From 55962c187b65a3a2020b8af9b7a6d1b49f4ce16f Mon Sep 17 00:00:00 2001 From: Gregor Riepl Date: Sat, 13 Aug 2022 20:43:52 +0200 Subject: [PATCH] Describe RS232 signal --- README.md | 29 ++++++++++++++++++++++++++--- 1 file changed, 26 insertions(+), 3 deletions(-) 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