C64 SERIAL BUS


 Serial Bus Pinouts


	Pin	Name	Description
	 1	SRQ	Serial Service Request In
	 2	GND	Ground
	 3	ATN	Serial Attention In/Out
	 4	CLK	Serial Clock In/Out
	 5	DATA	Serial Data In/Out
	 6	RESET	Serial Reset

	All signals are active low.


  SRQ: Serial Service Request In

 This signal is not used on the C64. On C128 it is replaced with Fast Serial
 Clock for the 1571 disk drive.


  ATN: Serial Attention In/Out

 Sending any byte with the ATN line low (sending under attention) causes it
 to be interpreted as a Bus Command for peripherals on the serial bus.

 When the C64 brings this signal LOW, all other devices start listening for
 it to transmit an address. The device addressed must respond in a preset
 period of time; otherwise, the C64 will assume that the device addressed is
 not on the bus, and will return an error in the STATUS word.

 Usually, the address byte will be followed by one to two commands for the
 device addressed, meaning the secondary address and channel on the peripheral.
 Such a command can be one of the following:

	20
	40
	60
	E0
	F0

 
  CLK: Serial Clock In/Out

  This signal is for timing the data sent on the serial bus. This signal is
  always generated by the active TALKER. RISING EDGE OF THE CLOCK means data
  bit is valid.


  DATA: Serial Data In/Out

  Data on the serial bus is transmitted bit by bit at a time on this line.


  RESET: Serial Reset

  You may disconnect this line to save your disk drive. The easiest way is to
  do that on the cable, thus avoiding any modifications on your peripherals.



  Serial Bus Timing

 ___
 CLK    |____|~~~~|	Ts Bit Set-up time
	: Ts : Tv :	Tv Bit Valid time



	 |<--------- Byte sent under attention (to devices) ------------>|

 ___	____                                                        _____ _____
 ATN	   |________________________________________________________|
	   :							    :
 ___	______     ________ ___ ___ ___ ___ ___ ___ ___ ___         :
 CLK	   : |_____|      |_| |_| |_| |_| |_| |_| |_| |_| |______________ _____
	   :       :        :				  :         :
	   : Tat : :Th: Tne :                             : Tf : Tr :
 ____	________ : :  :___________________________________:____:
 DATA	___|\\\\\__:__|    |__||__||__||__||__||__||__||__|    |_________ _____
	              :     0   1   2   3   4   5   6   7      :
	              :    LSB                         MSB     :
		      :     :				       :
		      :     : Data Valid	  Listener: Data Accepted
		      : Listener READY-FOR-DATA




		END-OR-IDENTIFY HANDSHAKE (LAST BYTE IN MESSAGE)
 ___	_______________________________________________________________________
 ATN
 ___	 ___ ___      ________________ ___ ___ ___ ___ ___ ___ ___ ___       __
 CLK	_| |_| |______|              |_| |_| |_| |_| |_| |_| |_| |_| |_______|_
	       :      :		     :	                             :       :
	       :Tf:Tbb:Th:Tye:Tei:Try:                               :Tf :Tfr:
 ____	__________:   :  :___:   :_______________________________________:   :_
 DATA	|__||__|  |______|   |___|   :                                   |___|_
	 6   7	      :	 :   :   :   :					 :
	    MSB	      :	 :   :   :   : Talker Sending			 :
		      :	 :   :   : Listener READY-FOR-DATA		System
		      :	 :   : EOI-Timeout Handshake		  Line Release
		      :	 : Listener READY-FOR-DATA
		      : Talker Ready-To-Send




		TALK-ATTENTION TURN AROUND (TALKER AND LISTENER REVERSED)
 ___	             _________________________________________________________
 ATN	_____________|
		     :
 ___	 ___ ___     :   _____   ________ ___ ___ ___ ___ ___ ___ ___ ___
 CLK	_| |_| |_________|   |___|      |_| |_| |_| |_| |_| |_| |_| |_| |_____
	       :     :   :   :	 :      :                               :
	       :Tf:Tr:Ttk:Tdc:Tda:Th:Try:                               :Tf :
 ____	__________:  :       :   :  :_______________________________________:
 DATA	|__||__|  |_________________|   :|__||__||__||__||__||__||__||__|   |_
	 6   7       :   :   :   :  :   : 0   1   2   3   4   5   6   7
	    MSB      :   :   :   :  :   :LSB                         MSB
		     :   :   :   :  :   :
		     :   :   :   :  :   : Data Valid
		     :   :   :   :  : Listener READY-FOR-DATA
		     :   :   :   : Talker Ready-To-Send
		     :   :   : Device acknowledges it's now TALKER.
		     :   : Becomes LISTENER, Clock = High, Data = Low
		     : Talker Ready-To-Send




 ___	_____________________________________________________________________
 ATN
 ___	    _________ ___ ___ ___ ___ ___ ___ ___ ___       ________ ___ ___
 CLK	____|       |_| |_| |_| |_| |_| |_| |_| |_| |_______|      |_| |_| |_
	    :       :				    :       :      :
	    :Th :Tne:                               :Tf :Tbb:Th:Tne:
 ____	    :   :___:___________________________________:      :_____________
 DATA	________|   :|__||__||__||__||__||__||__||__|   |______|
	    :   :   : 0   1   2   3   4   5   6   7     :
	    :   :   :LSB                         MSB    :
	    :	:   :					:
	    :	:   : TALKER SENDING		Listener: Data Accepted
	    :	: LISTENER READY-FOR-DATA
	    : TALKER READY-TO-SEND



  Serial Bus Timing


	Description			Symbol	 Min	 Typ	 Max

	ATN Response (required) 1)	 Tat	  -	  -	1000us
	Listener Hold-Off		 Th	  0	  -	 oo
	Non-EOI Response to RFD 2)	 Tne	  -	 40us	200us
	Bit Set-Up Talker  4)		 Ts	 20us	 70us	  -
	Data Valid			 Tv	 20us	 20us	  -
	Frame Handshake  3)		 Tf	  0	  20	1000us
	Frame to Release of ATN		 Tr	 20us	  -	  -
	Between Bytes Time		 Tbb	100us	  -	  -
	EOI Response Time		 Tye	200us	250us	  -
	EOI Response Hold Time  5)	 Tei	 60us	  -	  -
	Talker Response Limit		 Try	  0	 30us	 60us
	Byte-Acknowledge  4)		 Tpr	 20us	 30us	  -
	Talk-Attention Release		 Ttk	 20us	 30us	100us
	Talk-Attention Acknowledge	 Tdc	  0	  -	  -
	Talk-Attention Ack. Hold	 Tda	 80us	  -	  -
	EOI Acknowledge			 Tfr	 60us	  -	  -


   Notes:
	1)  If maximum time exceeded, device not present error.
	2)  If maximum time exceeded, EOI response required.
	3)  If maximum time exceeded, frame error.
	4)  Tv and Tpr minimum must be 60us for external device to be a talker.
	5)  Tei minimum must be 80us for external device to be a listener.