diff --git a/klippy/chelper/serialqueue.c b/klippy/chelper/serialqueue.c
index cc22d021..f9cfd148 100644
--- a/klippy/chelper/serialqueue.c
+++ b/klippy/chelper/serialqueue.c
@@ -1,6 +1,6 @@
 // Serial port command queuing
 //
-// Copyright (C) 2016-2020  Kevin O'Connor <kevin@koconnor.net>
+// Copyright (C) 2016-2021  Kevin O'Connor <kevin@koconnor.net>
 //
 // This file may be distributed under the terms of the GNU GPLv3 license.
 
@@ -627,7 +627,7 @@ retransmit_event(struct serialqueue *sq, double eventtime)
     pthread_mutex_lock(&sq->lock);
 
     // Retransmit all pending messages
-    uint8_t buf[MESSAGE_MAX * MESSAGE_SEQ_MASK + 1];
+    uint8_t buf[MESSAGE_MAX * MAX_PENDING_BLOCKS + 1];
     int buflen = 0, first_buflen = 0;
     buf[buflen++] = MESSAGE_SYNC;
     struct queue_message *qm;
@@ -665,13 +665,11 @@ retransmit_event(struct serialqueue *sq, double eventtime)
     return waketime;
 }
 
-// Construct a block of data and send to the serial port
-static void
-build_and_send_command(struct serialqueue *sq, double eventtime)
+// Construct a block of data to be sent to the serial port
+static int
+build_and_send_command(struct serialqueue *sq, uint8_t *buf, double eventtime)
 {
-    struct queue_message *out = message_alloc();
-    out->len = MESSAGE_HEADER_SIZE;
-
+    int len = MESSAGE_HEADER_SIZE;
     while (sq->ready_bytes) {
         // Find highest priority message (message with lowest req_clock)
         uint64_t min_clock = MAX_CLOCK;
@@ -689,13 +687,13 @@ build_and_send_command(struct serialqueue *sq, double eventtime)
             }
         }
         // Append message to outgoing command
-        if (out->len + qm->len > sizeof(out->msg) - MESSAGE_TRAILER_SIZE)
+        if (len + qm->len > MESSAGE_MAX - MESSAGE_TRAILER_SIZE)
             break;
         list_del(&qm->node);
         if (list_empty(&cq->ready_queue) && list_empty(&cq->stalled_queue))
             list_del(&cq->node);
-        memcpy(&out->msg[out->len], qm->msg, qm->len);
-        out->len += qm->len;
+        memcpy(&buf[len], qm->msg, qm->len);
+        len += qm->len;
         sq->ready_bytes -= qm->len;
         if (qm->notify_id) {
             // Message requires notification - add to notify list
@@ -707,23 +705,21 @@ build_and_send_command(struct serialqueue *sq, double eventtime)
     }
 
     // Fill header / trailer
-    out->len += MESSAGE_TRAILER_SIZE;
-    out->msg[MESSAGE_POS_LEN] = out->len;
-    out->msg[MESSAGE_POS_SEQ] = (MESSAGE_DEST
-                                 | (sq->send_seq & MESSAGE_SEQ_MASK));
-    uint16_t crc = crc16_ccitt(out->msg, out->len - MESSAGE_TRAILER_SIZE);
-    out->msg[out->len - MESSAGE_TRAILER_CRC] = crc >> 8;
-    out->msg[out->len - MESSAGE_TRAILER_CRC+1] = crc & 0xff;
-    out->msg[out->len - MESSAGE_TRAILER_SYNC] = MESSAGE_SYNC;
+    len += MESSAGE_TRAILER_SIZE;
+    buf[MESSAGE_POS_LEN] = len;
+    buf[MESSAGE_POS_SEQ] = MESSAGE_DEST | (sq->send_seq & MESSAGE_SEQ_MASK);
+    uint16_t crc = crc16_ccitt(buf, len - MESSAGE_TRAILER_SIZE);
+    buf[len - MESSAGE_TRAILER_CRC] = crc >> 8;
+    buf[len - MESSAGE_TRAILER_CRC+1] = crc & 0xff;
+    buf[len - MESSAGE_TRAILER_SYNC] = MESSAGE_SYNC;
 
-    // Send message
-    int ret = write(sq->serial_fd, out->msg, out->len);
-    if (ret < 0)
-        report_errno("write", ret);
-    sq->bytes_write += out->len;
+    // Store message block
     if (eventtime > sq->idle_time)
         sq->idle_time = eventtime;
-    sq->idle_time += out->len * sq->baud_adjust;
+    sq->idle_time += len * sq->baud_adjust;
+    struct queue_message *out = message_alloc();
+    memcpy(out->msg, buf, len);
+    out->len = len;
     out->sent_time = eventtime;
     out->receive_time = sq->idle_time;
     if (list_empty(&sq->sent_queue))
@@ -732,8 +728,9 @@ build_and_send_command(struct serialqueue *sq, double eventtime)
     if (!sq->rtt_sample_seq)
         sq->rtt_sample_seq = sq->send_seq;
     sq->send_seq++;
-    sq->need_ack_bytes += out->len;
+    sq->need_ack_bytes += len;
     list_add_tail(&out->node, &sq->sent_queue);
+    return len;
 }
 
 // Determine the time the next serial data should be sent
@@ -815,12 +812,24 @@ static double
 command_event(struct serialqueue *sq, double eventtime)
 {
     pthread_mutex_lock(&sq->lock);
+    uint8_t buf[MESSAGE_MAX * MAX_PENDING_BLOCKS];
+    int buflen = 0;
     double waketime;
     for (;;) {
         waketime = check_send_command(sq, eventtime);
-        if (waketime != PR_NOW)
-            break;
-        build_and_send_command(sq, eventtime);
+        if (waketime != PR_NOW || buflen + MESSAGE_MAX > sizeof(buf)) {
+            if (buflen) {
+                // Write message blocks
+                int ret = write(sq->serial_fd, buf, buflen);
+                if (ret < 0)
+                    report_errno("write", ret);
+                sq->bytes_write += buflen;
+                buflen = 0;
+            }
+            if (waketime != PR_NOW)
+                break;
+        }
+        buflen += build_and_send_command(sq, &buf[buflen], eventtime);
     }
     pthread_mutex_unlock(&sq->lock);
     return waketime;