bobkil@ibmpcug.UUCP (Bob Kilgore) (08/10/89)
Greg Wohletz asks; > I have heard conflicting reports as to the maximum length of > tranciever cable allowed. I had previously thought it was 50 > meters, but recently I heard that it was 20 meters. Which is > correct? The simple answer is both, and I am making an assumption that you are talking about Ethernet, (8802-3). Like so many things in the standard (8802-3) there is no statement that the AUI (tranciever) cable shall be of length N. There are two governing statements in section 7, 7.4.3.1 which states that the "dc" resistance shall not exceed 1.75 ohms. The other statement is 7.4.3.7 which states the signal (propagation) delay of the cable shall not exceed 257 nano seconds. Any cable that meets these requirements is legal in the context of 8802-3. It is generally accepted that 50 meters of cable will cause a delay of 257 nano seconds. If you choose to place some type of component in the signal path then the delay of that component must be subtracted from the cable length. The most frequently used device is the "multi-port transceiver". This device introduces a delay of some value into the AUI signal path. The magnitude of the delay is variable and usually associated with a vendor. I have found that in the design of a network that will use these devices a good rule of thumb is to subtract 15 meters of cable length for each device in the path. This implies that the person that told you 20 meters is maximum had two of these devices in a cascaded pair. This is allowed in the design rules for these devices. I might also point out that the "multi-port transceiver" is not specified in 8802-3 and not part of a "standard" network. > Also, Is there a MINIMUM length for tranciever cables? No. In fact many of the "ethernet" cards for a PC contain an on board MAU (transceiver). The signal lines from the MAC are usually fed through some type of jumper to the AUI input of the MAU. These connections can be viewed as the AUI cable and rarely exceed an inch or two in length. Good Luck and Best Regards; bob -- Automatic Disclaimer: The views expressed above are those of the author alone and may not represent the views of the IBM PC User Group.