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+++ b/content/hp34970a_protocol.rst Sun Sep 18 22:40:28 2016 +0200
@@ -0,0 +1,214 @@
+==========================================================
+ HP 34970A Data Acquisition Unit - communication protocol
+==========================================================
+
+:Author: David Douard
+:Category: Electronics
+:Tags: HP, 34970A, DMM, repair, test equipment
+:series: HP 34970A repair
+:JavaScripts: default.js, WaveDrom.js
+
+In order to build a replacement display for the HP34970A data
+acquisition unit, I needed to understand the communication protocol
+between the CPU board and the front panel assembly.
+
+General description
+===================
+
+The unit is built with four boards:
+
+- A1 is the main bard, with the PSU, the main controller and the floating logic,
+
+- A2 is the front panel with the display and the keypad,
+
+- A3 is the backplane on which I/O modules are plugged,
+
+- A4 is the (optional) internal 6.5 digits DMM.
+
+.. image:: {filename}/images/hp34970a/block_diagram.svg
+ :alt: System Block Diagram of the HP34970A.
+
+
+The commnucation between the different system blocks is done with
+asynchronous serial links. They use the rather uncommon bit rate of
+187500 baud, with a classic 8N1 schema.
+
+
+The CPU <-> Display Panel communication protocol
+================================================
+
+The communication protocol between the main controller (CPU) board and
+the display panel (DP) consists in "datagrams" sent using the general
+pattern:
+
+- when a device (CPU or DP) wants to take control of the communication
+ bus, it sends a ``Start of Transmission`` (SoT) signal (0x66),
+
+- each sent char (but the end of transmission) must be acknowledged
+ (ack value may vary),
+
+- at the end of a communication, the initiator send a "End of
+ Transmission" (EoT, 0x55). This sent value is not acknowledged.
+
+- the keyboard can interrupt a CPU->DP communication in progress by
+ not acknowledge a received byte, but sending a SoT instead of the
+ expected ACK value,
+
+- acknowledge values are:
+
+ - 0x99 as a response to the SoT,
+
+ - 0x00 otherise
+
+
+The CPU->DP transmission protocol looks like:
+
+.. wavedrom::
+
+ { signal: [
+ {name: "Rx", wave: "z3x4x4x=x=x|.=x3x", data: ["0x66", "CMD", "LEN", "D0", "D1", "Dn", "0x55"] },
+ {name: "Tx", wave: "zx5x5x5x5x5x|.5x.", data: ["0x99", "0x00","0x00", "0x00", "0x00", "0x00"] },
+ ]}
+
+Two (or more) datagrams can be transmitted in a single "transmission",
+ie. without sending the EoT byte, eg.:
+
+.. wavedrom::
+
+ { signal: [
+ {name: "Rx", wave: "z3x4x4x|=x3x4x|3x",
+ data: ["0x66", "0x0C", "0x03", "D3", "0x66", "0x0A", "0x55"] },
+ {name: "Tx", wave: "zx5x5x5x|5x5x5x|x",
+ data: ["0x99", "0x00", "0x00", "0x00", "0x99", "0x00"] },
+ ]}
+
+
+When the user press a key on the front panel, a slightly simpler "packet transmission" occurs:
+
+.. wavedrom::
+
+ { signal: [
+ {name: "Rx", wave: "zz5x5xx", data: ["0x99", "0x00"]},
+ {name: "Tx", wave: "z3x4x3x", data: ["0x66", "KP", "0x55"]},
+ ]}
+
+
+Sending data to the main display
+================================
+
+The main display consist in 13 17-segments digits, in which the
+character is displayed by a main 14-digits, and the punctuation with 3
+segments (2 dots and a comma, allowing to represent the signs ".",
+",", ":" and ";"). Punctuation signs are also very close to the
+preceding chracters.
+
+.. image:: {filename}/images/hp34970a/digit.jpg
+ :alt: 17-segments digit of the main display.
+
+The command used to send text to the main display is ``0x00``. The
+character ``0x09`` (tabulation) has a special meaning: it marks the
+beginning and the end of a part of the text to be displayed darker
+than the usual. This is used to emphasis a portion of the displayed
+text. Also, as the punctuation signs do not consume a digit, the
+displayed text can be larger than 13 characters.
+
+
+Sending data to the Channels display
+====================================
+
+This area only allows to display 3 7-segments digits. The command is
+``0x0C``, the payload is thus 3 bytes long.
+
+.. image:: {filename}/images/hp34970a/channel.jpg
+ :alt: The display area dedicated to current channel.
+ :align: center
+
+Flags
+=====
+
+The display also has several flags. Display flags are selected by
+sending the ``0x0A`` command. The payload is 4 bytes long. Each bit
+of these 4 bytes represent a flag on the display.
+
+Let's consider the following (we don't represent the acknowledgements here):
+
+.. wavedrom::
+
+ { signal: [
+ {name: "Rx", wave: "z344====3x",
+ data: ["0x66", "0x0A", "0x04", "F1", "F2", "F3", "F4", "0x55"]}
+ ]}
+
+Then the flags I've identified so far are:
+
+.. wavedrom::
+
+ { signal: [
+ {name: "bit", wave: "z========z",
+ data: ["7", "6", "5", "4", "3", "2", "1", "0"]},
+
+ {name: "F1", wave: "z=3333333z",
+ data: ["", "HI", "Alarm", "LO", "Channels", "Ch. frame", "Mx+B", "<Bell>"]},
+ {name: "F2", wave: "z===33333z",
+ data: ["", "", "", "4W", "1", "3", "4", "2"]},
+ {name: "F3", wave: "z========z",
+ data: ["", "", "", "", "", "", "", ""]},
+ {name: "F4", wave: "z=3=33===z",
+ data: ["", "CONFIG", "", "MON", "VIEW", "", "", ""]},
+ ],
+ config: { hscale: 2 },
+ }
+
+
+
+
+
+Keypad
+======
+
+Note that the front panel sends a "key press event" and a "key
+released event", depending on the value of the bit 7:
+
+:0: key pressed
+:1: key released
+
+The bit 8 of the key event byte is set high for the knob.
+
+.. wavedrom::
+
+ {signal: [
+
+ {name: "bit", wave: "z========z",
+ data: ["7", "6", "5", "4", "3", "2", "1", "0"]},
+ {name: "KP", wave: "z34=.....z",
+ data: ["Knob", "Key", "Key Code"]},
+ ],
+ config: { hscale: 1 },
+ }
+
+
+The key codes are:
+
+:0x00: View
+:0x01: Mon
+:0x02: Sto/Rcl
+:0x03: Scan
+:0x04: Alarm
+:0x05: Mx+B
+:0x06: Measure
+:0x07: Interval
+:0x08: Card Reset
+:0x09: Close
+:0x0A: Open
+:0x0B: Read
+:0x0C: Shift
+:0x0D: Write
+:0x0E: Left
+:0x0F: Right
+
+:0x10: Advanced
+:0x11: Step
+
+For the knob (including the "Knob" bit):
+:0x80: Knob left
+:0x81: Knob right 