diff --git a/artiq/gateware/serwb/kusphy.py b/artiq/gateware/serwb/kusphy.py
index 8abcb08d3..e0a0bd58b 100644
--- a/artiq/gateware/serwb/kusphy.py
+++ b/artiq/gateware/serwb/kusphy.py
@@ -205,8 +205,12 @@ class KUSSerdes(Module):
             decoders[3].input.eq(rx_bitslip.o[30:40]),
             self.rx_k.eq(Cat(*[decoders[i].k for i in range(4)])),
             self.rx_d.eq(Cat(*[decoders[i].d for i in range(4)])),
-            self.rx_comma.eq((decoders[0].k == 1) & (decoders[0].d == K(28,5)))
-        ] 
+            self.rx_comma.eq(
+                (decoders[0].k == 1) & (decoders[0].d == K(28,5)) &
+                (decoders[1].k == 0) & (decoders[1].d == 0) &
+                (decoders[2].k == 0) & (decoders[2].d == 0) &
+                (decoders[3].k == 0) & (decoders[3].d == 0))
+        ]
 
         idle_timer = WaitTimer(32)
         self.submodules += idle_timer
diff --git a/artiq/gateware/serwb/s7phy.py b/artiq/gateware/serwb/s7phy.py
index df4930572..967991aea 100644
--- a/artiq/gateware/serwb/s7phy.py
+++ b/artiq/gateware/serwb/s7phy.py
@@ -216,8 +216,12 @@ class S7Serdes(Module):
             decoders[3].input.eq(rx_bitslip.o[30:40]),
             self.rx_k.eq(Cat(*[decoders[i].k for i in range(4)])),
             self.rx_d.eq(Cat(*[decoders[i].d for i in range(4)])),
-            self.rx_comma.eq((decoders[0].k == 1) & (decoders[0].d == K(28,5)))
-        ] 
+            self.rx_comma.eq(
+                (decoders[0].k == 1) & (decoders[0].d == K(28,5)) &
+                (decoders[1].k == 0) & (decoders[1].d == 0) &
+                (decoders[2].k == 0) & (decoders[2].d == 0) &
+                (decoders[3].k == 0) & (decoders[3].d == 0))
+        ]
 
         idle_timer = WaitTimer(32)
         self.submodules += idle_timer