The firmware version and update issue is rather confusing. Unlike computer software, where the latest version is always better, "it ain't necessarily so" when it comes to our products. The simple reason is this: the firmware is very secondary to the hardware. Most firmware "updates" are made because of hardware changes, some of which are forced on us by outside suppliers. Here is just one example of many. For the UM400 we have 3 branches of the firmware. The 2.x branch is for units using a Philips 7026 phase locked loop, the 3.x branch is for units with the Texas Instruments TI2050 PLL and the 4.x branch is for units with the National LMX2353 branch. Obviously the 3.x branch is not better than the 2.x branch and the 4.x branch is not twice as good as the 2.x branch. Almost all of our firmware revisions have to do with hardware changes and not improvements in the product. We have to do revisions when sometimes the only change is that a company has discarded one IC package for a different one that has more or fewer pins and more or fewer functions. Just as often, we are informed that a part is being dropped from production by an IC manufacturer because the 100 thousand a year that they sell to 4 or 5 companies is not enough to keep the part in production. So we find a similar part, change the PC board and revise the firmware to handle a new command set for the new part. We also do many revisions to make our manufacturing and testing simpler or simply better. It may be easier to put a correction factor for modulation at different frequencies into the firmware than to select varactors and resonators to make deviation uniform across a block. That doesn't mean that older firmware with select parts is better for the customer than standard parts and newer firmware with a correction factor. All this is to say, we've pared down the hundreds of firmware revisions to those that truly affect the end user. Some of these listed changes are the fixing of firmware bugs and some are added features. If the bug has never affected your system and/or if you never need the feature then it may not be worth your effort to "upgrade" the firmware. Further, adding new features may also require hardware changes that may not be possible or may be more expensive than it is worth.
To summarize, a higher firmware number by itself is meaningless. It does not mean a better product. We have listed changes by date of manufacture since serial numbers are not reliable indicators of firmware version. This is because our products are manufactured on many frequency blocks and serial numbers are assigned to units weeks before actual shipping. Our service department can help you with specific questions about your unit. Please have your serial numbers available so they can check the date of manufacture.