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From: sjc@mordor.UUCP
Newsgroups: net.ham-radio
Subject: Xerox 820 (Big Board) update
Message-ID: <2227@mordor.UUCP>
Date: Tue, 3-Apr-84 04:17:15 EST
Article-I.D.: mordor.2227
Posted: Tue Apr  3 04:17:15 1984
Date-Received: Thu, 5-Apr-84 01:06:15 EST
Organization: S-1 Project, LLNL
Lines: 206

Thanks to a good samaritan at SRI, I have a copy of some Xerox 820
documentation. For brevity, I will detail primarily the aspects that
differ from the original "Big Board" from Digital Research Computers of
Texas.  Documentation for the original is still available from DRC
(214) 271-3538.  Xerox 820 documentation (which I haven't seen) is
advertised by BG Micro (214) 271-5546.

Port Address Decoding: no differences

System PIO "A" side:
	Bit0	DVSEL 1
	Bit1	DVSEL 2
	Bit2	Side
	Bit3	Keyboard data available
	Bit4	8"/5.25" disk select
	Bit5	Assigned for future use
	Bit6	Display character set (?)
	Bit7	Bank switching (0=RAM, 1=ROM & CRT & RAM)

Interrupt structures:
	Monitor uses mode 2 interrupts; I register is loaded with 0FF hex.
	Priority chain is organized high to low as follows:

	SIO CHANNEL A
	SIO CHANNEL B
	SYSTEM PIO PORT A
	SYSTEM PIO PORT B
	GP PIO PORT A
	GP PIO PORT B
	CTC CHANNEL 0
	CTC CHANNEL 1
	CTC CHANNEL 2
	CTC CHANNEL 3

General purpose PIO strappings (J11) and pin assignments (J8):

	J11:
		odd numbered pins grounded
		4	Port B READY polarity
		6	Port B lower direction
		8	Port A READY polarity
		10	Port A upper direction
		12	Port B upper direction
		14	Port A STROBE polarity
		16	Port B STROBE polarity
		18	Port A lower direction

	J8:
		odd numbered pins are supposed to be grounded; they
		aren't, so you must ground at least one of them to the
		PIO chip so as to provide a signal ground to the
		peripheral device
		2	Port A STROBE
		4	Port A READY
		6	Port A bit 0
		...
		20	Port A bit 7
		22	Port B STROBE
		24	Port B READY
		26	Port B bit 0
		...
		40	Port B bit 7

CTC Strapping and I/O assignments (J10):

	System clock	2 1	Clock/trigger 0
	ZC-TO0		4 3	Clock/trigger 1
	ZC-TO1		6 5	Clock/trigger 2
	ZC-TO2		8 7	Clock/trigger 3

Video output connector pin assignments (J7):

	6-10 grounded
	3	Vertical sync
	4	Horizontal sync
	5	Video

Serial I/O connector pin assignments channel A (J4):

	1	Protective ground
	2	Transmit data
	3	Receive data
	4	Request to send
	5	Clear to send
	6	Data set ready
	7	Protective ground
	8	Carrier detect
	15	Xmit clock
	17	Receive clock
	20	Data terminal ready

Serial I/O strapping options for channel A (J9): No differences

Serial I/O connector pin assignments channel B (J3):

	1	Ground
	2	Receive data
	3	Transmit data
	4	Clear to send
	5	Request to send
	6	Data set ready
	7	Ground
	8	Terminal ready
	20	Data carrier detect

Keyboard connector pin assignments (J2):

	14-25	grounded
	1-7	bits 0-7
	8	strobe
	13	+5 volts via resistor

Disk drive connector pin assignments (J1):

	20-37	grounded
	1	not connected
	2	8/5.25 select (grounded for 5.25)
	3	not connected
	4	index
	5	select 1
	6	select 2
	7	side
	8	HDLD
	9	step in
	10	step
	11	write data
	12	write
	13	trk 00
	14	write protect
	15	read data
	16	low current
	17	ready
	18	indep pin +12 or anything
	19	+5

CRT RAM: 3000 hex through 3FFF hex.

I/O ports:
	0-3	Channel A baud (writeonly)
	4	SIO channel A data
	5	SIO channel B data
	6	SIO channel A control
	7	SIO channel B control
	8	GP PIO A data
	9	GP PIO A control
	A	GP PIO B data
	B	GP PIO B control
	C-F	Channel B baud (writeonly)
	10	1771 status/command
	11	1771 track
	12	1771 sector
	13	1771 data
	14-17	CRT scroll register
		if 1, then loc 3000 is at bottom line; if 2, then
		loc 3000 is next to bottom line; if 17, then loc
		3000 is top line, etc.
	18	CTC 0
	19	CTC 1
	1A	CTC 2
	1B	CTC 3
	1C	system data
	1D	system control
	1E	keyboard data
	1F	keyboard control

Power (J5):
	1	-12
	2	+12 CPU
	3	+12 disk
	4-6	ground
	7	+12
	8	+5
	9	+5

Power fed out to monitor (if needed) (J6):
	1	ground
	2	+12

Original Xerox 820 disk formats:

		8"	5.25"
Tracks		77	40
Sectors		26	18
Bytes/sector	128	128
Tracks reserved	2	3
for OS
Capacity	241K	81K

5.25" format is CROMEMCO format, 8" is IBM 3740 format

Monitor entry points are alleged to be the same as the DRC big board.o

Monitor commands are alleged to be the same, except:

	Test command is "X", not "T"
	Verify command is missing
	Boot command is "A", not "B"
	A "T" command connects the keyboard to the printer port like
	a "typewriter"

Setting MSB of displayed character gives blinking attribute.

	--Steven Correll
	(S-1 Project, Lawrence Livermore National Laboratory)
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