MIKE@vax1.esprit.southampton.ac.uk (08/03/89)
High performance optical fibre data links are becoming widely available which satisfy the demands of the emerging FDDI standard. These devices typically have a minimum bandwidth of 125Baud, and come as complete modules with ECL logic interfaces. Such devices are available from manufacturers such as At&T, Plessey and Sumitomo. As well as providing far more performance than is necessary to transmit a 20Baud link, they are also rather costly, the prices that I was quoted in Nov 1988 ranged from pounds 230 to pounds 670 per transmitter/receiver pair depending upon manufacturer and spec. Note also that since these deveices are equiped with ECL logic interfaces, then TLL-to-ECL and ECL-to-TTL conversion will have to be performed between the transputer link interfaces and these devices. This, however, is achieved rather simply using standard parts such as the Motorola MC10H124/125 translators. An alternative may be to use the Hewlett Packard HFBR-0400 series of devices. The technical data, dated Feb 1986, describes and gives circuit diagrams for logic interfaces various performance ranges up to 50MBaud, using the HFBR 1402/1404 transmitters and HFBR 2404 receiver. Also of interest may be the HFBR-0422 50MBaud Transceiver Evaluation Board, which appears to offer a TTL interface to a 50MBaud transmitter/receiver pair. I do not know the prices of any of the HP devices described here. In their 1988/1989 HP also state that they will introduce an FDDI compatible transmitter/receiver pair in 1988/89. A couple of general points should be made here regarding optical budget and data integrity. As I understand it, the FDDI devices use a wavelength of 1300nm because glass cables can be made with a very low loss at this wavelength, and FDDI requires transmission distances of several Km. Note that the HP devices described above operate at 820nm, and are thus less suitable for long distance communications. Obviously careful attention should be paid to the optical budget available in a fibre optic data transmission system. Check the transmitter output, the receiver sensitivity, and all losses from cable length, connectors etc. Also ensure that the manufactuers' parameters are not being compromised by mistreatment of the cable, for example by bending with a small radius. Secondly, most receivers seem to be specified with a sensitivity at a specified bit error rate. The BER usually used is 10E-9. I interpret this as meaning that if the incoming optical signal is not less than the specified minimum power then an error rate of not more than 1 bit in 10E9 is to be expected. Note that 10E9 bits represents about 8 minutes worth of operation of a 20Mbaud link. Clearly with this sort of error rate some form of link error correction would be necessary for serious applications. Mike Moore, Dept Electronics and Computer Science, University of Southampton UK MIKE@UK.AC.SOTON.ESP.V1 ***Share and Enjoy***