HP 34970A Data Acquisition Unit - communication protocol

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.

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 8E1 schema (note the even parity bit).

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:

  • one-complement of the received SoT byte, so

    • 0x99 for the 0x66 SoT byte and
    • 0xCC for the 0x33 SoT byte.

  • 0x00 otherise

• a special packet is sent by the Display Panel at startup, which SoT value is 0x33, which does not seem to follow the general pattern in which the first value after the SoT byte is the number of arguments.

If a key is pressed during while powering the unit on, then the initialization of the Display Panel is frozen (with all elements of the display lit) until the key is released. In this case, the initialization packet adds the key code of that key, typically used with the Shift key pressed in to force a full test of the device (in this datagram, 0x0C is the keycode of the Shift key):

Shutdown sequence

CPU -> DP trasmission

The CPU->DP transmission protocol looks like:

DP -> CPU transmission

When the user press a key on the front panel, a slightly simpler "packet transmission" occurs:

Commands

The known command bytes sent by the CPU for now are:

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.

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.

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):

Keypad

the keycode byte consists in: