Interesting stuff. I ran a BBS in the early 1990s - but I never really dug into the technicals of how a modem worked.
So, I'll describe a Parallel Modem like this: how the data from your PC is sent/received to/from the modem is independent of how that same data is communicated over the (then) copper phone line. As Will mentioned, if you didn't have a local UART, then you could instead put that hardware on the modem side and let it handle the byte-to-bit translation. But ever since the IBM PC (and really even a few years earlier, as I'm reading about the S-100 serial card of the POLY-88 c. 1976, or the 0.916Hz clocked UART used on late model PDP's), nearly every system had a serial card with a "local UART" (or similar, like the VIA/PIA shift-register some other economy systems used; these shifters can let you kind of "emulate a UART in software" by at least doing the byte-to-bit conversion for you, you just have to bit-twiddle in the start/stop bits yourself). So why add that cost into a modem (of an additional UART that an end-users probably already has), and also: what would you connect it to? If you connect it to a serial port, then you've defeated the whole purpose. Well, a UART itself is basically a parallel device (from the bus lines to the processor - or memory?). So if you built such a thing, it would make sense to connect it to the host systems parallel port. Hence, a parallel-modem is just one that send/receives its byte across a parallel-port connection. There is a cross-over point where the performance of the local system (a combination of its raw CPU speed, memory, and getting data from a hard drive into said memory) isn't suitable - maybe less so on the TX/send side, but moreso on the RX/receive side. The article mentioned is talking 486-era systems. In the early days, a 1MHz CPU system would be "bogged down" on something as "simple" as scrolling the screen. By the time of 486-days, a system would be bogged down by normal multi-tasking (not Win95 style, but maybe OS/2?). But as mentioned, a UART is a mostly "fire and forget" thing - write your byte, it gets packed into start/data/parity/stop sequence (the data probably reversed). But on the receive side, your system has to do something with the data, and do it before the UART FIFO becomes full (or better, before it becomes halfway full). The modem gets that byte, strips off the "extra" bits, reverses the data back, and modulates it on out (at a rate that it-and-whatever-modem-it-has-connected-to has negotiated as an acceptable speed; your serial port might be set to 19200, but that BBS on the other end might be using a 2400 baud modem; hence all that modem-noise during an initial connection as they fight it out, and the BBS-host software might spew a few garbage bytes as it tries to match on its serial connection and asks "can you comprehend me now?" type questions; in the end, your Carrier Detect lights goes on). But, you're right - I'm still not yet rationalizing out how this parallel-connected-modem would help (even if a corporate or classroom scenario, with a room of 50 systems). I do know that "laplink" style parallel cables were said to be faster than null-modem cables in raw data transfer between systems. BTW, every "DB25" connector in the PC world I've seen, it's got 8-pins for data. Even the UserPort in the Commodore world has that - I've not commonly seen 4-data-pin parallel cables? So yes, you could "talk to a modem faster" using a parallel exchange (is it faster because of the 8-data-pins or faster because of not having to deal with the start/parity/stop bits? or both). -Steve On Sun, Feb 9, 2025 at 1:35 PM Paul Koning <[email protected]> wrote: > > > > On Feb 9, 2025, at 1:08 PM, Steve Lewis via cctalk < > [email protected]> wrote: > > > > I was about to ask if anyone ever built a "Parallel Modem" - but I > searched > > around first, and lo and behold, Microcom did ! (v.fast / v.34 era, c. > > 1996) > > I don't know what "parallel modem" would mean. Can you explain? > > > ...Related but different question: > > > > Is there any "natural rate" (Hz) of a modem? Meaning is 1200/2400 > > baud-equivalent modem an accelerated-by-enhanced-encoding version of 300 > > bps? and 9600 likewise an accelerated-by-encoding version of 2400? is > > 300bps itself some kind of special accelerated-by-encoding? I see 1200 > > baud was also still sub 3KHz (did any modem protocol go above 3KHz?). > > > > Or maybe I need to ask it this way: did 300 baud modems use a more 1:1 > > translation of the data-word bits into Hz signal over the modem (giving a > > more "natural" translation rate?) But then beyond that speed, does a > modem > > need to "cache" a few bytes and determine some encoding scheme to then > give > > modems an apparent speed boost? (is that "kind-of" like USB's 8B/10B? > > (not in implementation, but in the general concept that a different > > encoding can result in improved data throughput, without actual faster > > movement of that data?) > > For the most part the answer is "no". > > The job of a modem is to carry a digital signal over a wire, at a given > speed and given level of data integrity, and with a given channel bandwidth. > > When the channel bandwidth in Hz is well above the bit rate in bps, the > job is easy, an FSK modem can do the job. That's what the first modems > looked like (and perhaps even earlier devices used to deal with radio > transmission for Baudot teleprinters, commonly referred to as "tuning > units"). > > When data rates go up and bandwidth doesn't, you need more complex > modulation schemes. Modulating a carrier produces sidebands, so roughly > speaking your baud rate can't exceed half the channel bandwidth. (I'm sure > I'm handwaving a lot here.) You can't do 9600 bps FSK in a voice channel, > it won't fit. It would fit just fine if you have a 40 kHz channel, so it's > certainly possible to do FSK over VHF radio at that speed if you're > authorized that much bandwidth. > > So for high speed on a telephone line the exercise becomes "more bits per > baud" -- not one bit per signal element as you get from FSK or PSK, but two > (QPSK) or 4 (QAM16) or even 8 (QAM256). Note that those hairy modulation > schemes require a pretty clean channel; you're not likely to find them on > shortwave radio systems for that reason. Indeed, doing data transmission > over radio is an entirely separate art with a host of interesting and > exotic methods. Some of them can reliably send data using transmission > weak enough you can't hear them if you listen to the signal with an > ordinary audio receiver. > > paul > >
