Tektronix hex in Lua

When debugging complex hardware interactions, directly probing signals can be time-consuming and introduce noise. Tektronix's hexadecimal display in Lua scripting offers a powerful solution for engineers needing precise control and visibility into digital data streams. This guide will walk you through leveraging Lua to configure and interpret hexadecimal outputs from Tektronix oscilloscopes and logic analyzers. You'll learn how to efficiently decode bus activity, identify data anomalies, and streamline your debugging workflow, gaining deeper insights into your system's behavior right from your script.

Interfacing with Hex Data in Lua

Lua treats strings as sequences of bytes, which simplifies working directly with hexadecimal data. You can manipulate hex strings by converting pairs of characters into their corresponding byte values.

For instance, to convert the hex string "FF0A" into its byte representation, you can use the following:

local hex_string = "FF0A"
local bytes = string.gsub(hex_string, "(..)", function(hexpair)
    return string.char(tonumber(hexpair, 16))
end)
-- Now, 'bytes' holds the byte representation of FF0A

A common pitfall is forgetting that string.char requires decimal ASCII values. You must convert each two-character hex pair (like "FF") to its decimal equivalent (255) using tonumber(hexpair, 16) before passing it to string.char. Mastering this conversion is key for accurate hex data processing in Lua.

Parsing Hexadecimal Strings

Working with hexadecimal data in Tektronix oscilloscopes often means dealing with strings representing byte values. You'll frequently need to convert these hex strings into a usable format, like Lua numbers or byte sequences. The core process involves iterating through the hex string, typically processing two characters at a time, and converting each pair into its decimal equivalent.

Here's a practical function to parse a hex string into a table of decimal byte values:

function hex_to_bytes(hex_string)
  local bytes = {}
  for i = 1, #hex_string, 2 do
    local hex_pair = hex_string:sub(i, i + 1)
    -- Ensure we don't try to convert an incomplete pair
    if hex_pair and #hex_pair == 2 then
      table.insert(bytes, tonumber(hex_pair, 16))
    end
  end
  return bytes
end

A common pitfall is handling hex strings with an odd number of characters. The loop incrementing by two might miss the final character or cause an error if it attempts to read beyond the string's boundary. Always ensure your parsing logic accounts for this, perhaps by padding or explicitly ignoring an incomplete final pair. This careful handling ensures your data integrity.

Generating Hexadecimal Output

Converting Lua data back into hexadecimal strings is crucial for many low-level operations and protocol implementations. You'll often need to represent numbers and byte sequences in a readable, standardized hex format.

A common task is converting a table of decimal byte values into a single hexadecimal string. The string.format function in Lua is your go-to for this. Using the %02X format specifier ensures each byte is represented by two uppercase hexadecimal characters, automatically padding with a leading zero if the value is less than 16 (e.g., 10 becomes "0A").

function bytes_to_hex(bytes_table)
  local hex_string = ""
  for _, byte_value in ipairs(bytes_table) do
    hex_string = hex_string .. string.format("%02X", byte_value)
  end
  return hex_string
end

-- Example usage:
local data = { 72, 101, 108, 108, 111 } -- ASCII for "Hello"
print(bytes_to_hex(data)) -- Output: 48656C6C6F

A common pitfall is omitting the zero-padding. Protocols that expect fixed-width byte representations will often fail if they receive, say, "F" instead of "0F". Always ensure your hex output for bytes is consistently two characters wide.

Working with Binary Data Structures

Lua's string manipulation capabilities, combined with hexadecimal conversion, allow you to parse and generate fixed-format binary data, such as network packets. This process involves mapping specific byte ranges within a hex payload to meaningful data fields.

For instance, consider extracting a 2-byte integer from a hex string:

local packet_hex = "01020A3B" -- Example packet
local start_byte_index = 3 -- Index for bytes 3 and 4 ('0A' and '3B')
local byte1 = tonumber(packet_hex:sub(start_byte_index, start_byte_index + 1), 16)
local byte2 = tonumber(packet_hex:sub(start_byte_index + 2, start_byte_index + 3), 16)
local integer_value = byte1 * 256 + byte2 -- Assuming big-endian
print("Extracted integer:", integer_value) -- Output: Extracted integer: 10

A common pitfall is byte order (endianness). Always verify if the data is big-endian or little-endian to avoid misinterpreting multi-byte values. Ensure you understand the data format specification to correctly interpret each byte.