PLIP: The Parallel Line Internet Protocol Device Donald Becker (becker@super.org) I.D.A. Supercomputing Research Center, Bowie MD 20715 At some point T. Thorn will probably contribute text, Tommy Thorn (tthorn@daimi.aau.dk) PLIP Introduction ----------------- This document describes the parallel port packet pusher for Net/LGX. This device interface allows a point-to-point connection between two parallel ports to appear as a IP network interface. What is PLIP? ============= PLIP is Parallel Line IP, that is, the transportation of IP packages over a parallel port. In the case of a PC, the obvious choice is the printer port. PLIP is a non-standard, but [can use] uses the standard LapLink null-printer cable [can also work in turbo mode, with a PLIP cable]. [The protocol used to pack IP packages, is a simple one initiated by Crynwr.] Advantages of PLIP ================== It's cheap, it's available everywhere, and it's easy. The PLIP cable is all that's needed to connect two Linux boxes, and it can be built for very few bucks. Connecting two Linux boxes takes only a second's decision and a few minutes' work, no need to search for a [supported] netcard. This might even be especially important in the case of notebooks, where netcards are not easily available. Not requiring a netcard also means that apart from connecting the cables, everything else is software configuration [which in principle could be made very easy.] Disadvantages of PLIP ===================== Doesn't work over a modem, like SLIP and PPP. Limited range, 15 m. Can only be used to connect three (?) Linux boxes. Doesn't connect to an existing Ethernet. Isn't standard (not even de facto standard, like SLIP). Performance ========== PLIP easily outperforms Ethernet cards....(ups, I was dreaming, but it *is* getting late. EOB) PLIP hardware interconnection ----------------------------- PLIP uses several different data transfer methods. The first (and the only one implemented in the early version of the code) uses a standard printer "null" cable to transfers data four bits at a time using data bit outputs connected to status bit inputs. The second data transfer method relies on both machines having bi-directional parallel ports, rather than output-only ``printer'' ports. This allows byte-wide transfers and avoids reconstructing nibbles into bytes, leading to much faster transfers. Parallel Transfer Mode 0 Cable ============================== The cable for the first transfer mode is a standard printer "null" cable which transfers data four bits at a time using data bit outputs of the first port (machine T) connected to the status bit inputs of the second port (machine R). There are five status inputs, and they are used as four data inputs and a clock (data strobe) input, arranged so that the data input bits appear as contiguous bits with standard status register implementation. A cable that implements this protocol is available commercially as a "Null Printer" or "Turbo Laplink" cable. It can be constructed with two DB-25 male connectors symmetrically connected as follows: STROBE output 1* D0->ERROR 2 - 15 15 - 2 D1->SLCT 3 - 13 13 - 3 D2->PAPOUT 4 - 12 12 - 4 D3->ACK 5 - 10 10 - 5 D4->BUSY 6 - 11 11 - 6 D5,D6,D7 are 7*, 8*, 9* AUTOFD output 14* INIT output 16* SLCTIN 17 - 17 extra grounds are 18*,19*,20*,21*,22*,23*,24* GROUND 25 - 25 * Do not connect these pins on either end If the cable you are using has a metallic shield it should be connected to the metallic DB-25 shell at one end only. Parallel Transfer Mode 1 ======================== The second data transfer method relies on both machines having bi-directional parallel ports, rather than output-only ``printer'' ports. This allows byte-wide transfers, and avoids reconstructing nibbles into bytes. This cable should not be used on unidirectional ``printer'' (as opposed to ``parallel'') ports or when the machine isn't configured for PLIP, as it will result in output driver conflicts and the (unlikely) possibility of damage. The cable for this transfer mode should be constructed as follows: STROBE->BUSY 1 - 11 D0->D0 2 - 2 D1->D1 3 - 3 D2->D2 4 - 4 D3->D3 5 - 5 D4->D4 6 - 6 D5->D5 7 - 7 D6->D6 8 - 8 D7->D7 9 - 9 INIT -> ACK 16 - 10 AUTOFD->PAPOUT 14 - 12 SLCT->SLCTIN 13 - 17 GND->ERROR 18 - 15 extra grounds are 19*,20*,21*,22*,23*,24* GROUND 25 - 25 * Do not connect these pins on either end Once again, if the cable you are using has a metallic shield it should be connected to the metallic DB-25 shell at one end only. PLIP Mode 0 transfer protocol ============================= The PLIP driver is compatible with the "Crynwr" parallel port transfer standard in Mode 0. That standard specifies the following protocol: send header nibble '8' count-low octet count-high octet ... data octets checksum octet Each octet is sent as >4)&0x0F)> To start a transfer the transmitting machine outputs a nibble 0x08. The raises the ACK line, triggering an interrupt in the receiving machine. The receiving machine disables interrupts and raises its own ACK line. Restated: (OUT is bit 0-4, OUT.j is bit j from OUT. IN likewise) Send_Byte: OUT := low nibble, OUT.4 := 1 WAIT FOR IN.4 = 1 OUT := high nibble, OUT.4 := 0 WAIT FOR IN.4 = 0