Digital Equipment Corporation

DECmate (VT-278)



DECmate VT278 Computer System


DECmate was the name of a series of PDP-8-compatible computers produced by the Digital Equipment Corporation in the early 1980s. All of the models used an Intersil or Harris 6100 or 6120 microprocessor which emulated the 12-bit DEC PDP-8 CPU. They were text-only and used the OS/78 or OS/278 operating systems, which were extensions of OS/8 for the PDP-8. Aimed for the word-processing market, they typically ran the WPS-8 word-processing program.


Later models optionally had Intel 8080 or Z80 microprocessors which allowed them to run CP/M.


The range was a development of the VT78 which was introduced in 1978.



DECmate Specifications:


  • CPU: Intersil 6120
  • 10 MHz Clock Speed
  • 32k Words Main Memory
  • Addition non-volatile control panel memory
  • ROM-Based RX02 Bootstrap and Terminal Emulation Software
  • Integral RX02 Disk Interface
  • Integral Serial Printer Port (9600 Baud, 8 bits, No Parity)
  • Optional DP278 Communications Interface (two RS-232c Ports)
  • Option RL02 Hard Disk Interface
  • CPU Mounted in VT-100 Terminal Enclosure
  • Typical printer was DEC Letterwriter 100 Daisy-Wheel Printer




(From the PDP-8 FAQ Maintained by Doug Jones)


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




The DECmate 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.




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 compatible with the VT78 baud-rate selection scheme. The dual-port data communications option was flexible but completely incompatible with all previous PDP-8 serial ports.


The console and printer ports are not fully compatible 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, compatible with the VT78 (device 32/33), and it had an RX02 dual 8 inch diskette drive, mounted in the short pedestal under the terminal/CPU box. A 100Hz clock was included, as in the VT78 and PDP-8/A.




This was a closed system, with limited options. Specifically, a second RX02 could be connected (or an RX01, because that had a compatible 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 accommodate 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 emulation and allow diskless operation. The terminal emulator is an extended VT100 subset that is essentially compatible 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 incompatible with earlier DEC transports).




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.