How common are remote radio controllers in historical or modern systems?
I think the most interesting part of this article is the use of radio for a wide-area network, even if it only got a passing mention in the article.
Some questions I'd love to know: What kind of protocol was used? Are computer uses of walkie-talkie radio bands allowed by the FCC? How do the receivers work? If they don't run on their own microprocessor, how were they designed?
Data on radio channels is common! All you need is a "radio" and a "modem," and there are many variants of either.
These kinds of radio channels are already allocated to 'business' or 'government' service by the FCC. The user has licensed the use of one or more in a specific geographical area. These channels are 2.5kHz wide, but the old fashion was 5kHz bandwidth. They were allocated with FM audio in mind, which is what the portable radios do. You're normally allowed to run data on these channels.
The radio used at each node is probably a "mobile" radio intended for mounting in a vehicle but configured as a kind of base station by adding a DC power supply. Motorola has been selling microprocessor controlled, frequency-synthesized radios since the early 1980's, and the earliest one I know of used a 6800 processor (actually Hitachi 6300 series clone of M's cpu). The channel is probably shared with the maintenance chatters because the county already had the license, a radio fleet, and a shop to maintain the radios. By changing a "CTCSS" or "PL" tone to one that is different to the ones used by the voice radios, the maintenance chatters don't have to hear squawks and whistles all day and night.
The modem is probably an FSK kind of modem with a rate of something like 1200 bps. This works by representing 0 and 1 with different audio tones and is very simple with complete integrated silicon available during the 1980's. It's interfaced to the radio's audio input and output, the computer/modem needs a way to cause the radio to transmit, and it is nice to have a signal to show whether the radio channel is busy or idle. Otherwise it is probably a serial interface to the local controller at each site. The modem itself may have a microprocessor to do simple tasks like ensure the transmitter does not get stuck on by a fault or to prevent transmitting when the radio is receiving a signal already.
As far as the protocol goes, it can be anything and is probably something extremely simple: My address, length, payload, checksum.
This application should not be allowed in Amateur Radio service, because it's being used as part of a business or government operation.
The piece of spectrum this system actually uses, since it is shared with their maintenance radio fleet, is definitely not part of any amateur service allocation ("band")
The term 'packet radio' also implies (to me anyway) certain kinds of applications and messages, which don't apply to a system like this. Anyway, if there was some kind of IP or other routable protocol on the air here, that would be overdoing it. The messages probably look more like what you'd find on a multidrop serial bus, with elements including source and destination address, length, payload, checksum. A bunch of FSK modems working on the same radio channel is not much different to a multidrop serial bus.
This is actually surprisingly novel, and especially impressive given the developer was in high school at the time.
Running ethernet to these locations can be challenging, and often an SMS based approach or mesh network is used instead - which is really not that far off from this approach (with some obvious benefits).
Thanks! I knew they used something but nobody ever told me what the old standards were. I'm adding it to my link farm as some call it. Bet I find a use for it, too. :)
I think the most interesting part of this article is the use of radio for a wide-area network, even if it only got a passing mention in the article.
Some questions I'd love to know: What kind of protocol was used? Are computer uses of walkie-talkie radio bands allowed by the FCC? How do the receivers work? If they don't run on their own microprocessor, how were they designed?