Martin Adams 
   One advantage the model 90 has over the 77 is the 8 SIMM slots. Eight 8M sims are allot cheaper than four 16M sticks right now. We also have the caching SCSI that could have its cache upgraded. You don't have to pull adapter cards to reconfigure RAM. I prefer the planar mounted bus connectors too. 

Video Ram
   The VRAM chips are Toshiba TC524256BZ-10 or NEC D42274V-10. If you have a different Video ZIP make that works, send me a note. The 8590 systems have 8 sockets for video memory available. The 9590 systems come with 4 VRAM chips (512KB) soldered to the planar.

NOTE: Even though IBM Canada site sez the 9590 has XGA-2 integrated into the planar, IT DOES NOT!!! It has 512K soldered on the planar, plus 4 sockets for the 512K video memory upgrade. The 95xx just means ISO compliant. That's why the 9590 came with XGA-2 cards in them. 

Video RAM Installation

   Place the insertion tool (1) over the emptyVRAM socket (If you have one!) 
   Align the beveled corner (2) of the VRAM chip towards the rear of the system. On most 90s, there is a dot on the planar that the beveled edge (marked with a colored dot sometimes) lines up with. Carefully align the pins with the socket (3) and firmly press the module straight down into place 
   Do not start one end before the other. You can slightly rock the chip side to side to install into a stiff socket, but be careful! 

Which Slot for the XGA-2?
   Generally, Slot 2 or Slot 4. Slot 3 is physically incompatible. For a full discussion, go HERE

Differences between 8590 and 9590 Planars
 The 9590 lacks the DBA artifacts, has 512K video memory soldered on the planar, and is a pretty green. The 9590 planar is identified as an XP 90 system board under setup. It does not support Synchrostream. The parallel port is a standard one, no Expressprint. No Wake on Ring. 

Three Floppy Experience
  Just because people said it isn't done, I threw three 1.44MB floppies into my 9590. All three showed up under setup as 1.44MB drives. Under DOS, they are accessable as A:, B:, and D:. Under W95, it blows the mind of the IOS driver and Win95 says you must shut the system down and restart Windoze. But under safe mode, I was able to access and read off the D: floppy. 
  I will eventually try it under NT 4.0 just for snorts 'n grins... 

* Marked Floppy Drives on 8590s
   Older 8590s may have their floppy controllers FRIED if you use a 2.88MB floppy that has an asterisk ( * ) on the upper surface of the eject button. The 9590 is not affected by this charming quirk. There has to be an earlier floppy controller other than the 82077AA, which supports an asterisk marked floppy on my 9590. 

SIMM Holder Clips
There also was a problem with local power-drops on the early Mod. 90 memory riser cards (the ones with all-plastic SIMM-sockets). Improved versions had metal holder clips. And - logically - you should not mix the two versions. 

Memory Riser Card Support Bracket
There *must* be a plastic Support Bracket clipped over the 2 (two !) memory riser boards to properly fix them. This part is called "memory riser card support bracket" and is FRU 57F3029. It also has a "bay" to guide the SCSI-cable surface wave filter (that large heavy ferro-oxide block). 

ECA084 - Model 90 Memory Riser Card
   If memory riser card FRU p/n 33F4905 is populated with mixed 
SIMMs and any of the following errors occur:
  DOS NMI
  OS/2 TRAP 0002
  POST or Diagnostic memory errors
then replace both memory risers with FRU p/n 81F8823.

Original scan from Al Savage out on the left coast

   The "bad" riser (33F4905) has six electrolytic capacitors on the front. The "good" riser (81F8823) has only the silk screen outlines for the caps (also a lot more SMD resistors and caps on the back).
   Both risers have metal clips and white SIMM sockets.
 

Error 201
    Error code 201 says "Reseat system board memory" and can afflict the systemboard as well as the memory only. I would suggest to remove the memory risers, reseat all modules, plug them back and see if they are seated properly. 
    I would also suggest that you start with one single pair of matching memory modules in the connectors J1 + J3 (Peter, pay more attention when you post!!!) on riser J11 - the one closer to the processorboard. This is just to test out if your problem is memory- or systemboard related. 
    If the machine comes up fine (counts memory) - install the next pair in sockets J1 + J3 in Riser J14 - the on closer to the power supply to keep balanced load of the memory decoder lines. As I wrote: the Mod. 90 has a sensible feeling for imbalanced memory modules and may "spin out" with somewhat strange and unexplainable errors by no obvious reason. There once was a recommendation from IBM on that topic and they explicitely mentioned it for the Mod.90 - particularly for those cases where double-sided memory modules are used (which put a higher load on the decoder lines). 

Memory Expansion Boards
   You can't. Sort of.  The 90 does not cache the memory on expansion boards. So in addition to the overhead in negotiating for control of the microchannel bus, you have to give up the advantage of the 486 cache...

Serial Port
   Serial port can be set as Serial 1 through 8 or disabled.  If you are using an ASCII terminal as your system console, do not disable this port.
      <"SERIAL 1 (03f8h-03ffh 083f8h-083ffh, int 4)>, 2 (02f8-02ff 082f8-082ff, int 3), 3 (3220-3227 0b220-0b227, int 3), 4 (3228-322f 0b228-0b22f, int 3), 5 (4220-4227 0c220-0c227 int 3), 6 (4228-422f 0c228-0c22f  int 3), 7 (5220-5227 0d220-0d227 int 3), 8 (5228-522f 0d228-0d22f int 3),  Disabled

Serial Transmit Arbitration Level
   The serial port can be set to any one of the arbitration levels for transmitting data. If the level is shared then other devices can be set at the same level. If the level selected is dedicated then only this device can be set to that level.
      <"Shared 4" >, 3, 1, 0, 7, 6, 5, Dedicated 7, 6, 5, 4, 3, 1, 0, Disabled

Serial Receive Arbitration Level
   The serial port can be set to any arbitration level for receiving data. If shared is selected, then other devices can use the same level. If the level selected is Dedicated then only this device can be set to that level.
      <"Shared 3>, 1, 0, 7, 6, 5, 4, Dedicated 7, 6, 5, 4, 3, 1, 0, Disabled

Parallel Port
   The parallel port can be set as Parallel 1 through 3 or disabled.
       <"PARALLEL 1"  (03bc-03bf 1278-127f int 7)>,  2 (0378-037f int 7), 3 (0278-027f int 7),  Disabled

Parallel Port Arbitration Level
   The parallel port can be set to any arbitration levels.  If the level selected is shared then other devices can be set at the same level.  If the level selected is dedicated then only this device can be set to that level.  Select <Disabled> to use the port in compatibility mode.
      <"Shared 7">, 6, 5, 4, 3, 1, 0, Dedicated 7, 6, 5, 4, 3, 1, 0, Disabled

Preempt Enable/Disable
   Lets the system board processor preempt continuous data transfers by other devices for its use of Micro Channel.
     <"Enable">, Disable

Video I/O Address
   I/O (Input/Output) address range for the display controller registers.  Also selects the exact location of the video coprocessor registers.
        <"Instance 6: 2160h - 216Fh">, 1 (2110-211F), 2 (2120-212F), 3 (2130-213F), 4 (2140-214F), 5 (2150-215F)

Video ROM Address Space
   Memory address range used for the system video ROM.
        <"C0000-C1FFF" >, C2000-C3FFF, C4000-C5FFF, C6000-C7FFF, C8000-C9FFF, CA000-CBFFF, CC000-CDFFF, CE000-CFFFF, D0000- D1FFF, D2000-D3FFF, D4000-D5FFF, D6000-D7FFF, D8000-D9FFF, DA000-DBFFF, DC000-DDFFF, DE000-DFFFF

Video Arbitration Level
   The video sub-system can be set to any one of these arbitration levels.
        <"Arbitration level 13">, 12, 11, 10, 9, 8, 14

Video Fairness
   Whether the video sub-system coprocessor will follow the fairness algorithm for bus usage.
        <"Fairness On">, Off

ADPItem 1 Usable System-Board Memory   (Exec)
   This field displays the type of Usable Memory on the system board.  The memory type is either parity or error-correcting-code (ECC).