diff --git a/doc/keybrd_library_developer_guide.md b/doc/keybrd_library_developer_guide.md
index d74bbbc..81aa2e0 100644
--- a/doc/keybrd_library_developer_guide.md
+++ b/doc/keybrd_library_developer_guide.md
@@ -24,7 +24,9 @@ Keybrd library class inheritance diagram
 	Scanner_uC  Scanner_IOE  Scanner_ShiftRegsPISO
 
 
-	       PortInterface
+                PortWriteInterface
+                 /           \
+	       PortInterface   Port_ShiftRegs       (Port class for MOSI shift registers)
 	         /        \
 	Port_PCA9655E  Port_MCP23S17                (one Port class for each IOE type)
  
diff --git a/src/LED_Port.h b/src/LED_Port.h
index c541751..d1d49d5 100644
--- a/src/LED_Port.h
+++ b/src/LED_Port.h
@@ -3,23 +3,29 @@
 #include <Arduino.h>
 #include <inttypes.h>
 #include <LEDInterface.h>
-#include <PortInterface.h>
+#include <PortWriteInterface.h>
 
 /* An LED_Port object is an I/O expander pin that is connected to an LED indicator light.
+LED_Port functions turn LED on and off.
 
 Example initialization:
     const uint8_t IOE_ADDR = 0x20;
     Port_MCP23S17 portA(IOE_ADDR, 0, 1<<0 | 1<<1 );
     LED_Port LED_fn(portA, 1<<5);
+
+Example initialization:
+    Port_ShiftRegs shiftRegs(8);
+    LED_Port LED_fn(shiftRegs, 1<<6);
+    
 */
 class LED_Port : public LEDInterface
 {
     private:
-        PortInterface& refPort;
-        const uint8_t pin;                      //bit pattern, 1 is IOE pin to LED
+        PortWriteInterface& refPort;
+        const uint8_t pin;                      //bit pattern, 1 is pin connected to LED
 
     public:
-        LED_Port(PortInterface& refPort, const uint8_t pin)
+        LED_Port(PortWriteInterface& refPort, const uint8_t pin)
             : refPort(refPort), pin(pin) {}
         virtual void on();
         virtual void off();
diff --git a/src/LED_ShiftReg.cpp b/src/LED_ShiftReg.cpp
deleted file mode 100644
index cfb3b17..0000000
--- a/src/LED_ShiftReg.cpp
+++ /dev/null
@@ -1,11 +0,0 @@
-#include "LED_ShiftReg.h"
-
-void LED_ShiftReg::on()
-{
-    refPort.write(pin, HIGH);
-}
-
-void LED_ShiftReg::off()
-{
-    refPort.write(pin, LOW);
-}
diff --git a/src/LED_ShiftReg.h b/src/LED_ShiftReg.h
deleted file mode 100644
index d4a9acf..0000000
--- a/src/LED_ShiftReg.h
+++ /dev/null
@@ -1,22 +0,0 @@
-#ifndef LED_SHIFTREG_H
-#define LED_SHIFTREG_H
-#include <Arduino.h>
-#include <inttypes.h>
-#include <LEDInterface.h>
-#include <PortWriteInterface.h>
-
-/* LED_ShiftReg turns LED on and off.
-shift register RCLK pin a.k.a. SS or ST
-*/
-class LED_ShiftReg: public LEDInterface
-{
-    private:
-        PortWriteInterface& refPort;
-        const uint8_t pin;              //bit pattern, 1 is shift-register pin connected to an LED
-    public:
-        LED_ShiftReg(PortWriteInterface& refPort, uint8_t pin)
-            : refPort(refPort), pin(pin) {}
-        virtual void on();
-        virtual void off();
-};
-#endif
diff --git a/src/PortInterface.h b/src/PortInterface.h
index d8338a4..0f37b1f 100644
--- a/src/PortInterface.h
+++ b/src/PortInterface.h
@@ -2,11 +2,15 @@
 #define PORTINTERFACE_H
 #include <Arduino.h>
 #include <inttypes.h>
+#include <PortWriteInterface.h>
 
 /*
 Port classes are the keybrd library's interface to microcontroller ports or I/O expander ports.
+
+Port classes that can read and write, inherit from PortInterface.
+Port classes that can only write, inherit from PortWriteInterface.
 */
-class PortInterface
+class PortInterface : public PortWriteInterface
 {
     public:
         virtual void beginProtocol()=0;               //SPI bus or I2C bus
diff --git a/src/PortWriteInterface.h b/src/PortWriteInterface.h
index 037be89..b7e3815 100644
--- a/src/PortWriteInterface.h
+++ b/src/PortWriteInterface.h
@@ -3,6 +3,12 @@
 #include <Arduino.h>
 #include <inttypes.h>
 
+/*
+Port classes are the keybrd library's interface to MOSI shift registers i.e. Port_ShiftRegs
+
+Port classes that can read & write, inherit from PortInterface.
+Port classes that can only write, inherit from PortWriteInterface.
+*/
 class PortWriteInterface
 {
     public:
diff --git a/src/Port_ShiftRegs.h b/src/Port_ShiftRegs.h
index f21f432..4f379d9 100644
--- a/src/Port_ShiftRegs.h
+++ b/src/Port_ShiftRegs.h
@@ -6,12 +6,12 @@
 #include <PortWriteInterface.h>
 
 /* Port_ShiftRegs 
-shift register RCLK pin a.k.a. SS or ST
+slaveSelect is controller-pin number connected to shift register RCLK pin a.k.a. SS or ST
 */
 class Port_ShiftRegs : public PortWriteInterface
 {
     private:
-        const uint8_t slaveSelect;  //controller-pin number connected to shift register RCLK
+        const uint8_t slaveSelect;              //controller-pin number
         uint8_t outputVal;                      //bit pattern for LEDs
     public:
         Port_ShiftRegs(const uint8_t slaveSelect);