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Aug 19, 2014

Digital's DECSYSTEM-20 - Part 8 - I/O Bulkheads


The DECSYSTEM-20 bulkheads for connecting peripheral devices were located on the back of the mainframe: Basically they were the MASSBUS (I/O processor cabinet) and UNIBUS (front end processor cabinet) bulkheads for connectors.

The following picture shows again where the MASSBUS and UNIBUS were located in the overall system block diagram.

DECSYSTEM-20 block diagram

The MASSBUS was used for high speed devices like disk and tape drives and the UNIBUS was used for lower speed peripheral devices like terminals, modems, printers, card readers, and even floppy disks. (See part 1 of this article for more details about such peripheral devices). The UNIBUS usually included the local I/O processor and mini computer PDP-11. The following picture shows the rear view of the DECSYSTEM-20 mainframe computer three cabinets.

Rear view (Cabinets 3, 2, and 1)

The following picture shows where the bulkheads were located (IO bulkhead and front end bulkhead).

Rear view of cabinets 3, 2, and 1 and bulkheads enlarged


Front End cabinet (# 1) bulkhead at the lower right corner

The front end bulkhead was dominated by the 4 distribution panels of type H3007 of which each could handle 8 terminal connections (RS-232 type). It distributed the DH11 connectors to outside world from the internal PCBs in DC20-F blocks (inside the expansion drawer of the front end cabinet [see part 2 of this article]). DC20-F was the name given for the DH11s in a KL10 front end. So I am using here more DH11 documentation which is available.

Front End Bulkhead

The 32 RS-323 connectors are visible. Below them the main power connector cable, card reader connector, line printer connectors, and various other connectors.

Below are some photos of the H3007 8-line connector panel.

H3007 panel front face

H3007 panel inside face


RS-232 was first introduced in 1962 by the Radio Sector of the EIA. The original data terminal equipments (DTEs) were electromechanical teletypewriters, and the original data communication equipments (DCEs) were (usually) modems. When electronic terminals (smart and dumb) began to be used, they were often designed to be interchangeable with teletypewriters, and so supported RS-232. The C revision of the standard was issued in 1969 in part to accommodate the electrical characteristics of these devices.

BC03M digital RS232 Null-modem cable

Since the requirements of devices such as computers, printers, test instruments, POS terminals and so on were not foreseen by the standard, designers implementing an RS-232 compatible interface on their equipment often interpreted the standard idiosyncratically. The resulting common problems were non-standard pin assignment of circuits on connectors, and incorrect or missing control signals. The lack of adherence to the standards produced a thriving industry of breakout boxes, patch boxes, test equipment, books, and other aids for the connection of disparate equipment. A common deviation from the standard was to drive the signals at a reduced voltage. Some manufacturers therefore built transmitters that supplied +5 V and -5 V and labeled them as "RS-232 compatible".

digital printer cable

In telecommunications, RS-232 is a standard for serial communication transmission of data. It formally defined the signals connecting between a DTE  such as a computer terminal, and a DCE (data circuit-terminating equipment, originally defined as data communication equipment), such as a modem. The RS-232 standard is commonly used in computer serial ports. The standard defines the electrical characteristics and timing of signals, the meaning of signals, and the physical size and pin out of connectors. The current version of the standard is TIA-232-F Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange, issued in 1997.

Front end bulkhead

An RS-232 serial port was once a standard feature of a personal computer, used for connections to modems, printers, mice, data storage, uninterruptible power supplies, and other peripheral devices. However, the low transmission speed, large voltage swing, and large standard connectors motivated development of the Universal Serial Bus, which has displaced RS-232 from most of its peripheral interface roles. Many modern personal computers have no RS-232 ports and must use either an external USB-to-RS-232 converter or an internal expansion card with one or more serial ports to connect to RS-232 peripherals. RS-232 devices are still found, especially in industrial machines, networking equipment, and scientific instruments.

DB-25 Connector

DB-25 male connector

The DB-25 connector (named for its "B"-size "D"-shaped shell and 25 pins) is practically ubiquitous in the electronics industry. The DB-25 connector is used for a variety of purposes. Two common applications are RS-232/EIA-232 (serial) connections, and the parallel printer interface on the IBM PC. The DB-25 connector is also used for SCSI connections.


I/O cabinet bulkhead at the bottom

The KLlO was, by comparison, a radical departure from previous PMS (Processor-Memory-Switch) structures. In order to gain more performance, four words from four low order interleaved memory modules were accessed each cycle. The effective processor-memory bandwidth was thus over four Mwords/sec. The processor could also connect to as many as four PDP-11 minicomputers. Most of the i/o was handled by these front-end computers.

I/O Bulkhead

Each PDP-11 could access the KLlO memory via indirect address pointers and transfers data in much the same manner as the peripheral processing units of a CDC 6600. Notice also that the KLlO's console was tied to a PDP-11. This PDP-11 could load the KLlO microprogram memory, run micro diagnostics, and provided a potential remotely operated console. Each of the PDP-11's could achieve a word rate of 70 Kchar/sec.


Up to eight DEC Massbus controllers were integrated into the processor. The Massbus was an 18-bit data width bus for block transfer oriented mass storage devices such as disks and magnetic tapes. Each Mass- bus could transfer 1.6 Mwords/sec. yielding a maximum 12.8 Mwords/sec. transfer rate for all channels. How- ever, contemporary disks needed about 250 Kwords/sec. so that all eight channels only require 2.0 Mwords/sec. of the 4 Mword/sec. memory bandwidth of 4 modules. Individual disks and tapes could be connected to a second port for increased concurrency. For larger memory configurations, a memory bandwidth of 16 Mwords/sec. was not uncommon. A 2 Kword processor cache provided roughly a 90% hit rate and reduces memory bandwidth demand by nearly a factor of ten.

MASSBUS connector

BC06S digital MASSBUS cable

Connector Conspiracy /4/

"The term connector conspiracy possibly began to acquire popularity with the arrival of a mainframe computer called DEC KL-10 in the 1970s. The connectors for DEC KL-10 were completely different from all available connectors at that time. In fact, DEC even got the patent for the KL-10 Massbus connector. DEC reputedly turned down the option to license the design, which successfully locked third parties out of healthy competition for the profitable Massbus peripherals industry. This plan frustrated the vendors of obsolescent tape and disk drives. They maintained older VAX or PDP-10 systems. Their CPUs functioned fine, but they were tied to perishing, obsolescent tape and disk drives with increased power requirements and low capacity

MASSBUS cables

Another phenomenon closely related to the connector conspiracy, but with a slightly different objective, is the invention of new screw heads by some vendors. These screws could only be removed by those designated technicians who possess magic screwdrivers. Also, they only have the option to remove covers for repairing the product. Older Apple Macintosh computers took one step ahead, requiring a tailor-made, case-cracking instrument to open the box.In more recent years, this term might also be applied to cellphone chargers; many manufacturers have switched to a standard USB plug, but others - most notably Apple - have failed to follow suit."


/1/ PDP-11/40 System Manual
/5/ The Evolution of  the DECsystem 10 - G. Bell, S. H. Fuller, and D . Siewiorek, Editors

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