7A16P: Difference between revisions

The '''Tektronix 7A16P''' is a single channel, programmable, vertical amplifier for [[:Category:7000_series_digitizer_mainframes|7000-series digitizing mainframes]]. All functions of the 7A16P can be remotely programmed via the [[GPIB]] 7000-series main interface bus, with exception of the variable gain control and the probe identify switch. It is special among the 7000-series vertical amplifiers since it offers a switchable 1 MΩ/50 Ω input impedance, only paralleled by the [[7A42]]. The 7A16P is also unique as it has two input BNC connectors, but is strictly a single channel plugin, having a switch that routes one of the BNC connectors to the amplifier circuits. This feature is handy in remote-controlled digitizing applications.

It was mainly intended for use in the [[7612D]] and the [[7912|7912 family]], but can also be used in the [[7854]] if paired with a [[021-0374-00]] and internally modified by adding cables for a 18 pin header and setting jumpers correctly. It is not recommended to use the 7A16P in any conventional 7000-series mainframe since it expects +5.1 V at pins [[7000_Series_plug-in_interface|B27 and A9 of the mainframe interface]], and shorts them together. A9 is the 5V light pin on conventional mainframes, and its current rating might be too low for successful operation of the 7A16P.  

The input stage of the 7A16P contains miniature relays switching between the two available input connectors and selecting the AC and DC coupling modes. Another relay is used to enter a 50 Ω shunt if the corresponding input impedance is selected. Relays then select an optional 10x attenuator before entering the input buffer. This buffer contains an AC coupled MOSFET common drain amplifier and a DC coupled operational amplifier, configured in a feed-beside scheme. Gain switching is accommodated by several junction FETs to enter another 10x attenuator in front of the MOSFET buffer, and to enter 2x and 5x low impedance attenuators after the input buffer.  

The digital circuitry is build around a [[Motorola 6800]] processor with several MC6821 to drive the switches of the analog circuits and interfacing the modified 7000-series internal GPIB bus.

When operated with the [[021-0374-00]] the 7A16P's modified GPIB bus is directly connected to the [[021-0374-00]] through a 18-pin, dual-row pin header and 17 cables. Following the labeling from the 021-0374-00 manual the cable is connected as follows:

| 1 ||  bgcolor="SaddleBrown" | ||  U1200 pin 10||I/O 0

| 2|| bgcolor="red"|  || U1200 pin 11||I/O 1

| 3 ||bgcolor="orange"|  || U1200 pin 12||I/O 2

| 4 || bgcolor="yellow"|  || U1200 pin 13||I/O 3

| 5 || bgcolor="Lime"| || U1200 pin 14||I/O 4

| 6 || bgcolor="DeepSkyBlue"| || U1200 pin 15||I/O 5

| 7 || bgcolor="violet"| || U1200 pin 16||I/O 6

| 8 || bgcolor="Darkgrey"| || U1200 pin 17||I/O 7

| 9 || bgcolor="SaddleBrown" | ||  U500 pin 5 || <span style="text-decoration: overline">ATN</span>

|  10|| bgcolor="red"|  || U1620 pin 6 || <span style="text-decoration: overline">NDAC</span>

|  11 ||bgcolor="orange"|  || U1620 pin 8 || <span style="text-decoration: overline">NRFD</span>

Additionally, Jumper P1622 in the 7A16P should be set according to the [[021-0374-00]] manual. This connects the regular +5V supply on A1 to the lights pin A9. In a [[7854]] this will cause no direct harm if the rear "Control Illumination" switch is not in the "high" position since the [[7854]] uses series diodes to drop the light voltage if set to medium. However, the full fight current for all plugins will run through the 7A16P, so setting the switch to "high" is recommended.