diff --git a/MBPROTO b/MBPROTO
index 07c7e16..1bf5782 100644
--- a/MBPROTO
+++ b/MBPROTO
@@ -73,14 +73,6 @@ NVM
     1- 2* ( [ tab ] ) LITERAL +
   ;
 
-  \ MB loop (xt) from mbp1 to mbp1+mbp2
-  : mbloop ( xt -- )
-    mbp1 mbp2 OVER + SWAP  DO
-      ( xt ) I OVER EXECUTE
-    LOOP
-    ( xt ) DROP
-  ;
-
   \ true if n in range mbp1,mbp2, else false and MODBUS EC=2
   : mbrange? ( n -- b )
     -1 SWAP mbp1 mbp2 + - 0< IF
@@ -88,15 +80,24 @@ NVM
     THEN
   ;
 
-  \ Quantity of bits / 8, if the remainder is different of 0 => N = N+1
-  : N* ( -- N* )
-    1- 8 / 1+
+  \ MB loop (xt) from mbp1 to mbp1+mbp2
+  : mbloop ( xt -- )
+    mbp1 mbp2 OVER + SWAP  DO
+      ( xt ) I OVER EXECUTE
+    LOOP
+    ( xt ) DROP
   ;
 
-  \ MB looped bit read with action xt, build response (FC01, FC02)
-  : mbrbits ( xt -- )
-    ( xt ) mbp2 N* DUP txc+       \ response: number of bytes
-    ( xt N* ) 1- FOR 0 txc+ NEXT  \ init packed bits in response with 0
+  \ MB looped bit read with action xt, build response (FC01, FC02, FC03, FC04)
+  : mbread ( xt bpu -- )
+    ( xt bpu ) DUP mbp2 * ( bpu bits )
+    \ N* as "quantity of bits / 8, if the remainder is different of 0 => N=N+1"
+    ( bits ) 1- 8 / 1+ ( N* ) DUP txc+
+    ( bpu N* ) SWAP ( bpu ) 1 = IF
+       ( N* ) 1- FOR 0 txc+ NEXT  \ init bytes in response with 0
+    ELSE
+       ( N* ) DROP  \ not a bitfield, no memory init
+    THEN
     ( xt ) mbloop
   ;
 
diff --git a/MBSERVER b/MBSERVER
index 5e67ce7..5eece14 100644
--- a/MBSERVER
+++ b/MBSERVER
@@ -29,52 +29,48 @@ NVM  \ compile to Flash memory from here on
 #require BF!
 #require BF@
 
+  VARIABLE coils   COILCELLS    1- 2* ALLOT
+  VARIABLE inputs  INPUTCELLS   1- 2* ALLOT
+  VARIABLE holding HOLDINGCELLS 1- 2* ALLOT
 
   \ --- FC01 "Read Coils"
 
-  VARIABLE coils COILCELLS 1- 2* ALLOT
-
   \ FC01 Read Coils transfer
   :NVM  ( i -- )
     ( i ) coils OVER ( i a i ) BF@
     ( i b ) SWAP mbp1 - ( b i0 )
     txbuf 3 + SWAP ( b a i0 ) LEBF!
-  ;RAM ALIAS FC01ACT NVM
+  ;RAM ALIAS FC01ACT
 
   \ FC01 handler
   :NVM  ( -- )
     [ COILCELLS 16 * ] LITERAL mbrange? IF
-      [ ' FC01ACT ] LITERAL ( xt ) mbrbits
+      [ ' FC01ACT ] LITERAL 1 ( xt bpu ) mbread
     THEN
   ;NVM ( xt ) 1 FC>XT !
 
   \ --- FC02 "Read Discrete Inputs"
 
-  VARIABLE inputs INPUTCELLS 1- 2* ALLOT
 
   \ FC02 input register transfer
   :NVM  ( i -- )
     ( i ) inputs OVER ( i a i ) BF@
     ( i b ) SWAP mbp1 - ( b i0 )
     txbuf 3 + SWAP ( b a i0 ) LEBF!
-  ;RAM ALIAS FC02ACT NVM
+  ;RAM ALIAS FC02ACT
 
   \ FC02 handler
   :NVM  ( -- )
     [ INPUTCELLS 16 * ] LITERAL mbrange? IF
-      [ ' FC02ACT ] LITERAL ( xt ) mbrbits
+      [ ' FC02ACT ] LITERAL 1 ( xt bpu ) mbread
     THEN
   ;NVM ( xt ) 2 FC>XT !
 
   \ --- FC03 "Read Holding Registers"
 
-  VARIABLE holding HOLDINGCELLS 1- 2* ALLOT
-
-  \ dummy
-  : mbread ;
 
   \ FC03 holding register iterated transfer
-  :NVM  ( i -- 1 )
+  :NVM  ( i -- )
     ( i ) DUP 0 EECELLS WITHIN IF
       2* EESTART + @
     ELSE
@@ -83,8 +79,7 @@ NVM  \ compile to Flash memory from here on
       ELSE
         ( i ) DROP 0
       THEN
-    THEN
-    tx+   1 ( inc )
+    THEN   tx+
   ;NVM ( xt )
 
   \ FC03 handler
@@ -96,8 +91,9 @@ NVM  \ compile to Flash memory from here on
   \ --- FC04 "Read Input Registers"
 
   \ FC04 input register iterated transfer
-  :NVM  ( i -- 1 )
-    2* inputs + @ tx+   1 ( inc )
+  :NVM  ( i -- )
+    2* inputs + @
+    tx+
   ;NVM ( xt )
 
   \ FC04 handler
diff --git a/README.md b/README.md
index 46caa44..0296f7e 100644
--- a/README.md
+++ b/README.md
@@ -39,7 +39,7 @@ It's easy to build custom targets, e.g. using the $0.80 [MINDEV board](https://g
 
 ![MINDEV](https://camo.githubusercontent.com/82bd480f176951de9a469e134f543a6570f48597/68747470733a2f2f616530312e616c6963646e2e636f6d2f6b662f485442314e6642615056585858586263587058587136785846585858362f357063732d6c6f742d53544d3853313033463350362d73797374656d2d626f6172642d53544d38532d53544d382d646576656c6f706d656e742d626f6172642d6d696e696d756d2d636f72652d626f6172642e6a70675f323230783232302e6a7067)
 
-When using PB5 for RS485 direction control (-> `BUSCTRL`) the C0135 code can be used unchanged.
+When using PB5 for RS485 direction control (-> `BUSCTRL`) the C0135 code can be used (refer to the [C0135 STM8 eForth Wiki page][C0135]).
 
 ## Supported MODBUS Function Codes
 
@@ -47,10 +47,10 @@ When using PB5 for RS485 direction control (-> `BUSCTRL`) the C0135 code can be
 
 FC | Description | Support
 -|-|-
-**1**| **Read Coils** | implemented
+**1** | **Read Coils** | implemented
 **2** | **Read Discrete Inputs** | implemented
-3 | Read Holding Registers | partial
-4 | Read Input Registers | partial
+**3** | **Read Holding Registers** | implemented 
+**4** | **Read Input Registers** | implemented
 **5** | **Write Single Coil** | implemented
 **6** | **Write Single (Holding) Register** | implemented
 **15** | **Write Multiple Coils** | implemented
diff --git a/test/FC01 b/test/FC01
index bbe8aaf..7bd4f88 100644
--- a/test/FC01
+++ b/test/FC01
@@ -22,9 +22,9 @@ NVM
 #require MBDUMP
 #require BF!
 #require BF@
-  
-  8 CONSTANT COILBYTES
-  VARIABLE coils COILBYTES 2- ALLOT
+
+  4 CONSTANT COILCELLS
+  VARIABLE coils COILCELLS 1- 2* ALLOT
 
   \ FC01 Read Coils transfer
   :NVM  ( i -- )
@@ -39,10 +39,10 @@ NVM
   :NVM  ( -- )
     \ write register address and value to the console
     ." Read coil bits: A:" mbp1 . ."  Q:" mbp2 . CR
-    ." bits to coils end:" COILBYTES 8 * mbp1 mbp2 + - . CR
+    ." bits to coils end:" COILCELLS 16 * mbp1 mbp2 + - . CR
 
-    COILBYTES 8 * mbrange? IF
-      [ ' FC01ACT ] LITERAL ( xt ) mbrbits
+    COILCELLS 16 * mbrange? IF
+      [ ' FC01ACT ] LITERAL 1 ( xt bpu ) mbread
     THEN
   ;NVM ( xt ) 1 FC>XT !
 
diff --git a/test/FC04 b/test/FC04
new file mode 100644
index 0000000..85022bd
--- /dev/null
+++ b/test/FC04
@@ -0,0 +1,62 @@
+\ Minimal MODBUS server with FC04 "Read Input Registers" handler
+\ Features:
+\ - prints debug infos to the console
+\ - interactive tests from the console, e.g. set inputs manually "7 inputs !"
+
+\ check if the MODBUS protocol core is already present
+\ hint: the development cycle will be faster if you PERSIST it
+#require MBPROTO
+
+\ Resetting the FC handler table can be helpful for development
+#require WIPE
+#require MBRESET
+MBRESET   \ Reset the MODBUS Function Code table
+
+#require ALIAS
+#require :NVM
+#require 'IDLE
+#require .OK
+
+NVM
+
+#require MBDUMP
+#require BF!
+#require BF@
+
+  4 CONSTANT INPUTCELLS
+  VARIABLE inputs INPUTCELLS 1- 2* ALLOT
+
+  \ --- FC04 "Read Input Registers"
+
+  \ FC04 input register iterated transfer
+  :NVM  ( i -- 1 )
+    DUP .
+    2* inputs + @ tx+
+  ;NVM ( xt )
+
+  \ FC04 handler
+  :NVM  ( -- )
+    \ write register address and value to the console
+    ." Read input registers: A:" mbp1 . ."  Q:" mbp2 . CR
+
+     [ ( xt xth ) SWAP ] LITERAL 16 ( xt bpu ) mbread
+  ;NVM ( xth ) 4 FC>XT !
+
+
+  \ custom default action handler
+  : showfc ( -- )
+    rxbuf C@ ." FC:" . CR
+    1 MBEC  \ set error code
+  ;
+
+  : init ( -- )
+    0 UARTISR                     \ init UART handler w/ default baud rate
+    1 mbnode !                    \ set node address
+    [ ' showfc ] LITERAL mbdef !  \ FC default action (optional feature)
+    [ ' MBDUMP ] LITERAL mbact !  \ show buffers (debug demo)
+    [ ' MBPROTO ] LITERAL 'IDLE ! \ run MB protocol handler as idle task
+    .OK
+  ;
+
+  ' init 'BOOT !
+WIPE RAM
diff --git a/test/FC15 b/test/FC15
index 676425d..36ebbec 100644
--- a/test/FC15
+++ b/test/FC15
@@ -22,29 +22,29 @@ NVM
 #require MBDUMP
 #require BF!
 #require BF@
-  
-  8 CONSTANT COILBYTES
-  VARIABLE coils COILBYTES 2- ALLOT
+
+  4 CONSTANT COILCELLS
+  VARIABLE coils COILCELLS 1- 2* ALLOT
 
   \ --- FC15 "Write Multiple Coils"
 
   \ FC15 Write Multiple Coils write transfer
   :NVM  ( i -- )
     \ uncomment "DUP ." lines to check index (this can cause a timeout)
-    \ DUP . 
+    DUP .
     ( i ) rxbuf 7 + OVER mbp1 - ( i a i0 )
-    \ DUP . CR
+    DUP . CR
     LEBF@ SWAP ( b i )
-    coils SWAP ( b a i ) BF!  
+    coils SWAP ( b a i ) BF!
   ;RAM ALIAS FC15ACT NVM
 
   \ FC15 handler
   :NVM  ( -- )
     \ write register address and value to the console
     ." Write register: A:" mbp1 . ."  Q:" mbp2 . CR
-    ." Write bits from end:" COILBYTES 8 * mbp1 mbp2 + - . CR
+    ." Write bits from end:" COILCELLS 16 * mbp1 mbp2 + - . CR
 
-    COILBYTES 8 * mbrange? IF
+    COILCELLS 16 * mbrange? IF
       [ ' FC15ACT ] LITERAL ( xt ) mbloop
       MBWR   \ MODBUS write response
     THEN