diff --git a/README.md b/README.md
index fc523d3..60c5a1c 100644
--- a/README.md
+++ b/README.md
@@ -36,3 +36,6 @@ The other pin mappings are as follows:
 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.
+
+For programming info, refer to this excellent document:
+http://www.6502.org/users/andre/petindex/local/pet-io-2.txt