The Rough Guide to MBus Modules
Introduction Buses Modules Systems Chips Miscellany

The SM52 Name Game

Q:  What is the difference between SM52, SM52X, SM520, SM521 and SM512?
A:  SM512 was SM52, but none of the others were!

Sun has a regrettable habit of retroactively renaming products and options, and of corrupting their own product naming schemes. Both of these practices can lead to great confusion.

Author's aside: if the E280 is a SunFire, is the E6000 still one too?

The problem for the dual-SuperSPARC CPU modules is made worse by their somewhat unconventional release-history.

Shortly after the SPARCstation-10 was launched, the quad-processor model 54 was announced. The SS10 model 54 was specified as having a pair of double-width "SM52" modules, each incorporating a pair of 45MHz SuperSPARC CPUs, a pair of MXCC cache controllers, and 1Mb of level-2 cache per CPU.

Unfortunately, the SS10 model 54 was never shipped. Some customers that had ordered a model 54 recieved (after some delay) a model 36 instead (ie: with a single cacheless 36MHz CPU), other customers may have recieved a model 41 (a single 40MHz CPU with 1Mb cache).

So did the SM52 module (dual-SuperSPARC @ 45MHz) ever actually exist? Yes!

Such modules were indeed manufactured by Texas Instruments and tested by Sun in SS10s.

Unfortunately, the yields were low, and the working SM52 modules ran so hot that they would damage the motherboard and PSU of the SS10. There is a Sun internal bug report that records just such an instance, using "production" parts (ie: not engineering samples).

However, in the better-ventilated and roomier SPARCcluster, configurations using a single SM52 module per system-board were cooled sufficiently to run without apparent damage, and passed extended QA without overheating. A few SPARCcluster systems fitted with a single SM52 per system-board actually shipped to customers. However it may be illuminating to note that Sun never shipped any SPARCcluster configurations with a pair of SM52 modules on a single system-board.

So although such SM52 modules were manufactured, and a few were actually shipped, these modules are very rare.

One unofficial line from Sun at that time was that the quad-processor SS10 and SS20 models had been withdrawn to stop them competing with the SPARCserver-1000. Believe it if you will.

Later, when SuperSPARC production had settled down, yields were up, and the single-processor SM41 and SM51 modules were selling like hot cakes, Sun introduced a double-width dual-SuperSPARC module with the CPUs clocked at 50MHz, initially calling it the SM512, presumably to distinguish it from the earlier SM52. This "SM512" was basically two SM51s on a single double-width module. From around that time, there are even press-releases mentioning the "SPARCstation-10 model 514", even though no systems in that configuration were actually shipped: the "SM512" was made available as an upgrade option only.

However, for some reason, the name of this later dual-CPU module was changed, and is now refered to in the official Sun "Field Engineers Handbook" as "SM52X", even though the official Sun Spares listings still refer to it as SM512 (both sources use the same part-numbers, but different modules names...)!

The problem for users is further compounded by the fact that Sun resellers and OEMs were also confused by the renaming - some resellers list these latter-day parts as SM512, some as SM52X, and some even as SM52 (because they do not think the "X" is significant, and do not remember that a few of the earlier 45MHz SM52 modules did actually ship.

Finally, there is the matter of the dual-SuperSPARC modules intended for the SPARCserver-600 series: the SM520 and SM521 (a mirror-image of the SM520, to fit in the horizontally-opposed second MBus slot on the SS600 systemboard).

Of course, the SM512 and SM521 are often confused with each other, even though the SM521 will not physically fit any system other than an SS600 series server.

Worse, some resellers seem to think that the "X" in SM52X means "either SM520 or SM521, as appropriate". Unfortunately, this is not the case: the SM520 really is different to SM52X. The latter runs as a dual-CPU module in the SS10, but the SM520 only runs as a single-CPU module, irrespective of PROM type, at least on some SS10 motherboards. Curiously, the SM520 does function as a dual-CPU module in the SS20 and some clones.

Summary:   if you are offered a dual-SuperSPARC module, be very careful. The only way to know what you are dealing with is to get the "501-" part-number and look it up in the table of SuperSPARC modules on this site.

Introduction Buses Modules Systems Chips Miscellany
Mike Spooner, revised 20th March 2001