Tektronix 11000-series scopes use a serial digital protocol for communication between the mainframe and the plug-ins. This communication mechanism is called Serial Digital Interface (SDI). SDI was designed by Lester Larson.
SDI is bidirectional: The mainframe can send messages to the plug-ins; the plug-ins can send messages to the mainframe. SDI uses three signals:
- SDI Clock
- SDI Mainframe to plug-in data
- SDI Plug-in to mainframe data
The signals are 5 V TTL. The clock is a 4 MHz square wave running all the time.
In the 11301 mainframe, SDI communication is controlled by the U1760 SDI IC (Tektronix part number 156-2622-00), which is on the A11 Main Processor board, at the top rear of the scope.
The communications use hardware handshaking at the byte level, so that overruns never happen. Data only flows as fast as the slowest party (mainframe or plugin) can handle it (except there are provisions for detecting outrageously long handshakes and resetting the communications links).
- 11000-Series Plug-in to Main Frame Interface Manual (Section 3 describes SDI)
- Example recording of SDI bus traffic: 11302 startup in Sigrok format
This is a trace of what goes over the SDI bus when an 11302 is powered up and allowed to complete its entire startup procedure. When this trace was captured, there was an 11A32 in the left plug-in compartment, and 11A71 plug-ins in the center and right compartments. The mainframe stops on an ROM checksum error at one point during the capture, and I quickly press the WAVEFORM button next to the screen to bring the scope to normal operating mode. The file is in Sigrok format which can be viewed using the free open-source program PulseView available on the Sigrok download page.