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| 1 ========================================================== | |
| 2 HP 34970A Data Acquisition Unit - communication protocol | |
| 3 ========================================================== | |
| 4 | |
| 5 :Author: David Douard | |
| 6 :Category: Electronics | |
| 7 :Tags: HP, 34970A, DMM, repair, test equipment | |
| 8 :series: HP 34970A repair | |
| 9 :JavaScripts: default.js, WaveDrom.js | |
| 10 | |
| 11 In order to build a replacement display for the HP34970A data | |
| 12 acquisition unit, I needed to understand the communication protocol | |
| 13 between the CPU board and the front panel assembly. | |
| 14 | |
| 15 General description | |
| 16 =================== | |
| 17 | |
| 18 The unit is built with four boards: | |
| 19 | |
| 20 - A1 is the main bard, with the PSU, the main controller and the floating logic, | |
| 21 | |
| 22 - A2 is the front panel with the display and the keypad, | |
| 23 | |
| 24 - A3 is the backplane on which I/O modules are plugged, | |
| 25 | |
| 26 - A4 is the (optional) internal 6.5 digits DMM. | |
| 27 | |
| 28 .. image:: {filename}/images/hp34970a/block_diagram.svg | |
| 29 :alt: System Block Diagram of the HP34970A. | |
| 30 | |
| 31 | |
| 32 The commnucation between the different system blocks is done with | |
| 33 asynchronous serial links. They use the rather uncommon bit rate of | |
| 34 187500 baud, with a classic 8N1 schema. | |
| 35 | |
| 36 | |
| 37 The CPU <-> Display Panel communication protocol | |
| 38 ================================================ | |
| 39 | |
| 40 The communication protocol between the main controller (CPU) board and | |
| 41 the display panel (DP) consists in "datagrams" sent using the general | |
| 42 pattern: | |
| 43 | |
| 44 - when a device (CPU or DP) wants to take control of the communication | |
| 45 bus, it sends a ``Start of Transmission`` (SoT) signal (0x66), | |
| 46 | |
| 47 - each sent char (but the end of transmission) must be acknowledged | |
| 48 (ack value may vary), | |
| 49 | |
| 50 - at the end of a communication, the initiator send a "End of | |
| 51 Transmission" (EoT, 0x55). This sent value is not acknowledged. | |
| 52 | |
| 53 - the keyboard can interrupt a CPU->DP communication in progress by | |
| 54 not acknowledge a received byte, but sending a SoT instead of the | |
| 55 expected ACK value, | |
| 56 | |
| 57 - acknowledge values are: | |
| 58 | |
| 59 - 0x99 as a response to the SoT, | |
| 60 | |
| 61 - 0x00 otherise | |
| 62 | |
| 63 | |
| 64 The CPU->DP transmission protocol looks like: | |
| 65 | |
| 66 .. wavedrom:: | |
| 67 | |
| 68 { signal: [ | |
| 69 {name: "Rx", wave: "z3x4x4x=x=x|.=x3x", data: ["0x66", "CMD", "LEN", "D0", "D1", "Dn", "0x55"] }, | |
| 70 {name: "Tx", wave: "zx5x5x5x5x5x|.5x.", data: ["0x99", "0x00","0x00", "0x00", "0x00", "0x00"] }, | |
| 71 ]} | |
| 72 | |
| 73 Two (or more) datagrams can be transmitted in a single "transmission", | |
| 74 ie. without sending the EoT byte, eg.: | |
| 75 | |
| 76 .. wavedrom:: | |
| 77 | |
| 78 { signal: [ | |
| 79 {name: "Rx", wave: "z3x4x4x|=x3x4x|3x", | |
| 80 data: ["0x66", "0x0C", "0x03", "D3", "0x66", "0x0A", "0x55"] }, | |
| 81 {name: "Tx", wave: "zx5x5x5x|5x5x5x|x", | |
| 82 data: ["0x99", "0x00", "0x00", "0x00", "0x99", "0x00"] }, | |
| 83 ]} | |
| 84 | |
| 85 | |
| 86 When the user press a key on the front panel, a slightly simpler "packet transmission" occurs: | |
| 87 | |
| 88 .. wavedrom:: | |
| 89 | |
| 90 { signal: [ | |
| 91 {name: "Rx", wave: "zz5x5xx", data: ["0x99", "0x00"]}, | |
| 92 {name: "Tx", wave: "z3x4x3x", data: ["0x66", "KP", "0x55"]}, | |
| 93 ]} | |
| 94 | |
| 95 | |
| 96 Sending data to the main display | |
| 97 ================================ | |
| 98 | |
| 99 The main display consist in 13 17-segments digits, in which the | |
| 100 character is displayed by a main 14-digits, and the punctuation with 3 | |
| 101 segments (2 dots and a comma, allowing to represent the signs ".", | |
| 102 ",", ":" and ";"). Punctuation signs are also very close to the | |
| 103 preceding chracters. | |
| 104 | |
| 105 .. image:: {filename}/images/hp34970a/digit.jpg | |
| 106 :alt: 17-segments digit of the main display. | |
| 107 | |
| 108 The command used to send text to the main display is ``0x00``. The | |
| 109 character ``0x09`` (tabulation) has a special meaning: it marks the | |
| 110 beginning and the end of a part of the text to be displayed darker | |
| 111 than the usual. This is used to emphasis a portion of the displayed | |
| 112 text. Also, as the punctuation signs do not consume a digit, the | |
| 113 displayed text can be larger than 13 characters. | |
| 114 | |
| 115 | |
| 116 Sending data to the Channels display | |
| 117 ==================================== | |
| 118 | |
| 119 This area only allows to display 3 7-segments digits. The command is | |
| 120 ``0x0C``, the payload is thus 3 bytes long. | |
| 121 | |
| 122 .. image:: {filename}/images/hp34970a/channel.jpg | |
| 123 :alt: The display area dedicated to current channel. | |
| 124 :align: center | |
| 125 | |
| 126 Flags | |
| 127 ===== | |
| 128 | |
| 129 The display also has several flags. Display flags are selected by | |
| 130 sending the ``0x0A`` command. The payload is 4 bytes long. Each bit | |
| 131 of these 4 bytes represent a flag on the display. | |
| 132 | |
| 133 Let's consider the following (we don't represent the acknowledgements here): | |
| 134 | |
| 135 .. wavedrom:: | |
| 136 | |
| 137 { signal: [ | |
| 138 {name: "Rx", wave: "z344====3x", | |
| 139 data: ["0x66", "0x0A", "0x04", "F1", "F2", "F3", "F4", "0x55"]} | |
| 140 ]} | |
| 141 | |
| 142 Then the flags I've identified so far are: | |
| 143 | |
| 144 .. wavedrom:: | |
| 145 | |
| 146 { signal: [ | |
| 147 {name: "bit", wave: "z========z", | |
| 148 data: ["7", "6", "5", "4", "3", "2", "1", "0"]}, | |
| 149 | |
| 150 {name: "F1", wave: "z=3333333z", | |
| 151 data: ["", "HI", "Alarm", "LO", "Channels", "Ch. frame", "Mx+B", "<Bell>"]}, | |
| 152 {name: "F2", wave: "z===33333z", | |
| 153 data: ["", "", "", "4W", "1", "3", "4", "2"]}, | |
| 154 {name: "F3", wave: "z========z", | |
| 155 data: ["", "", "", "", "", "", "", ""]}, | |
| 156 {name: "F4", wave: "z=3=33===z", | |
| 157 data: ["", "CONFIG", "", "MON", "VIEW", "", "", ""]}, | |
| 158 ], | |
| 159 config: { hscale: 2 }, | |
| 160 } | |
| 161 | |
| 162 | |
| 163 | |
| 164 | |
| 165 | |
| 166 Keypad | |
| 167 ====== | |
| 168 | |
| 169 Note that the front panel sends a "key press event" and a "key | |
| 170 released event", depending on the value of the bit 7: | |
| 171 | |
| 172 :0: key pressed | |
| 173 :1: key released | |
| 174 | |
| 175 The bit 8 of the key event byte is set high for the knob. | |
| 176 | |
| 177 .. wavedrom:: | |
| 178 | |
| 179 {signal: [ | |
| 180 | |
| 181 {name: "bit", wave: "z========z", | |
| 182 data: ["7", "6", "5", "4", "3", "2", "1", "0"]}, | |
| 183 {name: "KP", wave: "z34=.....z", | |
| 184 data: ["Knob", "Key", "Key Code"]}, | |
| 185 ], | |
| 186 config: { hscale: 1 }, | |
| 187 } | |
| 188 | |
| 189 | |
| 190 The key codes are: | |
| 191 | |
| 192 :0x00: View | |
| 193 :0x01: Mon | |
| 194 :0x02: Sto/Rcl | |
| 195 :0x03: Scan | |
| 196 :0x04: Alarm | |
| 197 :0x05: Mx+B | |
| 198 :0x06: Measure | |
| 199 :0x07: Interval | |
| 200 :0x08: Card Reset | |
| 201 :0x09: Close | |
| 202 :0x0A: Open | |
| 203 :0x0B: Read | |
| 204 :0x0C: Shift | |
| 205 :0x0D: Write | |
| 206 :0x0E: Left | |
| 207 :0x0F: Right | |
| 208 | |
| 209 :0x10: Advanced | |
| 210 :0x11: Step | |
| 211 | |
| 212 For the knob (including the "Knob" bit): | |
| 213 :0x80: Knob left | |
| 214 :0x81: Knob right |
