melnik@gnu.ai.mit.edu (Ofer Melnik) (10/13/90)
HI, A group of friends and I are in the process of creating an intelligent data acquisition system; We are currently in the specifications phase of the communications protocol and we seems to have run into an impass. This is a rundown of the system; It consists of one master (an MSDOS machine) and up to 256 slave terminals. The physical form of communications employed between the machines is analgous to radio-communications over a single frequency. Meaning there is one common half-duplex channel which everyone listens to. The flow of communications is basiclly polling; The master repeatedly polls each slave, either receiving data from it or reprogramming it. Broadcast communications are also possible (master initialization of all slaves). As we see it asynchronous communications is the only viable form of communications possible over this one-line for 2 reasons. The first being obviously that we have no way to send a clock over the line. The second is that synchronous communications without additional hardware is usually CPU expensive (constant processing of received information). If it werent for this asynchronus need we would have used simple SLDC. We would like to be able to implement the protocols using minimal hardware. Meaning the only addition to the PC would be a form of modem connected to the serial line. And on the terminas we would like to use the serial line already existant on a 8751 (8051 microcontroller + EPROM). And if the protocol is purchaseable all the better. What protocols would you recomend for our applications?? Any infrmation would be greatly appreciated. Thanks in advance. Ofer MElnik
rpw3@rigden.wpd.sgi.com (Rob Warnock) (10/19/90)
In article <11347@life.ai.mit.edu> melnik@gnu.ai.mit.edu (Ofer Melnik) writes: +--------------- | A group of friends and I are in the process of creating an intelligent | data acquisition system... It consists of one master (an MSDOS machine) | and up to 256 slave terminals... The flow of communications is basiclly | polling; The master repeatedly polls each slave,... | As we see it asynchronous communications is the only viable form... | The first being obviously that we have no way to send a clock over the line. | | What protocols would you recomend for our applications?? Any infrmation | would be greatly appreciated. +--------------- I would recommend using DDCMP (the DEC link-level protocol) in polled multi-drop mode, with multi-drop modems. This is what we did some 15+ years ago at Digital Communcations Associates to interconnect our stat muxes when we had to use multi-drop lines. (Some of our customers were replacing existing multi-drop IBM terminals with our gear -- we could change the terminals but couldn't change the line or the modems.) DDCMP is fairly easy to implement, and can be done in a small amount of memory. (We did it in an 8K PDP-8, *including* the async drivers and stat-mux functions for multiple terminals/node.) It explicitly supports multi-drop, including letting the master bury an <ACK> to one slave in a data packet to another (or something like that). If the systems are reasonably close, you can simulate multi-drop modems with a diode/resistor hack between the transmit line of each slave and the common line back to the master. (This is what we did in-house when testing.) -Rob ----- Rob Warnock, MS-9U/510 rpw3@sgi.com rpw3@pei.com Silicon Graphics, Inc. (415)335-1673 Protocol Engines, Inc. 2011 N. Shoreline Blvd. Mountain View, CA 94039-7311