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PDP-8 Summary of Models and Options (posted every other month)
Section - What is a VT78?

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Date of introduction:  1978
Date of withdrawal:    1980 (Displaced by the DECmate)
Also known as:
	DECstation
	DECstation 78
Price: $7,995 ($5,436 in lots of 100)

Technology:  Based on the Intersil/Harris 6100 microprocessor, packaged
	in a VT52 case.  The 6100 processor was able to run at 4 MHz, but
	in the VT78, it was only clocked at 2.2 MHz because of the speed
	of the DRAM used and the deliberate use of graded out chips.

Reason for introduction:  Using TTL MSI and LSI components, DEC could
	pack a system into the vacant space in a standard terminal case,
	allowing PDP-8 systems to compete with personal computers in the
	small business and office automation market.  This was a natural
	follow-on to the desk-mounted workstation configurations in which
	the PDP-8/A was already being sold.

Compatability:  The Group I OPR combinations RAL RAR and RTL RTR are
	no-ops.  Unlike all earlier PDP-8 models, autoindex locations
	10 to 17 (octal) only work in page zero mode; these operate like
	all other memory locations when addressed in current page mode
	from code running on page zero.  Other than this, it is fully
	PDP-8/E compatable, even at the level of I/O instructions for
	the standard periperals; this was the last PDP-8 to offer this
	level of compatability.

	It was not possible to continue from a halt without restarting
	the machine.  In addition, none of the peripherals available on
	this machine needed DMA (data break) transfers.

Standard configuration:  The VT78 was sold with 16k words of DRAM with
	the keyboard and display of the VT52 terminal.  An RX01 dual 8"
	diskette drive was included, packaged in a teacart pedestal under
	the terminal.  The console (device 03/04) and the serial ports
	(devices 30/31 and 32/33) are compatible with the M8650 KL8E,
	with the latter extended to allow software controlled baud rate
	selection.  There are two parallel ports; device 66 (compatible
	with the M8365 printer controller) and device 47, compatible with
	the nonstandard port on the M8316 DKC8AA.  There is also a 100Hz
	clock compatible with the clock on the M8316 DKC8AA.

	The standard ROM boots the system from the RX01 after setting the
	baud rates to match that selected by the switches on the bottom
	of the VT52 case.

Expandability:  This was a closed system, with few options.  The base
	configuration was able to support two RX01 drives (later RX02),
	for a total of 4 transports.  Various boot ROM's were available,
	including a paper-tape RIM loader ROM for loading diagnostics
	from tape.  Another ROM boots the system from a PDP-11 server in
	the client/server configuration used by WPS-11.

Survival:  There are probably many VT78 systems still in use.
	

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Top Document: PDP-8 Summary of Models and Options (posted every other month)
Previous Document: What is a PDP-8/A?
Next Document: What is a DECmate I?

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Last Update March 27 2014 @ 02:11 PM