diff --git a/README.md b/README.md
index 968ef89..411da54 100644
--- a/README.md
+++ b/README.md
@@ -2,6 +2,8 @@
 
 An ESP8266 expansion for the Commodore PET / CBM.
 
+## Description
+
 Unlike the C64, the Commodore PET does not have a serial port.
 For that reason, most serial port adapters for the C64 don't work, and an
 alternate interface must be found. Furthermore, the user port on the PET
@@ -33,9 +35,30 @@ The other pin mappings are as follows:
 | Cassette Motor | CTS     | PET->ESP  | PIA1 CB2             | VIA PB4              |
 | Cassette Sense | RTS     | ESP->PET  | PIA1 PA4             | PIA1 PA5             |
 
-A suitable serial signal (RS-232-like) must be generated/decoded in
-software. Make sure that no cassette port #1 access is generated while
-the adapter is connected.
+A suitable serial signal (RS-232-like) must be generated/decoded in software.
+Make sure that you don't access the cassette port #1 while the adapter is
+connected.
 
 For programming info, refer to this excellent document:
 http://www.6502.org/users/andre/petindex/local/pet-io-2.txt
+
+## RS232 Waveform
+
+| Logic |  L  |  H  |
+|-------|-----|-----|
+| CMOS  | 0V  | 3V  |
+| RS232 | +3V | -3V |
+| Start | +3V |     |
+| End   |     | -3V |
+
+   _   ___   ___     ____
+H   | |   | |   |   |
+    | |   | |   |   |
+L   |_|   |_|   |___|
+     S 1 1 0 1 1 0 0 1 E
+
+     _     _     ___
++3  | |   | |   |   |
+    | |   | |   |   |
+-3 _| |___| |___|   |____
+     S 1 1 0 1 1 0 0 1 E