Closes #6

README.md

@@ -532,9 +532,8 @@ one-based physical line number, including blank lines:
 example.dbc:3: Invalid CAN ID: invalid
 ```
 
-This change validates frame definitions. Signal syntax and unsupported DBC
-constructs retain their existing behavior; comprehensive validation is still
-planned. File-open errors contain the path but have no line number.
+This change validates frame definitions. Signal definitions are also validated as described below; comprehensive
+DBC grammar coverage is still planned. File-open errors contain the path but have no line number.
 
 ---
 
@@ -562,6 +561,35 @@ Parsed fields:
 
 ---
 
+### Signal validation and parse errors
+
+`SG_` statements are matched completely. Signal names must be nonempty and
+contain no whitespace. Spaces and tabs are accepted between syntax elements;
+byte order and signedness remain adjacent (`@0+`, `@0-`, `@1+`, or `@1-`).
+A quoted unit (which may be empty) and a nonempty comma-separated receiver
+list are required. Unexpected trailing fields are rejected.
+
+Start bits must be unsigned decimal integers fitting `uint32_t`; lengths must
+be within `1..64` bits. Signs, fractional values, numeric suffixes, and overflow
+are rejected for these fields. Every signal bit must fit within the frame's
+declared payload length, using Intel increasing bit positions or Motorola
+sawtooth traversal. This uses the stored payload length without introducing
+an additional CAN or CAN FD payload limit.
+
+Factor, offset, minimum, and maximum accept signed decimal numbers, fractions,
+and scientific notation. The entire numeric token must be valid; hexadecimal
+numbers, NaN, infinity, conversion overflow, and conversion underflow are
+rejected. Minimum must not exceed maximum. Zero and negative factors are allowed.
+
+Multiplexing markers (`M`, `mN`, and combinations) are currently rejected rather
+than interpreted as part of a signal name. Multiplexing support and duplicate
+frame/signal detection remain separate work.
+
+Invalid signal definitions stop parsing with `path:line: description`, including
+when the signal exceeds the declared payload length.
+
+---
+
 ## Comments
 
 Frame comment example:
@@ -805,8 +833,8 @@ RegisterType<DbcValueHandler>("VAL_"); // after implementing DbcValueHandler
 Registration creates one handler; replacing a registration replaces that instance.
 Register during initialization before dispatching. A line is routed using its
 full first whitespace-delimited keyword, so `BO_TX_BU_` cannot match `BO_`.
-Unregistered keywords remain ignored. Existing `SG_` and `CM_` syntax policies
-are preserved, including requiring a literal space after those keywords.
+Unregistered keywords remain ignored. `SG_` accepts spaces and tabs after its keyword; `CM_` retains its existing
+policy requiring a literal space.
 
 This factory handles statements inside DBC files. A separate format-level
 factory can be introduced when another file format is implemented. The decoder