T-PI PADS

T-PI PADS calculates component values for unbalanced T-pad and pi-pad resistive attenuator networks used in audio and RF circuits. Given input and output impedances (Zi and Zo), the program computes the minimum achievable pad loss and then calculates resistor values (r1, r2, r3) for both topologies at a user-specified dB loss, using the standard voltage-ratio formula K = e^(L/4.342944819). A custom rounding subroutine at lines 320–450, driven by two DEF FN functions, normalizes computed resistor values to a sensible number of significant figures depending on magnitude. The program draws schematic diagrams of both pad types on screen using PLOT, DRAW, and CIRCLE commands, with a UDG character (defined from DATA at line 500) used as an ohm symbol suffix on printed values. A COPY command at line 260 allows the results screen to be sent directly to a printer.

---

Program Structure

The program is organized as a menu-driven calculator with a clear flow:

1. Lines 4–5: Define two helper functions FN r and FN s for rounding.
2. Lines 30–60: Startup splash screen with tape-stop prompt.
3. Lines 70–300: Main calculation loop — gather Zi/Zo, compute minimum loss, prompt for desired dB, then branch to display.
4. Lines 320–450: General-purpose magnitude-aware rounding subroutine.
5. Lines 460–500: UDG loader (reads DATA into UDG “A” for use as an ohm symbol).
6. Lines 520–780: Screen drawing routine — renders schematic outlines for both T and pi pads using PLOT/DRAW/CIRCLE.
7. Lines 790–850: Short DRAW subroutines for resistor symbols (zigzag horizontally/vertically) and wire segments.
8. Lines 860–920: T-pad (pi topology in the code labeled for T) resistor value calculation.
9. Lines 930–990: Pi-pad resistor value calculation.
10. Lines 1000–1150: Result display — prints component values positioned near schematic nodes using AT and string-length centering.
11. Lines 1160–1210: Introduction/help screen.

Attenuator Mathematics

The core math uses the standard voltage ratio K = EXP(l / 4.342944819), where 4.342944819 is 20 / LN(10), converting dB to a linear voltage ratio. The minimum pad loss for a mismatched impedance pair is computed at line 150 using the standard formula involving the impedance ratio. Lines 860–920 compute T-pad resistors, and lines 930–990 compute pi-pad resistors, both involving SQR(K * Zi * Zo) as the geometric mean term.

Rounding Subroutine (Lines 320–450)

This subroutine normalizes a value in X to a meaningful number of significant figures depending on its magnitude. Two DEF FN functions are used:

• FN r(q) = INT(X/q + 0.5) / q — rounds to the nearest multiple of 1/q (used for large values to reduce precision).
• FN s(q) = INT(X*q + 0.5) / q — rounds to the nearest multiple of 1/q (used for small values to increase precision).

The cascade of IF statements from line 320 to 440 selects the appropriate rounding scale based on magnitude, covering values from 10,000,000 down to 0.00001. This ensures displayed resistor values appear with a sensible number of digits regardless of the impedance and loss inputs.

Schematic Drawing

The schematic diagrams for both T and pi topologies are drawn entirely with BASIC graphics primitives (PLOT, DRAW, CIRCLE). Resistor symbols are rendered as zigzag lines using sequences of short diagonal DRAW commands in subroutines at lines 790 and 800. Wire stubs are handled by short DRAW subroutines (lines 810–850). The layout draws both pads on screen simultaneously, occupying the upper and lower halves of the display.

UDG Usage

The subroutine at lines 460–480 loads an 8-byte pattern from the DATA statement at line 500 into UDG “A”. This UDG is then appended to printed resistor values via "\\a" in PRINT statements (lines 1030–1140), serving as an ohm (Ω) symbol. The DATA bytes 0,24,36,66,66,36,102,0 form a simple omega-like glyph.

Value Positioning

Resistor values are printed near their schematic positions using PRINT AT row, col. The column is calculated dynamically using LEN(STR$(value)) to approximately center each value on its node — a common BASIC centering idiom. For example, line 1030 uses (12 - LEN(a$)/2) as the column offset.

Input Validation and Error Handling

Line 130 enforces that Zo ≤ Zi, redirecting to a message and back to the input prompt if violated. Line 210 rejects a dB loss of 0 or ≥ 300 as impractical, pausing and looping back. There is no validation that the requested loss exceeds the computed minimum (line 180 only displays the minimum; the program proceeds regardless), which could yield negative or nonsensical resistor values if the user enters a loss below the minimum.

Notable Idioms

• PAUSE NOT PI at line 1210 is an efficient way to wait for a keypress: NOT PI evaluates to 0, so this is equivalent to PAUSE 0.
• COPY / at line 260 sends the screen to a printer in a single command, including the schematic diagram.
• POKE 23658,0 at line 60 disables the CAPS LOCK state after the initial keypress.
• VAL "number" is not used here; line numbers in GO TO/GO SUB are given as literals.
• Line 1240 saves the program with LINE 20, causing it to auto-run from line 20 (skipping the DEF FN lines at 4 and 5, which are handled before execution begins).

Potential Anomaly

The SAVE line (1240) specifies LINE 20 as the auto-run start, but the DEF FN statements at lines 4 and 5 are before line 20. Since DEF FN lines must be executed before the functions can be called, and the auto-run skips to line 20, the functions FN r and FN s would not be defined when the program runs automatically. This would cause an error the first time the rounding subroutine is called (line 320). Running the program manually from line 1 (or including the DEF FN lines after line 20) would be required for correct operation.