a programmer yesterday.

Flashed the Intel P28F001BX-T150 EPROM with the "TOM 1.0" BIOS , seems to have worked, other than I'm having an issue with SpeedSys and my hard drive, but I don't think that's related.

As a 486DX4 75:

Then, decided to take a gamble on Thermalwrong's advice to just bridge the pads on 174/175 (removing the resistor on 176) without using resistors and it worked! Have a 33Mhz FSB now.

As a 486DX4 100:

And a few more. Not shown, Quake timedemo had ~7fps with the DX475. Last one is Doom on high:

OVERK|LL wrote on 2025-02-08, 01:00:

Then, decided to take a gamble on Thermalwrong's advice to just bridge the pads on 174/175 (removing the resistor on 176) without using resistors and it worked! Have a 33Mhz FSB now.

As a 486DX4 100:

Please may you show the SpeedSys Cache chart too?

Disruptor wrote on 2025-02-08, 06:00:

OVERK|LL wrote on 2025-02-08, 01:00:

Then, decided to take a gamble on Thermalwrong's advice to just bridge the pads on 174/175 (removing the resistor on 176) without using resistors and it worked! Have a 33Mhz FSB now.

As a 486DX4 100:

Please may you show the SpeedSys Cache chart too?

This is from before SpeedSys got unhappy with my hard drive, I haven't been able to get it to run since, it hangs at hard drive detection (see other thread), but this was it as the DX4-75 (it froze after this part trying to do the hard drive bench and subsequent runs it wouldn't get past the initial hard drive detection):

Also made an effort to clean the board and inside of the case without resorting to complete disassembly and removal with washing. Turned out OK, a few dust smudges but it generally looks pretty clean. Also had my son (21) play a round of Doom 2, which he'd never played before. Runs perfectly on the system now full screen, super pleased with this project! This forum is a great resource.

Awesome project, it's always fun to push old systems to their limits. A few weeks ago I was tinkering with a very similar system, specifically an HP Vectra 486/25VL. This system also came with an integrated 486SX-25, 4MB RAM, 512KB CL5426 and no option to add L2 cache. Despite these shortcomings, the motherboard is implemented using the VLSI VL82C480 chipset, which performs surprisingly well in memory speed benchmarks. The inability to add L2 cache to motherboards with this chipset is both a curse and a blessing. Why a curse is obvious, but why a blessing you might ask, and the answer is quite simple - without the L2 cache, the system memory will be able to run at tighter timings. In fact, by making a small change to the RAM timing register in the chipset, it is possible to make the system memory run at similar speeds to the L2 cache. This can be done with a regular DOS DEBUG by unlocking the configuration registers and changing the contents of the "RAMTMG" register - the chipset documentation can be found here. I changed the "RAMTMG" register on my HP system from the default 96H to 80H and the change was evident in memory speed benchmarks and applications.

RAMTMG-96H:

1 CACHECHK v4 2/7/96  Copyright (c) 1995 by Ray Van Tassle. (-h for help)
2 CMOS reports: conv_mem= 640K, ext_mem= 31,744K, Total RAM= 32,384K
3 "GenuineIntel"  486 DX2 Clocked at 66.8 MHz
4 Reading from memory.   HP Vectra 486/25VL, RAMTMG=96h (DEFAULT)
5 MegaByte#:      --------- Memory Access Block sizes (KB)-----
6       1    2    4    8   16   32   64  128  256  512 1024 2048 4096 <-- KB
7 0:   16   16   16   16   34   34   34   34   34   34   --   --   --    æs/KB
8 2:   16   16   16   16   34   34   34   34   34   34   34   --   --    æs/KB
9
10 Extra tests----
11 Wrt  24   24   25   25   25   25   25   25   25   25   25   --   --<-Write mem
12 This machine seems to have one cache!? [read]
13    !! cache is   8KB --  67.6 MB/s  15.5 ns/byte  (211%)          4.0 clks
14    Main memory speed --  32.0 MB/s  32.8 ns/byte  (100%) [read]   8.4 clks
15    Effective RAM access time (read ) is 131ns (a RAM bank is 4 bytes wide). 
16    Effective RAM access time (write) is  93ns (a RAM bank is 4 bytes wide). 
17    "GenuineIntel"  486 DX2 Clocked at 66.8 MHz.  Cache ENABLED.
18 Options: -t3

RAMTMG-80H:

1 CACHECHK v4 2/7/96  Copyright (c) 1995 by Ray Van Tassle. (-h for help)
2 CMOS reports: conv_mem= 640K, ext_mem= 31,744K, Total RAM= 32,384K
3 "GenuineIntel"  486 DX2 Clocked at 66.8 MHz
4 Reading from memory.   HP Vectra 486/25VL, RAMTMG=80h (FAST RAM)
5 MegaByte#:      --------- Memory Access Block sizes (KB)-----
6       1    2    4    8   16   32   64  128  256  512 1024 2048 4096 <-- KB
7 0:   16   16   16   16   26   26   26   26   26   26   --   --   --    æs/KB
8 2:   16   16   16   16   26   26   26   26   26   26   26   --   --    æs/KB
9
10 Extra tests----
11 Wrt  24   24   24   24   24   24   24   24   24   24   24   --   --<-Write mem
12 This machine seems to have one cache!? [read]
13    !! cache is   8KB --  68.7 MB/s  15.3 ns/byte  (162%)          3.9 clks
14    Main memory speed --  42.2 MB/s  24.9 ns/byte  (100%) [read]   6.3 clks
15    Effective RAM access time (read ) is  99ns (a RAM bank is 4 bytes wide). 
16    Effective RAM access time (write) is  93ns (a RAM bank is 4 bytes wide). 
17    "GenuineIntel"  486 DX2 Clocked at 66.8 MHz.  Cache ENABLED.
18 Options: -t3

The how to using DEBUG:

1RAMTMG register setup
2
3-O FB 00	;unlocks configuration registers (only dummy write req.)
4-O EC 01	;sets index with RAMTMG reg. address
5-O ED 80	;sets RAMTMG reg. with "1000 0000b"
6-O F9 00	;locks configuration registers (only dummy write req.)
7-Q		;quits

Reading "RAMTMG":

1-O FB 00	
2-O EC 01	
3-I ED 		;reads RAMTMG
4-O F9 00	
5-Q

This program can also be assembled into an executable file using DEBUG and run from AUTOEXEC every time the system boots.

SSTV2 wrote on 2025-02-09, 02:12:

Awesome project, it's always fun to push old systems to their limits. A few weeks ago I was tinkering with a very similar system […]
Show full quote

Awesome project, it's always fun to push old systems to their limits. A few weeks ago I was tinkering with a very similar system, specifically an HP Vectra 486/25VL. This system also came with an integrated 486SX-25, 4MB RAM, 512KB CL5426 and no option to add L2 cache. Despite these shortcomings, the motherboard is implemented using the VLSI VL82C480 chipset, which performs surprisingly well in memory speed benchmarks. The inability to add L2 cache to motherboards with this chipset is both a curse and a blessing. Why a curse is obvious, but why a blessing you might ask, and the answer is quite simple - without the L2 cache, the system memory will be able to run at tighter timings. In fact, by making a small change to the RAM timing register in the chipset, it is possible to make the system memory run at similar speeds to the L2 cache. This can be done with a regular DOS DEBUG by unlocking the configuration registers and changing the contents of the "RAMTMG" register - the chipset documentation can be found here. I changed the "RAMTMG" register on my HP system from the default 96H to 80H and the change was evident in memory speed benchmarks and applications.

The attachment VECT96HM.GIF is no longer available

The attachment VECT80HM.GIF is no longer available

RAMTMG-96H:

Copy code to clipboard

1 CACHECHK v4 2/7/96  Copyright (c) 1995 by Ray Van Tassle. (-h for help)
2 CMOS reports: conv_mem= 640K, ext_mem= 31,744K, Total RAM= 32,384K
3 "GenuineIntel"  486 DX2 Clocked at 66.8 MHz
4 Reading from memory.   HP Vectra 486/25VL, RAMTMG=96h (DEFAULT)
5 MegaByte#:      --------- Memory Access Block sizes (KB)-----
6       1    2    4    8   16   32   64  128  256  512 1024 2048 4096 <-- KB
7 0:   16   16   16   16   34   34   34   34   34   34   --   --   --    æs/KB
8 2:   16   16   16   16   34   34   34   34   34   34   34   --   --    æs/KB
9
10 Extra tests----
11 Wrt  24   24   25   25   25   25   25   25   25   25   25   --   --<-Write mem
12 This machine seems to have one cache!? [read]
13    !! cache is   8KB --  67.6 MB/s  15.5 ns/byte  (211%)          4.0 clks
14    Main memory speed --  32.0 MB/s  32.8 ns/byte  (100%) [read]   8.4 clks
15    Effective RAM access time (read ) is 131ns (a RAM bank is 4 bytes wide). 
16    Effective RAM access time (write) is  93ns (a RAM bank is 4 bytes wide). 
17    "GenuineIntel"  486 DX2 Clocked at 66.8 MHz.  Cache ENABLED.
18 Options: -t3

RAMTMG-80H:

Copy code to clipboard

1 CACHECHK v4 2/7/96  Copyright (c) 1995 by Ray Van Tassle. (-h for help)
2 CMOS reports: conv_mem= 640K, ext_mem= 31,744K, Total RAM= 32,384K
3 "GenuineIntel"  486 DX2 Clocked at 66.8 MHz
4 Reading from memory.   HP Vectra 486/25VL, RAMTMG=80h (FAST RAM)
5 MegaByte#:      --------- Memory Access Block sizes (KB)-----
6       1    2    4    8   16   32   64  128  256  512 1024 2048 4096 <-- KB
7 0:   16   16   16   16   26   26   26   26   26   26   --   --   --    æs/KB
8 2:   16   16   16   16   26   26   26   26   26   26   26   --   --    æs/KB
9
10 Extra tests----
11 Wrt  24   24   24   24   24   24   24   24   24   24   24   --   --<-Write mem
12 This machine seems to have one cache!? [read]
13    !! cache is   8KB --  68.7 MB/s  15.3 ns/byte  (162%)          3.9 clks
14    Main memory speed --  42.2 MB/s  24.9 ns/byte  (100%) [read]   6.3 clks
15    Effective RAM access time (read ) is  99ns (a RAM bank is 4 bytes wide). 
16    Effective RAM access time (write) is  93ns (a RAM bank is 4 bytes wide). 
17    "GenuineIntel"  486 DX2 Clocked at 66.8 MHz.  Cache ENABLED.
18 Options: -t3

The how to using DEBUG:

Copy code to clipboard

1RAMTMG register setup
2
3-O FB 00	;unlocks configuration registers (only dummy write req.)
4-O EC 01	;sets index with RAMTMG reg. address
5-O ED 80	;sets RAMTMG reg. with "1000 0000b"
6-O F9 00	;locks configuration registers (only dummy write req.)
7-Q		;quits

Reading "RAMTMG":

Copy code to clipboard

1-O FB 00	
2-O EC 01	
3-I ED 		;reads RAMTMG
4-O F9 00	
5-Q

This program can also be assembled into an executable file using DEBUG and run from AUTOEXEC every time the system boots.

Very cool! How are you running SpeedSys without the hard drive benchmark if you don't mind me asking?

If you can't get past the HDD detection, you can try disabling HDD or IDE ports via BIOS setup and booting from a startup floppy with Speedsys on it. Clean boot might also help, press F8 while booting and select "step-by-step confirmation".

By the way, do you have a benchmark of the video card before and after the memory expansion? In my case, the motherboard allows for memory expansion, but the memory sockets are for ZIP20 DRAM, which I don't have at hand, so I would like to know if it's even worth the hassle of upgrading vid mem in terms of graphical performance.

SSTV2 wrote on 2025-02-10, 00:35:

If you can't get past the HDD detection, you can try disabling HDD or IDE ports via BIOS setup and booting from a startup floppy with Speedsys on it. Clean boot might also help, press F8 while booting and select "step-by-step confirmation".

By the way, do you have a benchmark of the video card before and after the memory expansion? In my case, the motherboard allows for memory expansion, but the memory sockets are for ZIP20 DRAM, which I don't have at hand, so I would like to know if it's even worth the hassle of upgrading vid mem in terms of graphical performance.

I did not unfortunately. I got the ZIP40's on E-bay for next to nothing, so it was a no brainer to get High Colour.