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 Ebyte M31 I/O
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The ``driver/m31`` package defines one host and up to fifteen expansion
modules. A module's slice index is its slot. The host has eight digital
inputs and eight digital outputs. Mixed expansions add eight of each,
input expansions add sixteen inputs, output expansions add sixteen
outputs, and analog expansions add eight voltage outputs.

Each point uses ``m{slot}.di{channel}``, ``m{slot}.do{channel}``, or
``m{slot}.ao{channel}``. Inputs, coils, and analog registers have
independent zero-based address ranges that continue across modules.
Analog registers represent zero through ten volts as zero through 10000.
Hardware pulse widths are whole milliseconds from 50 through 65535.
Their width registers start at ``0x30d4`` and their output-mode
registers start at ``0x32c8``, both indexed by coil address. Mode zero
is level control and mode one is pulse control.

Descriptors use manufacturer ``ebyte`` and firmware ``unknown``. The
package does not read the host identity registers. Pulse capabilities
accept commands but do not report transient coil state. Input
capabilities are read-only.

Tests verify topology and descriptor values without a physical module.
They do not confirm physical expansion limits or relay timing under
load. The conservative sixteen-module limit includes the host.

***************
 Modbus client
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``NewTCPClient`` supplies a caller-owned, serialized connection through
``Client``. Connect selects the configured unit and has a two-second
timeout. A request has a one-second write deadline and a two-second
response deadline, both measured from its start. Caller deadlines and
cancellation can stop it earlier. Each response must match the
transaction, unit, function, length, and write acknowledgement.
Supported functions are 01, 02, 03, 05, 15, and 16. No external Modbus
package is required.

In-memory stream tests check request bytes, response validation,
exceptions, truncation, cancellation, and timeouts. They do not verify a
physical host or TCP network behavior.