Chapter 20 The HP Model 9100A computing calculator 253
Program read only memory
The 32,768 bit read only program memory consists of 512 64-bit words. These words contain all of the operating subroutines, stored constants, character encoders, and CRT modulating patterns. The 512 words are contained in a 16 layer printer-circuit board having drive and sense lines orthogonally located. A drive line consists of a reference line and a data line. Drive pulses are inductively coupled from both the reference line and data line into the sense lines. Signals from the data line either aid or cancel signals from the reference line producing either a 1 or 0 on the output sense lines. The drive and sense lines are arranged to achieve a bit density in the ROM data board of 1000 bits per square inch.
The program ROM decoder/driver circuits are located directly above the ROM data board. Thirty-two combination sense amplifier, gated-latch circuits are located on each side of the ROM data board. The outputs of these circuits control the hard wired logic gates on the instruction logic board.
Side boards
The program ROM printed circuit board and the instruction logic board are interconnected by the side boards, where preliminary signal processing occurs.
The keyboard
The keyboard contains 63 molded plastic keys. Their markings will not wear off because the lettering is imbedded into the key body using a double shot injection molding process. The key and switch assembly was specifically designed to obtain a pleasing feel and the proper amount of tactile and aural feedback. Each key operates a single switch having gold alloy contacts. A contact closure activates a matrix which encodes signals on six data lines and generates an initiating signal. This signal is delayed to avoid the effects of contact bounce. An electrical interlock prevents errors caused by pressing more than one key at a time.
Magnetic card reader
Two complete 196 step programs can be recorded on the credit card size magnetic program card. The recording process erases any previous information so that a card may be used over and over again. A program may be protected against accidental erasure by clipping off the corner of the card, Fig. 9, page 249. The missing corner deactivates the recording circuitry in the magnetic card reader. Program cards are compatible among machines.
Information is recorded in four tracks with a bit density of 200 bits per inch. Each six-bit program step is split into two time-multiplexed, three-bit codes and recorded on three of the four tracks. The fourth track provides the timing strobe.
Information is read from the card and recombined into six bit codes for entry into the core memory. The magnetic card reading circuitry recognizes the 'END' program code as a signal to end the reading process. This feature makes it possible to enter subroutines within the body of a main program or to enter numeric constants via the program card. The END code also sets the program counter to location 0-0, the most probable starting location. The latter feature makes the Model 9100A ideally suited to 'linking' programs that require more than 196 steps.
Packaging and servicing
The packaging of the Model 9100A began by giving the HP industrial design group a volume estimate of the electronics package, the CRT display size and the number of keys on the keyboard. Several sketches were drawn and the best one was selected. The electronics sections were then specifically designed to fit in this case. Much time and effort were spent on the packaging of the arithmetic processing unit. The photographs, Figs. 11 and 14, attest to the fact that it was time well spent.
The case covers are die cast aluminum which offers durability, effective RFL shielding, excellent heat transfer characteristics, and convenient mechanical mounts. Removing four screws allows the case to be opened and locked into position, Fig. 14. This procedure exposes all important diagnostic test points and adjustments. The keyboard and arithmetic processing unit may be freed by removing four and seven screws respectively.
Any component failures can be isolated by using a diagnostic routine or a special tester. The faulty assembly is then replaced and is sent to a service center for computer assisted diagnosis and repair.
Reliability
Extensive precautions have been taken to insure maximum reliability. Initially, wide electrical operating margins were obtained by using 'worst case' design techniques. In production all transistors are aged at 80% of rated power for 96 hours and tested before being used in the Model 9100A. Subassemblies are computer tested and actual operating margins are monitored to detect trends that could lead to failures. These data are analyzed and corrective action is initiated to reverse the trend. In addition, each calculator is operated in an environmental chamber at 55° C for 5 days prior to shipment to the customer. Precautions such as these allow Hewlett-Packard to offer a one year warranty in a field where 90 days is an accepted standard.