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What is inside the Alto's ethernet transceiver? From the remark that interface between Alto and transceiver is TTL I gather that it is not same as 10M one (whi
by dfox 9y ago
What is inside the Alto's ethernet transceiver?
From the remark that interface between Alto and transceiver is TTL I gather that it is not same as 10M one (which transformer-coupled RS-422-ish differential interface) and also the collision signal detected by transceiver should probably be different.
- kens 9y agoThe transceiver basically has three parts. There's the logic side connected to the Alto, which XORs the sent data and received data to see if there's a collision. The cable-side circuitry has some analog stuff to put signals in and out of the coax. These two sides are connected via transformers for isolation. Finally there's a simple switching power supply to provide isolated 5.7V to the cable-side circuitry. (You want the Alto electrically isolated from the cable so you don't get ground loops and other nasty stuff from the cable.) The schematic for the transceiver is at: http://bitsavers.org/pdf/xerox/alto/schematics/209926B_Ethernet_Transceiver.pdf http://bitsavers.org/pdf/xerox/alto/schematics/209926B_Ether...
- CamperBob2 9y agoInteresting, I wonder what the funky magnetically-coupled circuit with Q1 and Q2 is for.
- kens 9y agoThat's the switching power supply for the circuitry on the coax side of isolation. The coils are coupled to E, F, G which feed the 5.7V power supply on the previous page.
- CamperBob2 9y agoThat's what I thought at first, but the pin numbers (5, 6, and 7) on the E/F/G windings at right overlap with the pin numbers for the primary, suggesting they're different parts altogether. Pretty confusing...
- kens 9y agoI think that's all "T3". Looking inside the transceiver, there are two toroids, each with 6 loose wires coming off. Since there are no pins, I think the pin numbers are fictional.
- dfox 9y agoThat is the aforementioned switching power supply. On the other hand what I see as more funky is IC1 and IC02, what exactly is that? Some kind of gated comparator? (Also of note is how IC3A and B has different, but equivalent, logic symbol to IC3C and D)
- kens 9y agoIC1 is a 75140 dual line receiver, where pin 6 is a reference and pin 2 is a strobe, so pretty much what you thought. Also of interest is IC101, a 75122 triple line receiver, which has a Schmitt trigger, two AND gates and a NOR for each line. Most of the pins are tied to ground (1, 2, 3, 4, 12) so it's used as a buffer, ignoring the rest of the logic. For IC3A-D, I think what's going on with the symbols is a briefly-popular way of representing positive logic and negative logic by changing the symbols so a wire has bubbles on both ends. In other words, if you have active-high inputs, represent it as a NAND gate, but if you have active low inputs, represent it as an OR gate with inverters on the inputs. It's the same gate, just drawn to indicate if it's performing an AND or OR. Note the way the bubbles move on the inverters too.
- bsder 9y agoWell, IC3A and IC3B are just functioning as inverters ... The thing with the bubbles is probably an old technique that we used to use called "pushing circles". You can shove all the circles on one side of a logic gate to the other side of the logic gate by exchanging the gate type (AND->OR, OR->AND). And then you can ignore lines with two bubbles in series (ie. a double inversion). There are a bunch of tricks that we used to use when you had to do everything on graph paper. It's actually a good thing that they are lost to the mists of time as it means we have progressed enough that they don't matter anymore.
- thrownhwn 9y agoCool, had to Google bubble pushing[1] [1] http://district.bluegrass.kctcs.edu/kevin.dunn/files/Simplification/mobile_pages/4_Simplification10.html http://district.bluegrass.kctcs.edu/kevin.dunn/files/Simplif...