The 9835A/B
The HP 9835A/B were compact descendants of the HP 9845A systems with only one single processor and without graphics option. First mentioning was in the 1979 HP Catalog and in the May 1979 issue of the HP Journal. Software architecture was almost the same as for the HP 9845A, so most 9845A BASIC programs could be run on a 9835A/B without change, provided that they were loaded as DATA files in ASCII format. Industrial design was completely borrowed from the 9825A/B siblings, however the internal circuit design was a compact version of the 9845A boards, with some special changes to the memory architecture which were mainly related to the single processor design.
Actually, those changes in turn influenced the later 9845B design, which incorporated the new hybrid processor with integrated Address Extension Chip (AEC) as well as the dual port/dual bus memory scheme. In fact, the 9845B had been a 9845A with enhancements which had been proven before with the 9835A/B.
The main design guidelines for the 9835A/B were:
- It should become a desktop computer with all the features of the 9845A standard model but with smaller footprint and weight, with attractive pricing and capable of running most 9845A programs,
- it should be user friendly and easy to use like the 9845A,
- it should work with all 9845A peripherals,
- it should offer a larger amount of user memory and
- it should support low level programming in assembly language, without sacrifying the ease and comfort of program development provided by the HP enhanced BASIC.
It was much more a 'personal computer' than the original 9845A, but still much more engineered than the home computers of the time. In fact, the customer target base was not easy to define (it was still too expensive for being used only as a programmable instrument controller, didn't provide usable applications like word processors needed for an office machine and completely lacked graphics capabilities as a scientific and engineering computer), the product life of the 9835A/B spanned only 3 years, and, compared to the 9845A/B, only few units were sold.
However, as stated above, some major enhancements like the memory architecture or the assembly programming of the 9835A/B were inherited in the 9845B design. And the 9835A/B wasn't badly designed. Actually, disregarding the fact that is was not possible to upgrade with the graphics option (which probably had been some kind of product placement strategy for protecting 9845A sales), the missing second processor hardly made any difference in performance for standard applications. What still makes the 9835A/B a fairly reliable machine, at least compared to the 9845B, is that HP engineers decided to use more robust plugin ROM modules not only for the option ROMs, but also for the whole operating system.
HP 9835B
- The 9835A, a system with 9825 case and top-mounted, 98750A alike, 25 lines/80 characters CRT monitor, and
- the 9835B, a 9825 alike looking system with built-in single line 32-character LED display.
The 9845A was priced at $9,900, whereas the 9835B was available for $8,700. Both 9835 models were discontinued during the first quater of 1983.
Built-in peripherals were a single DC100 tape drive and an optional 16-character printer for 57mm thermal paper rolls. The 9835 provided physically the same keyboard as the 9825A/B but with different layout, originally with flat calculator-like keys and later with standard keys looking more like like a typewriter.
The 9835 is a nice piece of hardware, with at the time probably unmatched performance per volume ratio. The 9835 really is much smaller than its 9845 siblings, and still has almost all features of the larger systems. And it consumes significantly less power. It makes fun to work with this "baby" 9845B. The CRT of the 9835A is in most aspects a slightly modified 98750A monitor. Parts of the alpha control logic were moved from the mainframe into the CRT housing, where an additional PCB does the job at the location where the graphics option can be installed in the 98750A. In fact, the 9835A CRT lacks both the "leg" interface edge connectors and the socket for the graphics option, so it really was physically impossible to upgrade the 9835A with a graphics option, although all required bus signals and power lines were available from the I/O backplane.
HP 9835A
Technical Specifications
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More Information
As background for the 9835A/B it is recommended to examine the May 1979 issue of the HP Journal (see here for download).
A service manual for the 9835A/B can be found at hpmuseum.net.
In contrast to the HP 9815A and the HP 9825A/B, hardware and software architecture of the HP 9835 is almost the same as those of the other members of the HP 9845 family. For this reason, the 9835 is now included in this site, whereas the 9815 and the 9825 are not. I recommend visiting Steve Leibson's great site www.hp9825.com for everything you ever wanted to know about both the HP 9825 (also includes much information about the HP 9815).