diff --git a/sw/lib/ocaml/serial.ml b/sw/lib/ocaml/serial.ml
index 8e42ff98d7..220e947086 100644
--- a/sw/lib/ocaml/serial.ml
+++ b/sw/lib/ocaml/serial.ml
@@ -93,7 +93,7 @@ let close = Unix.close
 type 'a closure = Closure of 'a
 
 let buffer_len = 256
-let input = fun f ->
+let input = fun ?(read = Unix.read) f ->
   let buffer = String.create buffer_len
   and index = ref 0 in
 
@@ -102,7 +102,7 @@ let input = fun f ->
     index := n in
 
   Closure (fun fd ->
-    let n = !index + Unix.read fd buffer !index (buffer_len - !index) in
+    let n = !index + read fd buffer !index (buffer_len - !index) in
     Debug.call 'T' (fun f -> fprintf f "input: %d %d\n" !index n);
     let rec parse = fun start n -> 
       Debug.call 'T' (fun f -> fprintf f "input parse: %d %d\n" start n);
diff --git a/sw/lib/ocaml/serial.mli b/sw/lib/ocaml/serial.mli
index e9b22ebe8c..532787c587 100644
--- a/sw/lib/ocaml/serial.mli
+++ b/sw/lib/ocaml/serial.mli
@@ -54,11 +54,13 @@ val close : Unix.file_descr -> unit
 val set_dtr : Unix.file_descr -> bool -> unit
 val set_speed : Unix.file_descr -> speed -> unit
 
-val input : (string -> int) -> (Unix.file_descr -> unit) closure
-(** Buffered input. [input f] Returns a closure which must be called when
+val input : 
+  ?read:(Unix.file_descr -> string -> int -> int -> int) ->
+  (string -> int) -> (Unix.file_descr -> unit) closure
+(** Buffered input. [input ?read f] Returns a closure which must be called when
 characters are available on the stream. These characters are stored in a
 a buffer. [f] is then called on the buffer. [f] must return the number
-of consumed characters. *)
+of consumed characters. Default [read] is [Unix.read] *)
 
 type payload