diff --git a/artiq/runtime/bridge.c b/artiq/runtime/bridge.c
index 82fe3a16a..9c2e6d921 100644
--- a/artiq/runtime/bridge.c
+++ b/artiq/runtime/bridge.c
@@ -1,6 +1,7 @@
 #include "mailbox.h"
 #include "messages.h"
 #include "rtio.h"
+#include "ttl.h"
 #include "dds.h"
 #include "bridge.h"
 
@@ -58,7 +59,7 @@ void bridge_main(void)
                 struct msg_brg_ttl_out *msg;
 
                 msg = (struct msg_brg_ttl_out *)umsg;
-                rtio_output_blind(msg->channel, 1, msg->value);
+                rtio_output_blind(msg->channel, TTL_O_ADDR, msg->value);
                 mailbox_acknowledge();
                 break;
             }
@@ -66,7 +67,7 @@ void bridge_main(void)
                 struct msg_brg_ttl_out *msg;
 
                 msg = (struct msg_brg_ttl_out *)umsg;
-                rtio_output_blind(msg->channel, 0, msg->value);
+                rtio_output_blind(msg->channel, TTL_OE_ADDR, msg->value);
                 mailbox_acknowledge();
                 break;
             }
diff --git a/artiq/runtime/ttl.h b/artiq/runtime/ttl.h
new file mode 100644
index 000000000..87fa74b7c
--- /dev/null
+++ b/artiq/runtime/ttl.h
@@ -0,0 +1,8 @@
+#ifndef __TTL_H
+#define __TTL_H
+
+#define TTL_O_ADDR 0
+#define TTL_OE_ADDR 1
+#define TTL_SENSITIVITY_ADDR 2
+
+#endif /* __TTL_H */