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I have the case of a dual 6551 ACIA board from the RM 65 family from Rockwell. I don't have the schematic. It only has a 1.8432 Mhz Xtal to drive both ACIAs. I think it is not possible to connect the Xtal directly to both ACIAs, so I assume that the card must incorporate a pierce oscillator with the Xtal, resistors, capacitors and TTL inverters.

In that case, could the oscillator's output inverter be connected to the XTLI pins of both ACIAs, leaving the XTLOs pins floating? And, by the way, would it make sense to assign the binary value 1 to bit 4 in the control register of one of the ACIAs? For example, $0E for 9600bps, 1 stop bit, 8 data bit.

For connecting two ACIAs to a single crystal, see the diagrams just below the middle of http://wilsonminesco.com/6502primer/IO_ICs.html .

As for your last question, the answer is yes, that's how you would do it.  For other options where you might for example want a 0 in ACIA_CTRL<4>, see the DIP-switch options I made in the 7th diagram on my workbench computer's QRG page at http://wilsonminesco.com/BenchCPU/B1QRG/ which has allowed me to get the 31.25kbps for MIDI and 115.2kbps for programming our bluetooth modules.

Just a little after posting my question I found that link you suggested, thanks anyway.

The Rockwel and Commodore 6551 data sheets confuse me a little. On the contrary I deduce that you have mastered the operation modes of the ACIA 6551. I would appreciate it if you could clarify some Control Register configuration doubts for me:

1. TX = RX, with XTAL 1.8432 MHz on pin6/pin7.

1.1.- CTRL_Reg <4> = 1 and for 9600 bps, CTL_Reg = $1E.
RXC pin 4, becomes an output at 153.6 kHz (9600 x 16)
1.2.- CTRL_Reg <4> = 1 and for 115200 bps, CTRL_Reg = $10
RXC pin 4, becomes an output at 1.8432 MHz

2.- TX = RX, input 500 kHz on pin 6 (for MIDI bidirectional standard case - 31.25 kHz -)

CTRL_Reg <4> = 1, CTRL_Reg = $10.
RXC pin 4, becomes an output at 500 kHz.

3.- RX, input 500 kHz on RXC pin 4 (for MIDI 31.25 kHz, only reception)
TX, output for Terminal or printer at 9600 bps. XTAL 1.8432 MHz on pin6/pin7

CTRL_Reg <4> = 0, CTRL_Reg = 0E$

I won't take the time to look at all of the many datasheets online; but in my own books, Rockwell has better data on the '51 than the others.


I have always ust followed the data sheet (which I do kind of obsessively, because I don't like troubleshooting), and everything I've ever done with the 65c51 worked on first try, except when I lacked a capacitor in the crystal circuit.  (If you're using just a crystal and not a separate oscillator, the crystal goes from pin 6 to pin 7 of the DIP, but you need a 22pF capacitor from pin 6 to ground.)  Nevertheless, there are still features or modes I have not tried.  I've never used serial echo mode, data lengths of 5 or 6 bits and maybe even 7 (only 8), and I don't think I've ever used parity or transmit interrupt.


Yes, but the RxC pin is pin 5, not 4.  (Pin 4 is RST\.)  I just checked again the frequency output, on my workbench computer.


Yes, but again pin 5 is the output.  I just checked my code for something I did at work a few years ago for 115.2kbps, and yes, I put $10 in the control register.


Right (other than the pin 4/5 thing above).


I believe that's correct too.

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Thank you very much GARTHWILSON for your extensive response. It helps me a lot. Also, you are right, RxC is 5.

Regards.