diff --git a/libraries/AP_SerialManager/AP_SerialManager.cpp b/libraries/AP_SerialManager/AP_SerialManager.cpp
index 762d2ff289..3aa3407b7c 100644
--- a/libraries/AP_SerialManager/AP_SerialManager.cpp
+++ b/libraries/AP_SerialManager/AP_SerialManager.cpp
@@ -114,7 +114,6 @@ extern const AP_HAL::HAL& hal;
 #define SERIAL9_PROTOCOL SerialProtocol_None
 #endif // HAL_BUILD_AP_PERIPH
 
-
 const AP_Param::GroupInfo AP_SerialManager::var_info[] = {
 #if SERIALMANAGER_NUM_PORTS > 0
     // @Param: 0_BAUD
@@ -807,7 +806,7 @@ void AP_SerialManager::disable_passthru(void)
 // accessor for AP_Periph to set baudrate and type
 void AP_SerialManager::set_protocol_and_baud(uint8_t sernum, enum SerialProtocol protocol, uint32_t baudrate)
 {
-    if (sernum <= ARRAY_SIZE(state)) {
+    if (sernum < SERIALMANAGER_NUM_PORTS) {
         state[sernum].protocol.set(protocol);
         state[sernum].baud.set(baudrate);
     }
diff --git a/libraries/AP_SerialManager/AP_SerialManager.h b/libraries/AP_SerialManager/AP_SerialManager.h
index e7c8097ef1..c60be5648a 100644
--- a/libraries/AP_SerialManager/AP_SerialManager.h
+++ b/libraries/AP_SerialManager/AP_SerialManager.h
@@ -38,6 +38,19 @@
 #define SERIALMANAGER_NUM_PORTS 8
 #endif
 
+/*
+  array size for state[]. This needs to be at least
+  SERIALMANAGER_NUM_PORTS, but we want it to be the same length on
+  similar boards to get the ccache efficiency up. This wastes a small
+  amount of memory, but makes a huge difference to the build times
+ */
+#if SERIALMANAGER_NUM_PORTS > 10 || SERIALMANAGER_NUM_PORTS < 5
+#define SERIALMANAGER_MAX_PORTS SERIALMANAGER_NUM_PORTS
+#else
+#define SERIALMANAGER_MAX_PORTS 10
+#endif
+
+
  // console default baud rates and buffer sizes
 #ifdef HAL_SERIAL0_BAUD_DEFAULT
 # define AP_SERIALMANAGER_CONSOLE_BAUD          HAL_SERIAL0_BAUD_DEFAULT
@@ -223,12 +236,13 @@ public:
 private:
     static AP_SerialManager *_singleton;
 
-    // array of uart info
+    // array of uart info. See comment above about
+    // SERIALMANAGER_MAX_PORTS
     struct UARTState {
-        AP_Int8 protocol;
         AP_Int32 baud;
         AP_Int16 options;
-    } state[SERIALMANAGER_NUM_PORTS];
+        AP_Int8 protocol;
+    } state[SERIALMANAGER_MAX_PORTS];
 
     // pass-through serial support
     AP_Int8 passthru_port1;