diff --git a/contrib/examples/example_app/test.c b/contrib/examples/example_app/test.c
index 95cca1f1..21b682cb 100644
--- a/contrib/examples/example_app/test.c
+++ b/contrib/examples/example_app/test.c
@@ -258,11 +258,11 @@ pppLinkStatusCallback(ppp_pcb *pcb, int errCode, void *ctx)
 
 #if PPPOS_SUPPORT
 static u32_t
-ppp_output_cb(ppp_pcb *pcb, u8_t *data, u32_t len, void *ctx)
+ppp_output_cb(ppp_pcb *pcb, const void *data, u32_t len, void *ctx)
 {
   LWIP_UNUSED_ARG(pcb);
   LWIP_UNUSED_ARG(ctx);
-  return sio_write(ppp_sio, data, len);
+  return sio_write(ppp_sio, (u8_t*)data, len);
 }
 #endif /* PPPOS_SUPPORT */
 #endif /* USE_PPP */
diff --git a/contrib/examples/ppp/pppos_example.c b/contrib/examples/ppp/pppos_example.c
index a3362dfb..d356eecc 100644
--- a/contrib/examples/ppp/pppos_example.c
+++ b/contrib/examples/ppp/pppos_example.c
@@ -156,11 +156,11 @@ ppp_link_status_cb(ppp_pcb *pcb, int err_code, void *ctx)
 }
 
 static u32_t
-ppp_output_cb(ppp_pcb *pcb, u8_t *data, u32_t len, void *ctx)
+ppp_output_cb(ppp_pcb *pcb, const void *data, u32_t len, void *ctx)
 {
   LWIP_UNUSED_ARG(pcb);
   LWIP_UNUSED_ARG(ctx);
-  return sio_write(ppp_sio, data, len);
+  return sio_write(ppp_sio, (u8_t*)data, len);
 }
 
 #if LWIP_NETIF_STATUS_CALLBACK
diff --git a/doc/ppp.txt b/doc/ppp.txt
index 797b4e0a..a713cc52 100644
--- a/doc/ppp.txt
+++ b/doc/ppp.txt
@@ -195,7 +195,7 @@ static void status_cb(ppp_pcb *pcb, int err_code, void *ctx) {
  *
  * Return value: len if write succeed
  */
-static u32_t output_cb(ppp_pcb *pcb, u8_t *data, u32_t len, void *ctx) {
+static u32_t output_cb(ppp_pcb *pcb, const void *data, u32_t len, void *ctx) {
   return uart_write(UART, data, len);
 }
 
diff --git a/src/include/netif/ppp/pppos.h b/src/include/netif/ppp/pppos.h
index 983da56c..f587498d 100644
--- a/src/include/netif/ppp/pppos.h
+++ b/src/include/netif/ppp/pppos.h
@@ -58,7 +58,7 @@ enum {
 };
 
 /* PPPoS serial output callback function prototype */
-typedef u32_t (*pppos_output_cb_fn)(ppp_pcb *pcb, u8_t *data, u32_t len, void *ctx);
+typedef u32_t (*pppos_output_cb_fn)(ppp_pcb *pcb, const void *data, u32_t len, void *ctx);
 
 /*
  * Extended asyncmap - allows any character to be escaped.