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Hit Counter    rev. 01/11/15

IMSAI History- The First IMSAI's
Copyright 1999 Thomas "Todd" Fischer
All rights reserved worldwide

IMS Associates/IMSAI Manufacturing Corporation produced the IMSAI 8080 between December 1975 and September 1979. Fischer-Freitas Company (which became a corporation in 1980) continued production from October 1979 through May of 1986. The last six I-8080 machines were delivered to the computer lab at University of California- Davis. The 8080 went through a number of internal evolutions, despite retaining the same exterior characteristics. The following information should help distinguish significant variances between changes.

First Article- The first twenty-five IMSAI 8080 kits shipped just before Christmas in 1975.  These machines, whether factory-built or kit form included two metal adhesive-backed labels; one which indicated the line voltage and frequency that the transformer was wired for, and a second that included the serial number, starting with about 001008.  This sequential numbering system would continue throughout IMS Associates/IMSAI Manufacturing Corporation's three year run. 

Documentation was erratic and incomplete, but still had a trademark air of quality in its presentation. The packing box was "oyster white" and imprinted with large blue lettering proclaiming the IMSAI 8080 as the "Standard of Excellence in Microcomputers". Slick four-color magazine advertising and professionally typeset promotional materials bore the lavishness Bill Millard visualized in his business.

The 8080 kit was a delight to open and arrange prior to starting construction. Individual parts groups were placed into clear poly bags with an individual parts list and heat-sealed. All circuit boards were similarly packaged with their respective components. The sheet metal was wrapped in brown paper to protect the brushed aluminum and painted finish. The two-inch thick ringed white binder was imprinted in blue with an attractive graphic and the IMSAI logo. Inside were the individual sections describing assembly, master parts list, large schematic and assembly drawing for each board. These included the power supply, motherboard, CP-A front panel and the MPU-A 8080 processor board. The typical purchaser found an immediate sense of quality in just opening the box.

The Enclosure- The sheet metal was often the first thing to be assembled since it provided the sense of a "real" computer, even if incomplete. The color combination was chosen to reflect a kind of "thumbing your nose" attitude toward Bill Millard's former employer IBM. The IBM Blue cover and Charcoal Gray switch escutcheon complimented the nicely proportioned box and front panel. Constructed of 16 gauge aluminum, the sheet metal kit consisted of the base plate, back panel, front sub-panel, switch escutcheon, two card cage side panels, motherboard riser plate (which also helped stiffen the base plate), CP-A (front panel board) switch bracket, and a cable clamp for the back panel (top). Except for the IBM Blue cover and Charcoal Gray switch escutcheon, the aluminum was typically finished in either clear or light gold alodyne, depending on vendor. Alodyne is a dipping process, not electroplating.

The first 100 or so units did not use threaded inserts (6-32 PEM nuts) as extensively as the later revisions did, choosing instead to employ #6 hex head self-tapping sheet metal screws to attach many of the panels together. Most notable is the rev. 1 front sub-panel which, along with the back panel, was attached to the base plate with the same sheet metal screws. The back panel featured a 4 1/2" cutout for the  optional 3-blade "whisper" fan.  A black plastic finger guard was provided with all units to keep debris and small mammals from entering the cabinet, whether supplied with a fan or not.

The early back panel has two 1/4" holes located to the right of the 4 1/2" fan cutout.  These were intended for accessory cables, most commonly the cassette data storage interface input/output audio cables that were offered as an option.  Ten DB-25 cutouts were provided for cables in addition to the cable clamp located at the top of the panel.  This was provided to add versatility in cabling to peripheral devices.

Back Panel Revision- The later revision for the back panel (around August 1976) dropped the two 1/4" mounting holes, and added four 7/16" "D" punch holes to allow BNC-style connectors to be used. Ten DB-25 and six 5/16" holes to allow use of RCA-style receptacles (commonly used with the early cassette interface for data storage and retrieval) rounded out the cabling access. The back and front panels were revised in early 1976 with slotted holes for mounting the card cage side rails, allowing for centering and alignment of the side panels with respect to the motherboard sockets. The side rails now used 6-32 PEM nuts for mounting with 5/16" 6-32 Phillips head screws, rather than the self-tapping screws. The later side rails could not be used with the early front sub-panel for some reason that escapes me for now, but the newer front sub-panel had the notches for the power switch and motherboard increased in clearance. I know of only a couple of instances where IMSAI Customer Service replaced old-style sheet metal, and those were because of shipping damage or claims of missing parts.

Base Plate Revision- The base plate was originally punched in the mother board area with six clearance holes for #6 sheet metal screws. Six #6 hex head sheet metal screws (HSMS) entered from the underside and engaged the motherboard mounting plate. This arrangement provided a functional method of stiffening the base plate, which was prone to flexing from the weight of the transformer in the power supply. The motherboard mounting plate was punched with 24 holes that allowed the builder to fasten the motherboard to it with #6 HSMS. The screws went through twelve holes along each side of the motherboard, through a nylon or black fiber insulation washer, and into the mounting plate. A thick Mylar insulation sheet slipped under the assembled motherboard to help insulate the socket pins from the base plate.

The base plate was punched on the power supply side for the Electrolytic capacitor mounting brackets, each of two small and two large requiring three holes.  These were dropped when the second revision base plate was released.  A few chassis have turned up with the later style PS-28 power supply, but still having the older hole configuration.  These were simply older chassis that had been upgraded by drilling the newer holes into the original base plate.

When the power supply was upgraded from the original 10 amp model to the new 28 amp PS-C in early 1976, new mounting holes for transformer and power supply board mounting were incorporated for the second major revision.

With the third and last major revision (about August of 1976), the motherboard mounting plate was eliminated as a cost-cutting measure (a big mistake in my estimation), and the base plate was now punched with twenty-four #6 clearance holes to match the motherboard. Twenty-six 6-32 nylon screws were put in from the bottom, and fastened with 6-32 threaded aluminum spacers. Black fiber washers were placed next, the motherboard fitted over the screws, another set of black fiber washers, a #6 internal star lock washer, and finally a 6-32 hex nut completed the assembly.

Motherboards- the first IMSAI 8080 units and kits shipped with either an EXP-6 (standard) or EXP-10 motherboard (optional), depending on what was ordered. Only two sockets (with two nylon card guides each) were supplied with the basic machine. Additional sockets with card guides were optional. The EXP-6 was standard, while EXP-10 and the EXP-4 were optional choices, depending on the customer's anticipated requirements. EXP is a contraction of "expansion", and the number indicated the socket or "slot" capacity of each. By early spring of 1976, the EXP-22 became available and, by September of 1976, became the standard offering.

For the most part, all IMSAI circuit boards shipped in December of 1975 and early 1976 were not solder-masked. As old stock of the first revision boards was used up and new stock ordered, masked boards became standard. Prototype boards were not solder-masked except for some early and mid-1977 exceptions.

IMSAI offered a terminated motherboard beginning in Spring of 1977 with the introduction of the VDP-80 (Video Display Processor) which used an EXP-7 (7 card slots), and the EXP-10 used in the 80xx and VDP4x series machines. Termination of address and data lines was provided with 220/330 ohm resistors referenced to regulated five volts, a standard industry practice at that time. In 1981, Fischer-Freitas Corporation entered into agreement with Morrow Designs to offer George Morrow's 20 slot terminated motherboard as an option. We only sold about 20.

The second (from the front) slot of the EXP was seldom usable since there was inadequate clearance between the front sub-panel and components of the second slot board. IMSAI specified Texas Instruments gold-plated phosphor-bronze contacts for the S-100 sockets used in motherboards (and board sockets where used). Since all board contact fingers heavy gold plated over nickel (for strength), contact was usually adequate and seldom caused problems. The exception to this came about from the unfortunate choice of service personnel and customer alike using rubber erasers to clean the gold contacts. Eventually the gold would be worn away- even the nickel plating, thus exposing the copper as a contact point. Oxidation and corrosion would then rapidly aggravate flaky operation of an otherwise reliable system. High humidity and salt air environments made for trying times for Customer Support!

The Power Supply- The first IMSAI's used a 10 amp power transformer, the PS-A choke/filter board, and rectifier/heatsink assembly (mounted to the rear of the front sub-panel). Two large electrolytic "can-style" capacitors were held to the base plate with circular clamps and connected to the rectifiers with ring terminals at the top screws. Connected in parallel, they provided filtering for the unregulated 8 volt DC supply, while two smaller axial lead electrolytics mounted on the PS-A board filtered the unregulated plus and minus 18 volt supplies. I estimate between 100 and 200 of these machines were shipped; a few languished in Engineering, Marketing, and Customer Service for a year or so before they were scrapped.

In early 1976, a new 28-amp power supply dubbed the PS-28 became standard issue, incorporating a TRANEX power transformer and 16 1/2" x 5 3/4" power supply board. This cumbersome arrangement included three line chokes, ceramic line filter caps, rectifier/heatsink assembly, two 80,000 mfd. and two 10,000 mfd. electrolytics that were now mounted to the PS-28 board with the screw terminals in contact with the board. A fuse clip arrangement provided for short-circuit protection. Sadly, the whole arrangement of line voltage layout was a dangerous design exposing the builder and user to potential shock in the area of the fuse clips, power switch, chokes and several other areas.

By June of 1976 the PS-28C U was introduced for the export market. It used a dual-primary winding on the transformer to allow the user to configure the supply for 110-120 volt, or 220-240 volt operation. In late 1982, FFC introduced a resonant circuit supply using a Zambre-designed transformer and non-polar capacitor. Although heavier than the PS-28 transformer, the new supply now produced over 30 amps on the unregulated 8 volt supply with inherent stability and self-regulating qualities. Only 100 of these improved supplies were shipped.

The CP-A Front Panel- Perhaps the most striking characteristic of the IMSAI is its front panel with "nibble-grouped" red and blue paddle switches and black masked LED's.   Occasionally, an IMSAI 8080 will turn up with switch handles of a different color combination.  IMSAI's early Marketing Development guru Bruce Van Natta had all-black switch handles on his in-house machine to signify his "unique-ness".  Unusual and impractical, this choice of one color made the task of front panel switch programming less precise than when dealing with groups of four colors.

The first CPA boards were not solder masked until late spring of 1976. Like all IMSAI circuit boards, the CP-A was not provided with sockets, although they were an option. As mentioned above, low-profile Texas Instrument tin-plated contact sockets were provided. Many builders opted to use their own sockets, opting for gold-plated contacts or higher quality parts. High-profile sockets caused a clearance problem for the CP-A when the acrylic front mask set was installed.

The IMSAI was designed so that the CP-A was not necessary for system operation. This explains the choice of designer Joe Killian to put the data bus pull-up resistors on the MPU-A processor board and to use a 16 conductor flat cable with 16 pin "Dip" connector to connect the two boards together. This also allowed the CP-A to be used on a bus extender board (like the EXT) elsewhere in the motherboard for flexibility in testing and hardware debugging. It was not uncommon to find machines with multiple EXP motherboards, or an EXP-22 with address, data, and status lines cut to isolate the board. This was often done to allow use of two or more processors to operate in the same box, sometimes linked to each other's memory or I/O with flat cable(s).

Early IMSAI advertising suggested applications using IMSAI's PIO-4 parallel interface board as the front panel board. The PIO-4 had provision for 8 vertically arranged LED's for each of the four latched output ports, allowing a visual status of port output. I've seen several 8080's with custom masks that were made to allow viewing and identification of the status LED's with the original configuration acrylic set.

Four significant ECO's (Engineering Change Order) apply to the CP-A. They are:

bulletECO 76-0061- This modification prevented generation of the MEMWRITE signal during the OUTPUT instruction.
bulletECO 77-0035- This modification insured that the front panel always came up in the STOP mode on power up.
bulletECO 77-0039- This modification made the RUN line agree with the bus definition, allowing use with IMSAI's dynamic RAM boards (RAM 16, RAM 32, RAM 64, RAM III)
bulletECO 77-0098- This modification prevented spurious triggering of the ONE-SHOTS during RUN mode, causing unpredictable program execution.

 

 

To be continued:

-TRF