The Tektronix 7854 Waveform Processing Oscilloscope, introduced in 1980, is a 400 MHz combined analog / digital mainframe in the 7000 series that takes two vertical and two horizontal 7000-series plug-ins. In addition to a conventional analog (real time) scope, it contains an equivalent-time sampling 10-bit digitizer and a programmable waveform processor with GPIB interface. Single-shot, pretrigger acquisition is possible with the special 7B87 time base using real-time sampling. The project manager for the 7854 was Tom Rousseau.
The waveform processor can be programmed using a detachable calculator-style keyboard, using postfix operators ("reverse Polish notation" like HP calculators) that operate on entire waveforms, not just individual numbers.
|Bandwidth||(analog) 400 MHz|
|Fastest cal. sweep||500 ps|
|Sample rate||500 kHz (stores repetitive waveforms up to 400 MHz)|
|Acquisition depth||128, 256, 512 or 1024 points|
|Memory capacity||2048 waveform points in up to 16 waveforms (optional expansion to 5120 points and 40 waveforms)|
|Single shot||events and pretrigger up to 50 µs/div ("2.5 µs/point") with the 7B87 Time Base|
|CRT||154-0644-05 (P-31) or 154-0893-09 (P-11); cathode −3 kV, anode +21 kV, distributed vertical deflection plates|
The acquisition system uses what Tektronix termed a "display-oriented random sampling digitizing technique". It simultaneously digitizes a pair of X and Y signal values at a rate of 3.5 µs/point, using Schottky diode bridge based sample/hold circuits and a single 10-bit successive-approximation ADC. The Y sample value is then written into the memory location addressed by the X value, using DMA.
The CPU in the 7854 is a Texas Instruments TMS9900 16-bit microprocessor, known from the infamous TI-99/4 home computer of the late 70s, with 32+6KByte ROM and 8KByte RAM.
The firmware consists of mask ROMs, an FPLA, and patch EPROMs. These ROMs (particularly the early ones from Mostek) are a common point-of-failure. Several remedies exist, outlined here.
Diagnostic Test Interfaces
- 067-0911-00 Diagnostic Test Interface
- 067-0912-00 Analog Test Card for 7854
- 067-0961-00 Diagnostic Memory Board
- 067-0913-00 Extender Board 44-Pin (used with the A30-GPIB and A31-ROM Boards)
- 067-0914-00 Extender Board 80-Pin (used with the A27-MPU and A28-RAM Boards)
- 067-0915-00 Extender Board 124-Pin (used with the A26-Control Logic and A29-Display Boards)
- Tek 7854 Design Team
- Tek 7854 @ curiousmarc (with links to videos)
- Tek 7854 @ amplifier.cd
- Tek 7854 @ barrytech.com
- Mostek ROM Repairs @ vintagetek.org
- overview of ROM repair options @ paulcarbone.com
Mainframe rise time measured with 067-0587-02