Signal description of ACIA 6551 ------------------------------- Pin(s) Signal Dir Description 1 Vss - Ground (0V). 2 CS0 in Chip Select 2. CS0=1 AND CS1=0 = coupled to data bus, otherwise tri-state. 3 /CS1 in Chip Select 1. 4 /RES in Reset. 0=reset the ACIA to initial state. 5 RXC in / out Receive Clock. This pin can be configured to be or either an input for the 16fold of the baud rate progr.? or to ouput the same(?). 6 XTAL1 in Crystal 1. Usually, the 6551 is operated with a 1.8432 MHz crystal between XTAL1 and XTAL2. Additionally, a 30 pF capacitor should be connected between pin 7 and GND (Vss). 7 XTAL2 in Crystal 2. 8 /RTS out Request To Send. Switches the transceiver of an attached modem on, thus telling it to get ready to receive data from the computer. 9 /CTS in Clear To Send. The modem indicates that it is ready, usually upon an RTS from the computer. 10 /TXD out Transfer Data. 11 /DTR out Data Terminal Ready. Signal for attached modem that ACIA/computer is ready. 12 /RXD in Receive Data. 13-14 RS0-RS1 in Register select. These two pins select one of the ACIA's internal registers and are usually connected with A0 and A1 of the address bus. 15 Vdd - Supply voltage. +5V DC. 16 /DCD in Data Carrier Detect. The modem tells the ACIA if it receives a carrier on the phone line. DCD stays active during the entire connection. On half duplex modems, only the receiving modem activates this line. 17 /DSR in Data Set Ready. The modem tells the ACIA that the preparation for connecting with the target is finished and it is ready to communicate with the ACIA. With most modems, this line is always active ( = LOW), so that this line can only be used to determine if the modem is switched on. 18-25 DB0-DB7 in/out Data bus. 26 /IRQ out Interrupt ReQuest. 27 ø 2* in Phi 2. System clock signal. Used for bus synchronization (not for baud rate!) 28 R/-W in Read/-Write. 0=read registers, 1=write registers. * The 'ø' is used as Greek 'phi' here. progr. = programmable Author: Marc-Jano Knopp on 98/2/23 Source: http://mjk.c64.org/