From e29af906c3237bf6dd7f92f9adbbc6b39a547dd0 Mon Sep 17 00:00:00 2001
From: David Snowdon <3860429+dcasnowdon@users.noreply.github.com>
Date: Mon, 11 Aug 2025 11:14:46 +1000
Subject: [PATCH 1/5] Change the default to 0, which is a no-op for
 data_bitrate.

---
 can/interfaces/slcan.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/can/interfaces/slcan.py b/can/interfaces/slcan.py
index 01ba9c995..2b1aa0ed6 100644
--- a/can/interfaces/slcan.py
+++ b/can/interfaces/slcan.py
@@ -166,7 +166,7 @@ def __init__(
 
         super().__init__(channel, **kwargs)
 
-    def set_bitrate(self, bitrate: int, data_bitrate: Optional[int] = None) -> None:
+    def set_bitrate(self, bitrate: int, data_bitrate: Optional[int] = 0) -> None:
         """
         :param bitrate:
             Bitrate in bit/s

From 669fb124b3dd9b30bd3deaae25e7b5c7d604b5c3 Mon Sep 17 00:00:00 2001
From: David Snowdon <3860429+dcasnowdon@users.noreply.github.com>
Date: Mon, 11 Aug 2025 11:31:55 +1000
Subject: [PATCH 2/5] Added a changelog fragment.

---
 doc/changelog.d/1978.added.md | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 doc/changelog.d/1978.added.md

diff --git a/doc/changelog.d/1978.added.md b/doc/changelog.d/1978.added.md
new file mode 100644
index 000000000..0eaca620d
--- /dev/null
+++ b/doc/changelog.d/1978.added.md
@@ -0,0 +1 @@
+Fix the optional setting of  data bitrate in slcan

From c5f1c96107ff9ff61b5db316f97df09a6d5443c0 Mon Sep 17 00:00:00 2001
From: David Snowdon <3860429+dcasnowdon@users.noreply.github.com>
Date: Mon, 11 Aug 2025 11:37:55 +1000
Subject: [PATCH 3/5] Fix black formatting for test_slcan.py

---
 test/test_slcan.py | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/test/test_slcan.py b/test/test_slcan.py
index 491800e24..b757ad04d 100644
--- a/test/test_slcan.py
+++ b/test/test_slcan.py
@@ -74,7 +74,13 @@ class slcanTestCase(unittest.TestCase):
     def setUp(self):
         self.bus = cast(
             can.interfaces.slcan.slcanBus,
-            can.Bus("loop://", interface="slcan", sleep_after_open=0, timeout=TIMEOUT),
+            can.Bus(
+                "loop://",
+                interface="slcan",
+                sleep_after_open=0,
+                timeout=TIMEOUT,
+                bitrate=500000,
+            ),
         )
         self.serial = cast(SerialMock, self.bus.serialPortOrig)
         self.serial.reset_input_buffer()

From df572d8f002452a981778640967a75374d58619e Mon Sep 17 00:00:00 2001
From: David Snowdon <3860429+dcasnowdon@users.noreply.github.com>
Date: Mon, 11 Aug 2025 20:34:08 +1000
Subject: [PATCH 4/5] Revert the function signature, and explicitly check for
 `None` in data_bitrate.

---
 can/interfaces/slcan.py | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/can/interfaces/slcan.py b/can/interfaces/slcan.py
index 2b1aa0ed6..d9eab5edf 100644
--- a/can/interfaces/slcan.py
+++ b/can/interfaces/slcan.py
@@ -166,7 +166,7 @@ def __init__(
 
         super().__init__(channel, **kwargs)
 
-    def set_bitrate(self, bitrate: int, data_bitrate: Optional[int] = 0) -> None:
+    def set_bitrate(self, bitrate: int, data_bitrate: Optional[int] = None) -> None:
         """
         :param bitrate:
             Bitrate in bit/s
@@ -180,6 +180,11 @@ def set_bitrate(self, bitrate: int, data_bitrate: Optional[int] = 0) -> None:
         else:
             bitrates = ", ".join(str(k) for k in self._BITRATES.keys())
             raise ValueError(f"Invalid bitrate, choose one of {bitrates}.")
+
+        # If data_bitrate is None, we set it to 0 which means no data bitrate
+        if data_bitrate is None:
+            data_bitrate = 0
+
         if data_bitrate in self._DATA_BITRATES:
             dbitrate_code = self._DATA_BITRATES[data_bitrate]
         else:

From 1f99d75aaa3b485099b7e175333c7f70d5ce85eb Mon Sep 17 00:00:00 2001
From: David Snowdon <3860429+dcasnowdon@users.noreply.github.com>
Date: Tue, 12 Aug 2025 13:28:09 +1000
Subject: [PATCH 5/5] Update the town crier news fragment.

---
 doc/changelog.d/1978.added.md | 1 -
 doc/changelog.d/1978.fixed.md | 1 +
 2 files changed, 1 insertion(+), 1 deletion(-)
 delete mode 100644 doc/changelog.d/1978.added.md
 create mode 100644 doc/changelog.d/1978.fixed.md

diff --git a/doc/changelog.d/1978.added.md b/doc/changelog.d/1978.added.md
deleted file mode 100644
index 0eaca620d..000000000
--- a/doc/changelog.d/1978.added.md
+++ /dev/null
@@ -1 +0,0 @@
-Fix the optional setting of  data bitrate in slcan
diff --git a/doc/changelog.d/1978.fixed.md b/doc/changelog.d/1978.fixed.md
new file mode 100644
index 000000000..1de50bb60
--- /dev/null
+++ b/doc/changelog.d/1978.fixed.md
@@ -0,0 +1 @@
+Fix initialisation of an slcan bus, when setting a bitrate. When using CAN 2.0 (not FD), the default setting for `data_bitrate` was invalid, causing an exception.
\ No newline at end of file