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

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Date of introduction:  1980
Date of withdrawal:    1984 (Phased out in favor of the DECmate II)
Also known as:  DECmate (prior to the DECmate II, no suffix was used)
		VT278

Technology:  Based on the Harris 6120 microprocessor, packaged in a
	VT-100 box with keyboard and display.

Reason for introduction:  This machine was aimed primarily at the market
	originally opened by the VT78, using a new gate-array implementation
	of the PDP-8 built under contract with DEC by Harris.  The Harris
	6120 was designed to run at 10 Mhz, and the new packaging was
	optimized for minimum cost and mass production efficiency.

Compatability:  A new feature was introduced in the 6120 microprocessor:
	The Group I OPR combination RAL RAR was defined as R3L, or rotate
	accumulator 3 places left, so that byte swap (BSW) is equivalent
	to R3L;R3L.  RTR RTL remained a no-op, as in the 6100.

	Also, the EAE operations not implemented in the basic CPU cause
	the CPU to hang awaiting completion of the operation by a
	coprocessor.  Unfortunately, no EAE coprocessor was ever offered.

	The printer port offered software baud-rate selection compatable
	with the VT78 baud-rate selection scheme.  The dual-port data
	communications option was flexible but completely incompatable
	with all previous PDP-8 serial ports.

	The console and printer ports are not fully compatable with the
	earlier PDP-8 serial ports.  Specifically, on earlier serial
	interfaces, it was possible to test flags without resetting them,
	but on the DECmate machines, testing the keyboard input flag
	always resets the flag as a side effect.  In addition, on the
	console port, every successful test of the flag must be followed
	by reading a character or the flag will never be set again.

	It was not possible to continue from a halt without restarting
	the machine.

	The large amount of device emulation performed by the CPU in
	supporting screen updates severely limits the ability of the
	system to run in real time.

Standard configuration:  The DECmate I was sold with 32k words of memory,
	with a small control memory added to handle control/status,
	console device emulation and boot options.  The console terminal
	keyboard and display functions are largely supported by code
	running in control memory (a less expensive alternative to
	dedicating hardware for this, as was done in the VT78).

	The DECmate I came with an integral printer port, compatable with
	the VT78 (device 32/33), and it had an RX02 dual 8 inch diskette
	drive, mounted in the short pedistal under the terminal/CPU box.
	A 100Hz clock was included, as in the VT78 and PDP-8/A.

Expandability:  This was a closed system, with limited options.
	Specifically, a second RX02 could be connected (or an RX01,
	because that had a compatable connector), the DP278A and DP278B
	communications boards (really the same board, but the DP278B had
	2 extra chips), and the RL-278 disk controller, able to accomodate
	from 1 to 4 RL02 rack mount disk drives.

	When the DP278A option is added, additional routines in control
	memory come alive to handle terminal emulaton and allow diskless
	operation.  The terminal emulator is an extended VT100 subset
	that is essentially compatable in 80 column mode.  The DP278A
	option could support both asynchronous and synchronous protocols,
	and the DP278B could handle SDLC and other nasty bit-stuffing
	protocols.

	Various pedestal and desk configurations were sold for housing
	the RX01 and RX02 drives, most being teacart style designs, but
	there was also a pedestal version that was essentially a
	repackaging of the RX02 with either 2 or 4 new 8 inch disk
	transports (physically incompatable with earlier DEC transports).

Survival:  Many DECmates are still in use, and they are fairly common on
	the surplus market.  They are found in small numbers just about
	anywhere large numbers of early PC vintage machines are found.

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Top Document: PDP-8 Summary of Models and Options (posted every other month)
Previous Document: What is a VT78?
Next Document: What is a DECmate II?

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Last Update March 27 2014 @ 02:11 PM