diff --git a/artiq/gateware/serwb/kusphy.py b/artiq/gateware/serwb/kusphy.py
index e0a0bd58b..395c4c0b7 100644
--- a/artiq/gateware/serwb/kusphy.py
+++ b/artiq/gateware/serwb/kusphy.py
@@ -215,4 +215,5 @@ class KUSSerdes(Module):
         idle_timer = WaitTimer(32)
         self.submodules += idle_timer
         self.comb += idle_timer.wait.eq(1)
-        self.sync += self.rx_idle.eq(idle_timer.done & (rx_bitslip.o == 0))
+        self.sync += self.rx_idle.eq(idle_timer.done &
+            ((rx_bitslip.o == 0) | (rx_bitslip.o == (2**40-1))))
diff --git a/artiq/gateware/serwb/s7phy.py b/artiq/gateware/serwb/s7phy.py
index 967991aea..cc7cc4e30 100644
--- a/artiq/gateware/serwb/s7phy.py
+++ b/artiq/gateware/serwb/s7phy.py
@@ -226,4 +226,5 @@ class S7Serdes(Module):
         idle_timer = WaitTimer(32)
         self.submodules += idle_timer
         self.comb += idle_timer.wait.eq(1)
-        self.sync += self.rx_idle.eq(idle_timer.done & (rx_bitslip.o == 0))
+        self.sync += self.rx_idle.eq(idle_timer.done &
+            ((rx_bitslip.o == 0) | (rx_bitslip.o == (2**40-1))))