henry@utzoo.UUCP (Henry Spencer) (06/15/83)
A little while ago we finally figured out the cause of a serious problem we'd been having with our Racal-Vadic 3451PA autodialing modem. The symptom was that we simply couldn't talk to our neighbor utcsstat at 1200 baud in any useful way, because noise bursts hit the line every few seconds. The symptoms were sort of odd, because the bursts were usually the same bit pattern, and they only came in one direction. They only affected communications with utcsstat, the only local 212-only site, which made us suspect the 212 circuitry... except that said circuitry worked perfectly when talking to decvax, which is 212-only and a Long Distance call to boot! Some other bits of evidence indicated that utcsstat's DH, cabling, and modems were OK. This really puzzled us, until we noticed an interesting timing coincidence. About 8 months ago, we were having great trouble talking to decvax. This turned out to be a timing problem, but one of the things we tried before we realized this was to change a couple of strapping options on the modem. There are two strapping options which enable an amplitude equalizer and a phase equalizer, indicated as appropriate for dealing with noisy phone lines. There is no other comment on these equalizers in the manual. They are off with the options set as they are when the modem is delivered; we tried turning them on. No effect on the decvax problem, since it was elsewhere. We pretty much forgot about them. Recently it occurred to us that the problems in talking to the local system at 1200 baud had started at roughly the same time. Naw, couldn't be... We went into the modem again and turned the equalizers off... and suddenly communications to utcsstat were fine again! The moral of the story is, beware the phase and amplitude equalizers in the RV3451PA (and lookalikes like the A-J 1259). Sometimes they make things worse (in very strange ways) rather than better. Henry Spencer U of Toronto