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PhoneNet, CapNet, ground, and static electricity
Posted by: Tashtari on 2026-09-12 07:26:59 So I was looking over the CapNet 'spec' and I noticed something curious: CapNet includes a pair of 1 kΩ resistors between the TxD± lines and ground. The document says that this is to prevent buildup of static electricity on the bus, but as far as I'm aware, PhoneNet has no such provision. I split apart a PhoneNet box a while back (picture below) and it does have a connection to the miniDIN's ground pin, but it is connected to a trace on the bottom of the board that doesn't appear to go anywhere except around the phone jack side circuitry. It seems like PhoneNet ought to have some kind of provision for static electricity along the lines of what 10base-2 Ethernet does (having a connection to ground on one end of the bus only), but this doesn't seem to be that. Anyone more electrical than I have any idea what's going on here?
Posted by: cheesestraws on 2026-09-14 03:14:43 No useful knowledge here, but the LocalTalk spec in IA has (2nd Ed. p. A-4) "The resistor (R1) provides static drain for the cable shield to ground."
Posted by: Tashtari on 2026-09-14 05:45:44 Good find! Actually, I find the resistor/capacitor pair to the left of what you circled more interesting... phone cables are unshielded so there's nothing to connect a 1 MΩ resistor to, but the 1 kΩ resistor connected to pin 2 of the miniDIN-3 connectors is similar to what the CapNet spec provides. Why the 0.1 µF capacitor, too, though, I wonder?
Posted by: obsolete on 2026-09-14 06:58:48 Above a certain frequency, the impedance through C1 to ground will actually be quite low (much less than 1k ohms), so this would effectively shunt high-frequency noise to ground. I'm guessing that was the goal here.
Posted by: pl212 on 2026-09-14 09:27:47 So I was looking over the CapNet 'spec' Man, what an amazing burst of late-80s/early-90s nerd enthusiasm. It's interesting how many projects sprung up around LocalTalk, including BMUG's role in PhoneNet and the resulting Farallon corporation. CapNet kinda takes that DIY ethos to an extreme.
Posted by: hwertz on 2026-09-14 10:51:02 I'll just say, just because phonenet doesn't have a resistor there to avoid static buildup, doesn't mean it shouldn't have. They ran some phonenet when I was in high school (mid 1990s) and didn't have any problems AFAIK (yes, this is pretty late for them to do this -- they ran "some" ethernet, kind of ran out of budget so they had an ethernet backbone with phonenet in each area of the building, and localtalk to ethernet bridges bridging it to the backbone.) I'm not aware of any big problems, but if they burned out equipment due to static I wouldn't be aware of it either since I just used it, I wasn't involved in installing it. After all, some of those Macs had several RS422 ports so if one toasted they could just use the next one AFAIK. I wouldn't be surprised if some modern hardware, you know, they were running 12V chips back then, I wouldn't be surprised if some modern hardware wasn't "12V tolerant" and won't put up with voltage spikes that the hardware back then was fine with. I must say, Localtalk was a pretty smart move -- not very fast, but a lot of stuff back then just didn't need that much speed, and they sure got a lot of use out of it.
Posted by: olePigeon on 2026-09-14 15:10:48 I must say, Localtalk was a pretty smart move -- not very fast, but a lot of stuff back then just didn't need that much speed, and they sure got a lot of use out of it. I was told by a friend of my dad's that LocalTalk (and then AppleTalk) was the primary reason why Macs were insanely popular with law offices back in the day. Back in the mid 80s through the early 90s, pretty much every lawyer had Macs in their office. It was the cheapest and most effective way for networking and sharing files. They were some of the first professionals to really embrace digitizations, and it t allowed them to move away from physical rooms filled large metal filing cabinets and replace them with just a handful of computers and some phone cables. It wasn't until ethernet became affordable and ubiquitous that law offices moved to Windows/DOS PCs. I don't know if anyone here completed their BAR exam in the early 2000s, but I seem to recall that by then it required a Windows PC if you were going to do it online. So you couldn't even use a Mac to take the BAR. Times changed.
Posted by: Tashtari on 2026-09-15 06:21:06 Above a certain frequency, the impedance through C1 to ground will actually be quite low (much less than 1k ohms), so this would effectively shunt high-frequency noise to ground. I'm guessing that was the goal here. Hm, could be. just because phonenet doesn't have a resistor there to avoid static buildup, doesn't mean it shouldn't have True, though the fact that there is a ground connection makes me wonder what it is they expected it to do. Maybe static was expected to jump the gap to the ring trace if it got bad enough? I must say, Localtalk was a pretty smart move -- not very fast, but a lot of stuff back then just didn't need that much speed, and they sure got a lot of use out of it. It's funny to think about how expensive networking was in those days. Even though LocalTalk was cheaper by comparison, it feels like it wasn't properly cheap. For only containing a handful of passives, if memory serves, each LT dongle cost something like $70, and that was in late-80s dollars. Small wonder PhoneNet was so popular when it came out. In other news, I made a little PCB that acts as a PhoneNet terminator and has a ground connection, to go with my homemade PhoneNet dongle PCBs. Not sure if that'll be useful to anyone, but there it is. I've had a handful of people inquire about homemade PhoneNet dongles lately, apparently they're getting expensive on eButt. I only have a handful of the transformers left, I may need to go hunting for a still-manufactured replacement...
Posted by: Tashtari on 2026-09-15 06:22:52 each LT dongle cost something like $70 Wikipedia says "In 1989, these boxes typically cost US$90 (equivalent to $230 in 2025)". Yeowch. And that's not even counting the cost of the proprietary shielded miniDIN-3 cable used to connect them together...
Posted by: cheesestraws on 2026-09-15 07:06:26 Wikipedia says "In 1989, these boxes typically cost US$90 (equivalent to $230 in 2025)". Yeowch. And that's not even counting the cost of the proprietary shielded miniDIN-3 cable used to connect them together... good grief. how much was phonenet?
Posted by: hwertz on 2026-09-15 17:19:50 Per Google, about $50 initially... I think they were closer to $25 after they were out for a while. I'll just comment, the $90, this was Apple being Apple, I mean in present day they sell $120 and $250 pairs of earbuds; they were well known back then for, lets say, having very healthy margin on their products. Farallon did as well, just not to the same extent. The bigger deal really was being able to use phone line rather than the localtalk cabling. (Localtalk cabling was fairly expensive because it almost certainly was more expensive to produce than phone line; healthy markups; and there were tons of vendors making phone line in truly mass quantities so that drove prices to be quite low as well.)
Posted by: pl212 on 2026-09-16 14:39:35 Here is the original article from the Fall 1985 BMUG Newsletter on what was then called BMUGNet. Written by Reese Jones, who would go on to found Farallon. The diagrams were created in MacDraw and contain a lot of details about specific electronic components. For what it's worth, the BMUGNet kits were sold for US$15, or about US$46 today.
Posted by: ymk on 2026-09-16 16:14:06 Why the 0.1 µF capacitor, too, though, I wonder? The R in the RC network prevents any static buildup. The value is high enough to prevent any significant current flow when there's a 50/60Hz ground loop between the two ends of the cable. The C presents a very low resistance path to ground for a static discharge from a person handling the cable.
Posted by: Tashtari on 2026-09-17 06:52:49 Here is the original article from the Fall 1985 BMUG Newsletter on what was then called BMUGNet. Oh, interesting! Looks like the idea of using the second pair of wires on the phone cable came later and they're using a third wire to... balance the grounds of all connected systems, I guess? Unlike LocalTalk, the RC isn't connected to the bus lines. That's curious. I guess Farallon decided this wasn't necessary. I'm actually a bit disappointed in the doc, I was hoping they'd reveal some details of the isolation transformer they used, but they just refer to it as "custom transformer". Maybe they invested some resources in getting transformers made that follow the spec in Inside AppleTalk, though my use of the token ring transformers has shown that this doesn't seem to be strictly necessary. I'm not electrical enough to know exactly which transformer parameters can be bent and how much, but IA only specifies a minimum inductance (20 mH)... and the token ring transformers violate that pretty egregiously, having an inductance of 500 µH. The C presents a very low resistance path to ground for a static discharge from a person handling the cable. Aha, neat. I'll add that to my terminator PCB, just for belt and suspenders.
Posted by: Tashtari on 2026-09-17 07:42:26 The C presents a very low resistance path to ground for a static discharge from a person handling the cable. One thought... is it important that the cap be polarized?
Posted by: ymk on 2026-09-17 10:31:15 One thought... is it important that the cap be polarized? It's not clear what the DC offset might be, but a 25V non-polar ceramic should do it.
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