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

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Date of introduction:  1982
Date of withdrawal:    1986 
Also known as:
	PC27X series.
Price: $1,435

Technology:  Based on the 6120 microprocessor, this shared the same
	packaging as DEC's other competitors in the PC market, the
	Rainbow (8088 based) and the PRO-325 (PDP-11 based).

Reason for introduction:  This machine was introduced in order to allow
	more flexibility than the DECmate I and to allow more sharing of
	parts with the VT220 and DEC's other personal computers.

Compatability:  Same as the DECmate I, except it could continue from a
	halt.  There was better hardware for device emulation support,
	allowing for somewhat better real-time performance.  The data
	communications port was an incompatable improvement on the
	incompatable DECmate I communications port.  No built-in terminal
	emulation was provided, and the data communications port supported
	only one line, but aside from this, the data communications port
	is essentially as powerful as the DP-278B on the DECmate I.

Standard Configuration:  The DECmate II was sold with 32K of program
	memory, plus a second full bank for dedicated control panel
	function emulation.  Code running in the second bank is sometimes
	referred to as slushware; it looks like hardware to the PDP-8
	user, but it is actually device emulation software that is loaded
	from the boot diskette.

	An integral RX50 dual 5 1/4 inch diskette drive with an 8051
	controller chip was included, along with a printer port, a 100Hz
	real-time clock, single data communications port, and interfaces
	to the monitor and keyboard.  The diskette drive can read
	single-sided 48 track-per-inch diskettes, so it might be possible
	to read (but not write) IBM PC diskettes on it.

Expandability:  This was the most open of the DECmate systems, with a
	number of disk options:  An additional pair of RX50 drives could
	be added, and with the RX78 board, it could support a pair of
	dual 8 inch drives, either RX01 or RX02.

	As an alternative to the RX78, there was a controller for an
	MFM hard drive.  The interface to the RX78 board wasn't fully
	compatable with earlier interfaces to RX01 and RX02, and there
	was no way to have both an RX78 and an MFM drive.  The MFM drive
	could be up to 64 MB, with 16 sectors per track, 512 bytes each
	and at most 8 heads and 1024 (or possibly 4096) cylinders.  A
	power supply upgrade was needed to support the MFM drive.  DEC
	sold this machine with 5, 10 and 20 meg hard drives, Seagate
	ST-506, 412, and 225 respectively.

	A graphics board supporting a color monitor could be added in
	addition to the monochrome console display; two variants of
	this board were produced during the production run, all slightly

	A coprocessor board could be added, with communication to and
	from the coprocessor through device 14.  DEC sold three boards,
	an APU board (Z80 and 64K), and two XPU boards (Z80, 8086 and
	either 256K or 512K).  If these added processors are used, the
	6120 processor is usually used as an I/O server for whatever
	ran on the coprocessor.  The XPU boards used a Z80 for I/O
	support, so 8086 I/O was very indirect, particularly if it
	involved I/O to a PDP-8 device that was emulated from control
	memory.  Despite this, the DECmate version of MS/DOS is generally
	faster than MS/DOS on more recent 80286 and 80386 based IBM PCs
	because of effective use of the coprocessors (but they couldn't
	run MS/DOS code that bypasses MS/DOS for I/O).

Survival:  As with the DECmate I.

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Top Document: PDP-8 Summary of Models and Options (posted every other month)
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Last Update March 27 2014 @ 02:11 PM