In our previous article we discussed the hardware details of Allen-Bradley’s first programmable controller, the PDQ-II. Now in today’s article we’ll take a detailed look at the program side, including programming options and procedures, as well as what mnemonics (aka instructions) were available for use in PDQ-II programs.

PDQ-II programs were stored in the system’s on-board memory, which was expandable from 1K (1024 8-Bit words) to 4K (4096 8-Bit words) by means of up to four 1K Memory Modules installed in the system’s Memory Chassis.

The program’s themselves supported standard Boolean Functions as well as Counting, Timing, Shift Registers and Retentive memory. And the PDQ-II was fast for it’s time with an average scan time of 3 to 5 ms per 1024 words.

Now before we jump into how to program the PDQ-II, it’s important to note that programming back in the late 60’s and early 70’s was accomplished without the use of a Personal Computer, as they would not come to market until the mid to late 70’s, and the IBM PC (grandfather of all Microsoft/Intel based PCs) wouldn’t come out until the summer of 1981.

So to program a device like the PDQ-II back in the late 60s and erarly 70s, you typically have two choices: You could either use a specifically designed Programming Device (aka a Programmer,) or use software written for a Mainframe Computer.

Option 1: The PDQ-II’s Manual Programming Panel (MPP)

The first step to programming a PDQ-II with its MPP would be to convert your machine control logic (often in the form of Relay Ladder Diagrams) into the PDQ-II Mnemonic equivalents using its Basic Instruction Set of 12 mnemonics list below:

• EØN – Energize Output Non-holding
• DØN – De-energize Output Non-holding
• XIC – eXamine Input Closed (also works as XTC)
• XIØ – eXamine Input Open (also works as XTØ)
• XØE – eXamine Output Energized (also works as XTE)
• XØD – eXamine Output De-energized (also works as XTD)
• BBX – Begin Branch eXamine
• BLC – Branch List Complete
• HLT – Halt (Stop Memory Cycle)
• NØP – No OPeration
• TMR – TiMer Start
• JMA- JuMp Absolute

Note: Some of the mnemonics listed above will be immediately recognizable to long time users of A-B programmable controllers, including the XIC (eXamine If Closed,) XIØ (eXamine If Open,) and the NØP (No OPeration.)

Once you’ve created your program on paper, you’d then use the MPP to load each instruction into the PDQ-II’s memory one at a time by (1) pressing the Instruction Push Button, (2) Setting the IO Address, and (3) pressing the Load Memory Push Button.

In addition to loading your program into the PDQ-II’s memory, you could also use the MPP to Verify Memory, Modify Memory, and Monitor the Status of Inputs and Outputs. You could even create a Paper Tape of the current program by way of the optional Tape Reader/Punch.

Option 2: Computer Aided Programming

When it came to programming the PDQ-II from a “Computer,” it was nothing like in the 80’s and early 90’s when IBM Compatible Personal Computers running DOS were used with PLC Programming Software like ICOM’s A.I. Series, or Allen-Bradley’s 6200 and APS Software.

But if you did have a Computer to aid you in programming your PDQ-II, you would unlock the use of PDQ-II’s Extended Instruction Set (in additional to the Basic Instructions) listed below:

• EØH – Energize Output Holding
• DØH – De-energize Output Holding
• SSA – Sequence Step Activate
• SSD – Sequence Step De-activate
• JMM – JuMp and Mark
• TIS – Transfer Input Status
• TØS – Transfer Output Status
• TRN – TRansfer Number
• XTC – eXamine Timer Closed (use XIC on MPP)
• XTØ – eXamine Timer Open (use XIØ on MPP)
• XTE – eXamine Timer Energized (use XØE on MPP)
• XTD – eXamine Timer De-energized (use XØD on MPP)

To start programming the PDQ-II using a computer, just like with the Manual Programming Panel you first needed to convert your existing machine control code (typically documented in Relay Ladder Diagrams) into the PDQ-II Mnemonic equivalents:

Once this is done, you’d then need to type your program in on a standard 8-channel Teletype-Writer. Once completed, you’d then dial your company Computer with the PDQ-II Translation Software loaded, and feed your PDQ-II program into it.

The Computer would then output a Punched Tape of your program in the PDQ-II’s Machine Language.

To transfer the Punched Tape program into the Memory of your PDQ-II, you’d typically remove the Memory Chassis from your PDQ-II and insert it into the (optional) Memory Loader device.

Then you’d run through the procedure to transfer the Computer generated Punched Tape into the Memory of your PDQ-II, after which you reinstall the Memory Chassis into your PDQ-II Logic Package chassis.

While the above procedure may seem tedious, on the bright side A-B actually made the PDQ-II Programming Software available free to customers.

Since the email standard (SMTP) wouldn’t premiere on ARPANET until 1983, and the Internet wouldn’t go public until 1991, customers would actually request a copy of the PDQ-II Software by writing and mailing a physical letter to Allen-Bradley (this also allowed users to avoid the exorbitant cost of long distance phone calls of the time.)

In return, Allen-Bradley would mail the user back the program they requested on either Punch Cards or Perforated Tape used to load the translation program into the company’s Computer.

Editors Note: Looking back, I’m very thankful Floppy Disks were a thing by the time I started using PLCs in 1990!

During my research into the software I also found mention of multiple software packages, including one that would allow a Computer to either Monitor the status of the PDQ-II’s inputs and outputs, or actually take over complete control of the system (Skynet anyone? – Ed.)

Unfortunately I wasn’t unable to find any further information as to how this was done and what connections were used. But if you have any PDQ-II information you’d like to share, or experience using it, I’d love to hear them! Just use the comment area below this article to post them or the contact link on the top of the page to get in touch.

Until next time, Peace ✌️ 

If you enjoy this episode please give it a Like, and consider Sharing as this is the best way for us to find new guests to come on the show.

Shawn M Tierney
Technology Enthusiast & Content Creator

Eliminate commercials and gain access to my weekly full length hands-on, news, and Q&A sessions by becoming a member at The Automation Blog or on YouTube. You'll also find all of my affordable PLC, HMI, and SCADA courses at TheAutomationSchool.com.

• Author
• Recent Posts

Shawn Tierney

I'm a lifelong Technology Enthusiast who began programing in the early 80's on the Apple II and VIC-20 personal computers. After earning a degree in Electronics, I worked as a Certified Controls and Information Automation Specialist for 25 years before leaving to become a full time Industrial Automation instructor and mentor at TheAutomationSchool, as well as publishing industry interviews, news, and product how-to’s at TheAutomationBlog.com. If you're interested in my training, mentoring, or becoming a sponsor, please schedule a teams meeting with me via https://calendly.com/shawntierney/business-meeting

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It was the summer of 1969, and Information Instruments, Inc (3I) became the second company (after DEC) to supply a “Standard Machine Controller” to the GM’s Hydra-Matic division, several weeks ahead of Bedford Associates and their legendary Modicon 084.

While GM Hydra-Matic Computer Group was extremely happy with how the PDQ-II preformed on the Forward Clutch Line, earning it the nickname “Pretty Dang Quick,” overall GM would go on to favor the 084 over the PDQ-II.

Taking part in GM’s “Standard Machine Controller” product evaluation is said to have convinced the president of 3I that there was great potential for these products, and for his company to be competitive in this emerging market they were going to need additional funding.

This is said to be the driving factor behind 3I signing a deal with Allen-Bradley in which 3I sold A-B a 25% share in the company, along with the option to acquire the rest of the company at a later date.  And as part of that deal, Allen-Bradley also gained the exclusive rights to sell and service the PDQ-II product line.

The Bulletin 1760 Programmable Controller from Allen-Bradley

The PDQ-II (along with PDQ-II Compact) are featured in the above image, which I scanned in from the front cover of A-B Publication SD27 from 1973 (superseded the 1970 version.)

This brochure touts the PDQ-II as a solid state programmable controller designed to be rugged and reliable enough to be installed in industrial applications. It also touts that it’s a device which can be installed, programmed, and maintained by existing plant personal.

As a solid state equivalent of relay control panels, the main advantage of the PDQ-II (and literally all programmable controllers) was that changes can be made via its program, instead of the laborious process of editing volumes of wiring schematics followed by the time consuming process of actually rewriting the relay control panels to match the updated drawings.

And while the PDQ-II is a “first generation” PLC, the product’s block diagram (from the same brochure mentioned above) clearly shows the very familiar representation of the components that would go on to become standard with most all programmable controllers:

Editor’s Note: One term in the above diagram that did not become widely used is  “Control Logic.” In most PLCs that component is labeled as the “CPU” or “Processor.” While it may just be a coincidence, I found it interesting that thirty years later A-B would release a new generation of programmable controllers called the “ControlLogix,” which could in part in homage to their original PLC?

Each PDQ-II System consisted of eight types of components. As shown in the above image, these included the Logic Package (aka PLC Chassis or Rack) into which Logic Cards and a Memory Chassis would be installed.

The Logic Package itself included a power supply, cooling fans, ports to connect to multiple Input Output Packages and Programming Devices, two slots for Logic Cards, and a large slot for the Memory Chassis.

The two Logic Cards (aka Control Logic Modules) supported up to four internal timers, ranging from 0.1 ms to 70 hour time periods. And the Memory Chassis had slots for up to four 1K 8-Bit Random Access Core Memory Modules.

The Input/Output Package, also known as the I/O Chassis, is where the Input and Output Modules were installed, with each modules having its own Plug-in Blocks (aka Removable Terminal Blocks.)

The chassis included a total of eight slots. The first four slots were narrower than the second four, and were used for the input modules. The second four wider slots could accept any combination of Output, Indicating Control Storage, or Analog Timer modules.

As far as I/O Module types are concerned, the PDQ-II system supported 120vac and 24vdc Inputs, as well as 120vac, 24vdc, and Relay Outputs. Each module type was capable up supporting up to 16 I/O Points, and each I/O Point had its own Indicator Lamp.

Output Modules were also individually fused and included Fuse Blown Indicator Lamps. The 120vac TRIAC and 24vdc Open Collector Output Modules supported up to 2 amps, and the N.O. Relay Output Module supported 35va (1 amp fused.)

The PDQ-II also supported up to a total of Four I/O Chassis, with a maximum of 250 Inputs and 250 Outputs. Editor’s Note: I’m not sure why the maximum wasn’t 256 In and 256 Out, as that would have seemed to make more sense?

As far as power, the PDQ-II supported either 120vac or 240vac at 50 or 60hz, and consumed 500 watts itself (in addition to any I/O load requirements.)

The Logic Package with one I/O Package measured 46” high, 32” wide, and 9.5” inches deep, and they had a combined weighed 197lbs, and supported a maximum ambient operating temperature of 60 C.

Optional Accessories

There were also a number of optional accessories available for the PDQ-II. The Manual Programming Panel (MPP) was primarily used to program the controller, and included a Main Programming Panel, a Connector Cable, and a Card to interface with the Control Logic and Memory.

In addition to programming the PDQ-II, the MPP could also be used to Verify Memory, Modify Memory, Monitor the Status of Inputs and Outputs, and create a Paper Tape of the current program by way of the optional Tape Reader/Punch.

The Memory Loader Accessory was also available to load a program from Tape into the PDQ-II’s Memory, as well as manually Write, Edit, or Verify the memory of the PDQ-II. It could also be used with a Computer to write a program to Paper Tape.

While the PDQ-II was obviously a very capable product for its time, as we look at the programming options in our next article I think it will become clear why Allen-Bradley ultimately took another path.

That said, if you’d like to leave some feedback or share your experience with the PDQ-II, or if you’d like to donate a working unit for us to use in future stories, please use the comment area at the bottom of this page to let us know what’s on your mind.

Until next time, Peace ✌️ 

If you enjoy this episode please give it a Like, and consider Sharing as this is the best way for us to find new guests to come on the show.

Shawn M Tierney
Technology Enthusiast & Content Creator

Eliminate commercials and gain access to my weekly full length hands-on, news, and Q&A sessions by becoming a member at The Automation Blog or on YouTube. You'll also find all of my affordable PLC, HMI, and SCADA courses at TheAutomationSchool.com.

• Author
• Recent Posts

Shawn Tierney

I'm a lifelong Technology Enthusiast who began programing in the early 80's on the Apple II and VIC-20 personal computers. After earning a degree in Electronics, I worked as a Certified Controls and Information Automation Specialist for 25 years before leaving to become a full time Industrial Automation instructor and mentor at TheAutomationSchool, as well as publishing industry interviews, news, and product how-to’s at TheAutomationBlog.com. If you're interested in my training, mentoring, or becoming a sponsor, please schedule a teams meeting with me via https://calendly.com/shawntierney/business-meeting

Latest posts by Shawn Tierney (see all)

• First Look: Siemens S7-1200 G2 – Unbox, Wire, Program (S2E13) - April 17, 2025
• MaintainX: Prevent Unplanned Downtime (P236) - April 9, 2025
• First Time Using Ignition: Tags and Views (P235) - March 24, 2025