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In the early seventies, companies like HP and Wang sold 'programmable desktop calculators' that were really personal computers in the time before what is usually thought of as the dawn of personal computers.

At the same time, there was a demand for small computers to control lab and factory equipment, and this demand would tend to occur in the same sort of places as would be buying desktop calculators. So it seems likely that people would be wanting to use programmable calculators for equipment control.

The de facto – and indeed de jure – standard interface for computers controlling random equipment was RS-232.

But looking over the specs of the early HP and Wang programmable calculators, I can't find any mention of them having RS-232 ports.

Did any of the early programmable calculators have RS-232 ports? (Or if not, why not?)

In the early seventies, companies like HP and Wang sold 'programmable desktop calculators' that were really personal computers

Not really, as they stood firmly on the calculator side. If at all, systems like the Cogar 4 and Datapoint 2200 are the origin of desktop computing. Complete units with a CPU, mass storage, CRT display and a full figured typewriter keyboard, all in one unit to be placed on a desk.

The de facto – and indeed de jure – standard interface for computers controlling random equipment was RS-232.

Not really. For one, RS-232 is still a somewhat complicated interface to build (that's the time before integrated circuits) (*1), but equally important, it lacked standardized protocols and application. If at all, HP's parallel HP-IB (HP-Interface Bus), available since the mid 1960s, ruled the area. It became soon known as GPIB (General Purpose Interface Bus), a term coined by companies building compatible devices but trying to avoid the HP name like hell. Later standardization in 1975, as IEEE-488, eased the burden.

Sustainable definition of the HP-IB enabled to not only ease the development of applications, but it also allowed to operate multiple devices on one interface (*2). An HP-IB enabled computer did only need to have a single interface to control (almost) as many devices as needed, thus enabling to handle a whole setup, not just a single instrument. With a predefined protocol device manufacturers needed only to add application specific data/protocol.

Serial in contrast is a point-to-point interface without any protocol support at all. It needs a separate interface for each and every device to be connected, making it quite expensive on the hardware side - not to mention, that there's usually only a finite and rather small number of serial interfaces that can be added to a given computer. On the protocol side applications had to do everything from arbitration and framing to message sequencing and command separation.

Did any of the early programmable calculators have RS-232 ports? (Or if not, why not?)

If they were intended to control measuring devices, a GPIB interface was the way to go. Serial was only good for data transmission. So for example many radio applications used it - and of course terminals. Both not necessarily applications for calculators.

In fact, HP (and many others) even offered serial controllers to be operated via GPIB. Making it easy to extend any device with a GPIB interface to handle many serial connections.

Serial only became a cheap and popular solution when integrated controllers became available (*3). Even then most of the downsides persisted. Still today it's confined to the topic of cheap low level point-to-point connections. The stuff hobbyists prefer as they can replace functionality by investing their time to add hardware hacks and software layers.

On professional measurement equipment GPIB is still today (2019) the most important interface, even though many affords are made to replace the hardware layer by an IP or USB based connection.

No company in the measurement business could afford to deliver devices without GPIB from the 1970s until today. Just do a search for PCIe based GPIB controllers and you'll be in for a surprise. That market is so big, that many manufacturers crank out new interface cards whenever there new desktop systems become available. Similar on the device side.

Of course, all of this happens more on the professional side, where devices easily carry 5 digit price labels, but not so much in the sub-1000 USD realm where hobbyists fight for pennies.

HP reused the GPIB idea even as the serial implementation HP-IL, for their pocket calculators, when they became powerful enough to act as controllers for other devices. Most notably the HP-41 series.

*1 - For a serial interface bit timing circuitry, sequencing logic and a shift register is needed for sending and the same again plus additional word synchronizing on the receiving side. All plus the optional parity logic. Quite a lot. A parallel interface in contrast is just a set of two latches (one can even be just a buffer) with clocked input and output enable. Quite less and much more simple hardware.

*2 - Features that enabled the success of USB half a century as well.

*3 - Serial interfaces of the 1960s were shoe box sized or larger.

The de facto – and indeed de jure – standard interface for computers controlling random equipment was RS-232.

Some would argue with that, at least in certain industries — Hewlett-Packard’s HPIB (GPIB, IEE-488) was (and is) also commonly used to connect control and/or measurement equipment. It is simpler to implement than RS-232.

So some early programmable calculators did have external connectivity options, but using HPIB, not RS-232; examples include the HP 9800 series, the HP 80 series, and later calculators using HP-IL and its HPIB adapter. HP-Collection has an extensive selection of photos of HP-IL peripherals.

RS-232 did appear in calculator-like devices, but they tended to be marketed more as handheld computers — HP’s 94 series for example, and of course Psion’s Organiser II with its CommsLink.

RS-232 became "standard peripheral interface" later, when most of these "peripheral devices" adopted microprocessors.
In early days, parallel ports were more common because they:

• can read-write data from logic elements like latches and d-triggers


• can control state of output triggers and even relays

Actually they were a window to real life for CPU's register - all 8 bits can be exposed to physical state. This allows you to create a lot of stuff (almost anything) without its own expensive processing units, using only your computer's one.

Serial port requires all data to be processed by CPUs on both side and triggers to work on higher speeds (you can send 8 bits in parallel port in one time switching 8 triggers, while you need to switch 1 trigger 11 times to send 8 bits via typical serial interface with start and stop bytes). Also you need to control trigger open-closed time very carefully because in parallel port you can simply register trigger state change events or have external sync, while in serial all your synchronization is time-based - depending on transfer speed your bit must be sent in specified time. Even now, in era of super-high-speed CPUs, serial ports have to be controlled by hardware or by microprocessors with "real time" (actually hardware interrupt is ok) abilities, because operating system's clock is not precise enough for this.

All this serial stuff allows you just 2 things:

• limit number of wires to 2 (most common now), 4 (easy duplex) or 2+2/4+2 (2 addition wires for external sync of transfer direction or bus master selection and ready-to-read state in some implementations)


• have loooong wires

RS-232 was not able to offer you long wires in basic implementation and speed, in early days, was something about that LPT was able to offer, so it was not very popular until many devices required thin wires (human input devices like mouse) and limited wires data transfer ability (modem devices, where physical interface was also serial, so data could be processed on PC for serial transmission, transferred to modem via 232 and then outside without any impact) and generic standard (at least in physical implementation).

By the way - serial ports are not required to be point-to-point as said in other answers - RS-485 for example is multipoint.

I don't know how 'early' this is, but in 1990, the HP48 series had built in RS232. In addition to the IR used by the immediately preceding HP28, there was also a four pin serial connector on the device. With the right kind of cable, this would provide up to 9600bps to another RS232 device.

At the time, HP also sold a companion PC connection kit that that bundled the cable along with some software for editing HP programs on a DOS PC, as well as transferring binary programs back and forth. It was rather expensive, so there was also a homebrew community that showed how to build the cables yourself. This is a page of the HP48 FAQ that includes directions on building cables HP->PC, HP->HP, and HP->MODEM:

https://www.hpcalc.org/hp48/docs/faq/48faq-12.html

I'm sure there are people that used this feature for various kinds of device control, but the bidirectional link to a PC also sat at the foundation of the early emulator and low level programming community. Tools for dumping the ROM worked by sending the contents over the serial port to an attached PC. The serial port also provided easier and more automatic tools for loading programs into the calculator that that normal user interface wouldn't let you enter. (This includes RPL programs using words that are not normally part of the UI as well as machine code.)