diff --git a/artiq/coredevice/grabber.py b/artiq/coredevice/grabber.py
index 6406578ea..7a00549db 100644
--- a/artiq/coredevice/grabber.py
+++ b/artiq/coredevice/grabber.py
@@ -40,13 +40,13 @@ class Grabber:
         Advances the timeline by 4 coarse RTIO cycles.
         """
         c = int64(self.core.ref_multiplier)
-        rtio_output(now_mu(), self.channel_base, 4*n+0, x0)
+        rtio_output((self.channel_base << 8) | (4*n+0), x0)
         delay_mu(c)
-        rtio_output(now_mu(), self.channel_base, 4*n+1, y0)
+        rtio_output((self.channel_base << 8) | (4*n+1), y0)
         delay_mu(c)
-        rtio_output(now_mu(), self.channel_base, 4*n+2, x1)
+        rtio_output((self.channel_base << 8) | (4*n+2), x1)
         delay_mu(c)
-        rtio_output(now_mu(), self.channel_base, 4*n+3, y1)
+        rtio_output((self.channel_base << 8) | (4*n+3), y1)
         delay_mu(c)
 
     @kernel
@@ -67,7 +67,7 @@ class Grabber:
 
         :param mask: bitmask enabling or disabling each ROI engine.  
         """
-        rtio_output(now_mu(), self.channel_base+1, 0, mask)
+        rtio_output((self.channel_base + 1) << 8, mask)
 
     @kernel
     def gate_roi_pulse(self, mask, dt):