0 or 1 ? 

 PS/55 model numbering 

Some models of PS/55s are described as xxx0 and at the same time these models are also described as  xxx1. 
For example, model  "5551" is sometimes called as "5550",  "5561" is called as "5560" and so on. 
What is the trick ( or logic whatever )  there ?   I am not sure about the logic or trick. 
I may use 5551 in some case and  I may use 5550 in another occasion.  So please don't think  these 
are different models. 

Model 5530, 5510, 5580  These all  do not have xxx1 number.  They are always described as xxx0 . 
On the other hand,  5550, 5570, 5560, 5540 these models have xxx1 number  with specific type-suffix.    ( Recently I noticed that 5560-N has also 5561 number with type suffix ). 
Really strange.... .   There is NO 5520 number for model 5521-Y.

Wild assumption for 0 and 1 and model coding. 

During email transaction with Mr. Tahara who knows  a lot about  IBM Multistation 5550 series We reached following assumption about numbering of PS/55.

Model numbers of Multistation series and PS/55.
 Fact  #1 :  5550, 5540, 5530, 5560 had been used for Multistation series. 
                 IBM  used same numbers for new series.
                 "55" in PS/55  was took after Multistation "55xx".

 Fact  #2 :  Different alphabet indicating  CPU speed were given to new models.

 Fact  #3 :  IBM Multistation 5550 had been consisted with following units and xxx1 was already
                 used for indicating PC unit. 

                 5551: PC unit with  various type suffix code began in A to P.
                 5553: Printer
                 5555: Display
                 5556: Keyboard
                 5557: Printer
                 5559: Expansion unit.

 Fact  #4 : Model 5530 which was defined as "Entry class" had no xxx1 number for Multistation.
                PS/55 5530 also doesn't have xxx1 number. 
                PS/55 5510 defined as " Entry model " doesn't have xxx1 number.

Coding rule for PS/55 should be;
xxx0  was used as general number indicating "system" as a whole and at the same time  it was  used as model  number. 
On the other hand xxx1 was used for PC unit ( with type suffix consisted with combination of alphabet  and numeric ).

The rule might be explained as follows.

For desktop models ( Including tower model 5570 ) 
 55x0 Stands for model number such as "Model 5550",  Model 5560", "Model 5570", " Model 5540".
 55x1 Number xxx1 is used to describe each specific model-type with suffix indicating CPU,  memory, drive arrangement and installed OS and so on

Server model ( 5580 )
Server model was a *new* segment which had not been available in Multistation series. 
They no need to follow existing numbering rule. Thus 5580 was always 5580 like 9595 and 9585/8595. 

Entry class machine 
Figure 1 to 3 for 3rd digit ( xxNx ) was kept for  *Entry class machine such as xx10, xx20 and xx30.    xxx1 was not used for *Entry class* machines. Model 5510 and 5530 has no xxx1 number even for any specific types.  Exception was xx20 for 5521-Y. 

The last desktop model derived from 5550-R/L ( same body, almost same planar ). 
This model was also described as "Entry model"  when they were introduced.  The 3rd digit left unused for entry class machine between "1" to "3" was 2 ( "1" was used for  5510-S/Z/T,  "3" was used for 5530-T/Z ).
Thus 552x was left for the model. But the model was actually a model 5550 486 family. 5550 should be a root number for this system.  So IBM might give "5521" for it but not 5520.

Why it was not coded as 5551-xxx then ? 
 According to PS/55 coding,  486 model were coded as ;
       N :  DX2/66 IML 
       Y :  DX/SX33
       R :  DX2/66 IDE,
       L :  BL2/66
       W : DX/SX25
Y should be suitable for i486SX/33 but 5551-Y was already marketed .  Alphabet left unused were  "I",  "O",  "P", "Q", "X" and  "Z" .   IBM din' t use  "I",  "O" and  "Q" because these alphabet might cause confusion between alphabets and numeric    "X" and "Z " were supposed to be suitable for "High end" (or Ultimate )machines but not for Entry level machine and these letters might be kept for future coming models.    And anyway it has 486SX33 for which "Y"  was defined.  Thus IBM had no choice but name it as 5521-Y.



   Hmm.... confusing.             Anyway nobody cares for such story these days. 

                                          We are now in GHz era . 

PS/55 Index