Custom IC Electronics for a Low Cost Plotter

it was apparent eariy in the development of'he 7J70A that there mas a need to integrate much of trie electronics. The digita. portion most readily lends itself to integration in some form of custom IC The anatog circuits, on the other hand, were designed with off-the-shelf parts, but nevertheless benefited from the use of commercially available tCs

In deciding which part of the digital electronics should be made into a custom IC. we noticed that the microprocessor, memory. I/O controller and buffers and the servo controllers were all in LSi (large-scale integration) form already Our strategy, then, was to try to eliminate all ol the 74LS-series ICs from the printed circuit board This has many attractions: A custom LSI IC saves a tremendous amount of board space Substantially ess power is required from the power supply and less heat Is generated ■ Electromagnetic compatibility requirements can be met more readily because the radiation caused by the relatively short rise and fall times of LSTTL* signals and many Interconnecting traces on the printed circuit board are eliminated

Gate Arrays

After the decision was made to develop a custom IC, we had to select between the different technologies available The selection process was based on many Important factors that include per piece part cost, development cost, development time, expected volume, circuit complexity, reliability, and design risk, A fully custom NMOS (n-channel metal-oxide-semiconductor) integration effort was considered. This approach promised relatively low part cost using proven technology and design methods However, the very high development cost and lengthy development time could not be justified at our projected moderate volumes.

We turned to another approach—semicustom gate arrays, which are rapidly gaining in popularity. Gate arrays are Integrated circuits prefabricated In wafer form up to the final processing steps. These wafers are then customized at the metal masking stage by applying a unique Interconnect pattern to implement the logic design Because of the lower level of customization, shorter development time can be expected along with significant savings m development cost

Based on the 'arge amount of circuitry to be mplernerrted in LSI form and the number of signal pins required, two silicon-gate CMOS ■ complementary metai-o> ¡de-semiconductor) gate arrays were selected. The silicon-gate process provides the necessary performance level, and the use of CMOS logic reduces power consumption to a very low level Of the two arrays chosen one is a 770-gate array and the other contains lOOO gates Both are housed In low-cost 40-pinduai-mime plastic packages. Tnese two arrays are the equivalent of 80 LSTTL ICs that would nave required a current of 0.7A at 5V. As a result, the HP-IB version of the 7470A contains only eleven digital tCs. quite amazing considering its performance and capabilities

Armed with the strategy of attempting to put every little bit of miscellaneous digital circuitry in the gate array implementations, we began development of the array ICs. First, the circuits were designed using standard 74LS-series ICs. After breadboarding and testing was performed to check out the design, a gate count was done to determine the array size required. The resulting total gate count of 1200 precluded the use of one arge array Instead, two moderate sized arrays were chosen. The circuits were then partitioned for a good fit In the following way. The circuit functions for gate array A inciude the baud-rate generator, Interrupt timer, servo control and status ports, memory decode, Y-axis motor drive control, and Y-axis servo gain adjust. The circuit functions In gate array B Include the pen solenoid pulse-width modulator, power supply voltage measurement circuit, front-panel input ports, memory decode, X-axis motor drive control, and X-axis servo gain adjust.

During the development of the gate array ICs, the LSTTL version of the gate array circuitry was made into printed circuit boards. These boards served as logic simulators for the ICs, allowing the prototype 7470A Instrument to operate while the ICs were In development. These boards plugged directly into two 40-pin sockets on the instrument board via ribbon cables. When the gate array ICs were ready, the boards were replaced

-Peter Ma

Peter L. Ma

Peter Ma joined HP In 1978 and designed the I/O processor system lor the 7310A Printer and the digital circuits and gate arrays for the 7470A. He is a graduate of the University of Washington. where tie received a BSEE degree in 1978. and Stanford University, where he received an MSEE degree in 1982 Peter was born in Hong Kong and grew up in Seattle, Washington. He is a high-fidelity audio enthusiast, enjoys tennis, skiing, and going to the beach, and lives in San Diego, California.

David M. Ellement

David Ellement received the BSEE degree in 1976 from the University of California at Berkeley and joined HP shortly after. Besides his work on the 74 70A. he developed the 7245A's power supply. David was born in Buffalo, New York and is a member of the IEEE. He is married, has one daughter, and lives in Escondido, California. When not busy improving his mastery of the local Spanish dialect. David enjoys soccer, cross-country skiing, and boulder ing

Neal J. Martini

Neal Martini has been with HP since 1978 and was the electronics project leader for the 7470A. He received a BSEE degree from the University of Detroit In 1969 and the MSEE degree from the University of Missouri at Rolla in 1971. Born in Buffalo, New York, Neal now Irves in San Diego, California. He is married, has two children, and enjoys playing racquetball, tennis, basketball, and piano.


Special thanks to the following for their contributions to the 7470A development: Dave Paulsen and Janie Chintala for their outstanding software contributions, Gary Skrabatenas for his motor driver and power supply designs, Richard Murray for his work in the I'O area of the 7470A, Keith Cobbs for his analysis and testing of the servo design. Dee Setliff for her contributions to the RS-232-C code development, and Bruce Jenkins for his help in following through with the HP-lb version of the 7470A.


1. C.C. Lo, "Plotter Servo Electronics Contained on a Single IC," Hewlett-Packard Journal, Vol. 32, no. 10, October 1981,

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