diff --git a/firmware/motion-head/src/config.h b/firmware/motion-head/src/config.h
index eaea7b2a139dc1f53dfae1f1ffa212515ecd4ac5..facb76c2ad0b617f9a2973ef0ee0e8415416a6e1 100644
--- a/firmware/motion-head/src/config.h
+++ b/firmware/motion-head/src/config.h
@@ -1,8 +1,13 @@
+// compile-time configur8or 
 #ifndef CONFIG_H_
 #define CONFIG_H_
 
+// for the bus: head, or drop? 
 #define UCBUS_IS_HEAD
 //#define UCBUS_IS_DROP
+// and D21 or D51?
+#define UCBUS_IS_D51
+//#define UCBUS_IS_D21 
 
 // if you're using the 'module board' https://gitlab.cba.mit.edu/jakeread/ucbus-module
 // the first (og) revision has an SMT header, and some of the RS485 pins are varied, 
diff --git a/firmware/motion-head/src/main.cpp b/firmware/motion-head/src/main.cpp
index 534da6e32334fe538d42cf0c2881984a24156992..8ec6413dcd305ba1666b0893f913b12eacd2ad8d 100644
--- a/firmware/motion-head/src/main.cpp
+++ b/firmware/motion-head/src/main.cpp
@@ -8,8 +8,8 @@
 
 #include "osape-d51/osape/osap/osap.h"
 #include "osape-d51/vertices/vt_usbSerial.h"
-#include "osape-d51/vertices/vt_ucBusHead.h"
-#include "osape-d51/vertices/ucBusHead.h"
+#include "osape-d51/vertices/ucbus/vt_ucBusHead.h"
+#include "osape-d51/vertices/ucbus/ucBusHead.h"
 // -------------------------------------------------------- SMOOTHIE HANDLES 
 // *should* go to smoothieRoll.h, duh 
 
@@ -228,7 +228,8 @@ void setup() {
   smoothieRoll->init(20000);
   // 25kHz base (40us period) or 
   // 20kHz base (50us period)
-  d51ClockBoss->start_ticker_a(50); 
+  // 5kHz base (200us period)
+  d51ClockBoss->start_ticker_a(200); 
 }
 
 void loop() {
diff --git a/firmware/motion-head/src/osape-d51 b/firmware/motion-head/src/osape-d51
index 50426e4ff771eeb2c94180e59a05258d2daf24ff..935536e05193d3808b831df3f0e664d3379dd1d4 160000
--- a/firmware/motion-head/src/osape-d51
+++ b/firmware/motion-head/src/osape-d51
@@ -1 +1 @@
-Subproject commit 50426e4ff771eeb2c94180e59a05258d2daf24ff
+Subproject commit 935536e05193d3808b831df3f0e664d3379dd1d4
diff --git a/firmware/motion-head/src/syserror.cpp b/firmware/motion-head/src/syserror.cpp
index a650704bee1384f9e6475b5da6b8b9e1f030f29a..bdf769f459f03e4d07a001a9192f9e33970002ae 100644
--- a/firmware/motion-head/src/syserror.cpp
+++ b/firmware/motion-head/src/syserror.cpp
@@ -1,7 +1,7 @@
 #include "syserror.h"
 #include "indicators.h"
 #include "osape-d51/osape/osap/ts.h"
-#include "osape-d51/vertices/ucBusDrop.h"
+#include "osape-d51/vertices/ucbus/ucBusDrop.h"
 #include "osape-d51/osape/utils/cobs.h"
 #include "config.h"