diff --git a/bmspy/__init__.py b/bmspy/__init__.py
index 1e17e0b..aca74e2 100644
--- a/bmspy/__init__.py
+++ b/bmspy/__init__.py
@@ -8,7 +8,9 @@ import pprint
 from bmspy.utilities import debugger
 
 
-def parse_args():
+
+
+def parse_args() -> argparse.Namespace:
     parser = argparse.ArgumentParser(
         description="Query JBD BMS and report status",
         add_help=True,
@@ -115,7 +117,7 @@ def parse_args():
     return args
 
 
-def main():
+def main() -> None:
     try:
         args = parse_args()
 
@@ -163,7 +165,7 @@ def main():
             client.handle_registration(args.socket, "bmspy", debug)
             atexit.register(client.handle_registration, args.socket, "bmspy", debug)
 
-            # {ups_name: JBDUPS}
+            # {ups_name: UPS}
             data = client.read_data(args.socket, "bmspy", ups=args.ups, debug=debug)
 
             if args.report_json:
diff --git a/bmspy/classes.py b/bmspy/classes.py
index 85f8a73..1427c2e 100644
--- a/bmspy/classes.py
+++ b/bmspy/classes.py
@@ -1,4 +1,7 @@
-from dataclasses import dataclass, fields as dataclass_fields
+from abc import ABC, abstractmethod
+from collections.abc import Iterator
+from dataclasses import dataclass
+from typing import Any
 
 
 @dataclass
@@ -10,7 +13,7 @@ class BMSScalarField:
     value: str
     units: str | None = None
 
-    def get(self, key, default=None):
+    def get(self, key: str, default: Any = None) -> Any:
         return getattr(self, key, default)
 
 
@@ -20,11 +23,11 @@ class BMSMultiField:
 
     help: str
     label: str
-    raw_values: dict
-    values: dict
+    raw_values: dict[int, float | bool]
+    values: dict[int, str]
     units: str | None = None
 
-    def get(self, key, default=None):
+    def get(self, key: str, default: Any = None) -> Any:
         return getattr(self, key, default)
 
 
@@ -35,5 +38,44 @@ class BMSInfoField:
     help: str
     info: str
 
-    def get(self, key, default=None):
+    def get(self, key: str, default: Any = None) -> Any:
         return getattr(self, key, default)
+
+
+type BMSField = BMSScalarField | BMSMultiField | BMSInfoField
+
+
+def _field_from_dict(d: dict[str, Any]) -> BMSField:
+    """Reconstruct a BMSField from its JSON-serialized dict."""
+    if "raw_values" in d:
+        return BMSMultiField(**d)
+    elif "info" in d:
+        return BMSInfoField(**d)
+    else:
+        return BMSScalarField(**d)
+
+
+class UPS(ABC):
+    """Abstract base class for all UPS/BMS device data containers."""
+
+    @abstractmethod
+    def items(self) -> Iterator[tuple[str, BMSField]]:
+        ...
+
+    def __bool__(self) -> bool:
+        return next(iter(self.items()), None) is not None
+
+    @classmethod
+    def from_dict(cls, data: dict[str, Any]) -> "UPS":
+        """Reconstruct a UPS snapshot from a JSON-decoded field dict."""
+        return _UPSSnapshot({name: _field_from_dict(v) for name, v in data.items()})
+
+
+class _UPSSnapshot(UPS):
+    """Generic UPS snapshot reconstructed from serialized data by the client."""
+
+    def __init__(self, fields: dict[str, BMSField]) -> None:
+        self._fields = fields
+
+    def items(self) -> Iterator[tuple[str, BMSField]]:
+        yield from self._fields.items()
diff --git a/bmspy/client.py b/bmspy/client.py
index eeebe7d..cf0f6b9 100644
--- a/bmspy/client.py
+++ b/bmspy/client.py
@@ -5,18 +5,21 @@ import sys
 import struct
 import json
 import socket
+from typing import Any
+
 from bmspy.utilities import debugger
+from bmspy.classes import UPS
 
 
 is_registered = False
 
 
-def handle_registration(socket_path, client_name, debug=0):
+def handle_registration(socket_path: str, client_name: str, debug: int = 0) -> dict[str, Any]:
     global is_registered
-    data = dict()
+    data: dict[str, Any] = dict()
 
     if is_registered:
-        message = {"command": "DEREGISTER", "client": client_name}
+        message: dict[str, Any] = {"command": "DEREGISTER", "client": client_name}
     else:
         # fork server if it's not already running
         message = {"command": "REGISTER", "client": client_name}
@@ -41,8 +44,8 @@ def handle_registration(socket_path, client_name, debug=0):
     return data
 
 
-def socket_comms(socket_path, request_data, debug=0):
-    response_data = dict()
+def socket_comms(socket_path: str, request_data: dict[str, Any], debug: int = 0) -> dict[str, Any]:
+    response_data: dict[str, Any] = dict()
 
     # Create a UDS socket
     sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
@@ -106,9 +109,9 @@ def socket_comms(socket_path, request_data, debug=0):
     return response_data
 
 
-def read_data(socket_path, client_name, ups=None, debug=0):
-    """Return {ups_name: JBDUPS} for all UPSes, or just the named one."""
-    request = {"command": "GET", "client": client_name}
+def read_data(socket_path: str, client_name: str, ups: str | None = None, debug: int = 0) -> dict[str, UPS]:
+    """Return {ups_name: UPS} for all devices, or just the named one."""
+    request: dict[str, Any] = {"command": "GET", "client": client_name}
     if ups is not None:
         request["ups"] = ups
 
@@ -117,7 +120,7 @@ def read_data(socket_path, client_name, ups=None, debug=0):
     if data is None:
         raise RuntimeError("No data received from daemon")
 
-    return data
+    return {name: UPS.from_dict(fields) for name, fields in data.items()}
 
 
 if __name__ == "__main__":
diff --git a/bmspy/influxdb.py b/bmspy/influxdb.py
index 04e58e3..5280a43 100644
--- a/bmspy/influxdb.py
+++ b/bmspy/influxdb.py
@@ -5,26 +5,27 @@ import sys
 import time
 from influxdb_client_3 import InfluxDBClient3, Point
 from bmspy import client
+from bmspy.classes import UPS
 from bmspy.utilities import debugger
 
 DAEMON_UPDATE_PERIOD = 30
 
 
-def influx_shutdown(influxclient):
+def influx_shutdown(influxclient: InfluxDBClient3 | None) -> None:
     if influxclient is not None:
         influxclient.close()
 
 
 def influxdb_export(
-    bucket,
-    url=None,
-    org=None,
-    token=None,
-    socket_path=None,
-    ups=None,
-    daemonize=True,
-    debug=0,
-):
+    bucket: str,
+    url: str | None = None,
+    org: str | None = None,
+    token: str | None = None,
+    socket_path: str | None = None,
+    ups: str | None = None,
+    daemonize: bool = True,
+    debug: int = 0,
+) -> None:
     if not url:
         url = os.environ["INFLUXDB_V2_URL"]
         org = os.environ.get("INFLUXDB_V2_ORG")
@@ -46,7 +47,7 @@ def influxdb_export(
     atexit.unregister(influx_shutdown)
 
 
-def influxdb_write_snapshot(influxclient, bucket, ups_data, debug=0):
+def influxdb_write_snapshot(influxclient: InfluxDBClient3, bucket: str, ups_data: dict[str, UPS], debug: int = 0) -> None:
     if debug > 1:
         debugger("influxdb: creating snapshot")
     points = influxdb_create_snapshot(ups_data, debug)
@@ -58,8 +59,8 @@ def influxdb_write_snapshot(influxclient, bucket, ups_data, debug=0):
         debugger(e)
 
 
-def influxdb_create_snapshot(ups_data, debug=0):
-    """Build InfluxDB points from {ups_name: JBDUPS}, tagging each point with the UPS name."""
+def influxdb_create_snapshot(ups_data: dict[str, UPS], debug: int = 0) -> list[Point]:
+    """Build InfluxDB points from {ups_name: UPS}, tagging each point with the UPS name."""
     points = []
     now = datetime.datetime.now(datetime.timezone.utc)
 
diff --git a/bmspy/server.py b/bmspy/server.py
index 86a8ee7..7c0b9f0 100755
--- a/bmspy/server.py
+++ b/bmspy/server.py
@@ -3,6 +3,7 @@
 # Daemon: listens on a Unix socket and serves JBD BMS data to clients
 #
 import os
+import socket
 import sys
 import stat
 import time
@@ -11,6 +12,7 @@ import signal
 import json
 import struct
 from dataclasses import asdict as dataclass_asdict
+from typing import Any, NoReturn
 
 from bmspy.utilities import debugger
 from bmspy.jbd_ups import collect_data, initialise_serial
@@ -33,15 +35,15 @@ from bmspy.jbd_ups import collect_data, initialise_serial
 connected_clients = list()
 
 
-def signalHandler():
+def signalHandler() -> NoReturn:
     raise SystemExit("terminating")
 
 
-def socket_cleanup(socket_path, debug=0):
+def socket_cleanup(socket_path: str, debug: int = 0) -> None:
     os.unlink(socket_path)
 
 
-def read_request(connection, debug=0):
+def read_request(connection: socket.socket, debug: int = 0) -> dict[str, Any]:
     # get length of expected json string
     request = bytes()
     try:
@@ -72,7 +74,7 @@ def read_request(connection, debug=0):
     return request_data
 
 
-def send_response(connection, response_data, client, debug=0):
+def send_response(connection: socket.socket, response_data: Any, client: str, debug: int = 0) -> None:
     if debug > 2:
         debugger("socket: sending {!r}".format(response_data))
     try:
@@ -94,7 +96,7 @@ def send_response(connection, response_data, client, debug=0):
         raise OSError("unable to encode response: {}".format(e))
 
 
-def parse_device(device_str):
+def parse_device(device_str: str) -> tuple[str, str]:
     """Parse 'name:/dev/path' or '/dev/path' into (name, path)."""
     if not device_str.startswith("/") and ":" in device_str:
         name, path = device_str.split(":", 1)
@@ -105,7 +107,6 @@ def parse_device(device_str):
 
 def main():
     import argparse
-    import socket
     import pwd
     import grp
 
diff --git a/bmspy/ups.py b/bmspy/ups.py
index 83936e5..1b71d99 100644
--- a/bmspy/ups.py
+++ b/bmspy/ups.py
@@ -2,6 +2,7 @@ from collections import deque
 import argparse
 import atexit, datetime, os, re, sys, time
 import smtplib, ssl, socket
+from typing import Any
 from bmspy import client
 
 DAEMON_UPDATE_PERIOD = 30
@@ -11,7 +12,7 @@ critical_sent = False
 warning_sent = False
 alert_sent = False
 
-def handle_shutdown(action = 'cancel', delay = 0, debug = 0):
+def handle_shutdown(action: str = 'cancel', delay: int = 0, debug: int = 0) -> None:
     global scheduled_shutdown
 
     if action == 'shutdown':
@@ -25,7 +26,7 @@ def handle_shutdown(action = 'cancel', delay = 0, debug = 0):
     return
 
 
-def handle_email(text, level, recipient = "root", mailserver = "localhost", port = 25, mailuser = None, mailpass = None, debug = 0):
+def handle_email(text: str, level: str | None, recipient: str = "root", mailserver: str = "localhost", port: int = 25, mailuser: str | None = None, mailpass: str | None = None, debug: int = 0) -> None:
     isSSL = False
     hostname = socket.gethostname()
 
@@ -53,7 +54,7 @@ def handle_email(text, level, recipient = "root", mailserver = "localhost", port
     return
 
 
-def main():
+def main() -> None:
     global alert_sent, warning_sent, critical_sent
 
     parser = argparse.ArgumentParser(
diff --git a/bmspy/utilities.py b/bmspy/utilities.py
index fde696b..4146931 100755
--- a/bmspy/utilities.py
+++ b/bmspy/utilities.py
@@ -4,9 +4,10 @@
 #
 import datetime
 import pprint
+from typing import Any
 
 
-def debugger(data, pretty: bool = False):
+def debugger(data: Any, pretty: bool = False) -> None:
     if pretty:
         pp = pprint.PrettyPrinter(indent=4)
         pp.pprint(