Date: Saturday, 14 April 1984 From: Russ Smith To: net.micro Re: modems and heat problems A while back I posted a request for help concerning problems I was having using MDM7xx on a homebuilt z80 based machine with a Hayes 1200 Smartmodem and an h-19 terminal. The problem was that I could hook the Hayes directly to the terminal and talk both ways to another machine without any problem but if I hooked the Hayes and the h-19 to my machine I could only send stuff to the other machine, received stuff being totally garbaged. The problem seemed to go away after I just turned everything off and reseated boards...it didn't. Well, the recent net traffic about modem heating problems got my synapses firing... I took the front cover off the Hayes, placed a muffin fan blowing at it about 4" away, and all symptoms have "permanently" disappeared. Apparently my chicken-wire-and-spit homebuilt machine is close to the limit with the "receive data from modem" baudrate. Hence when the Hayes started drifting due to heat, the received characters started getting garbaged (framing error). This caused the modem-to-computer-to-terminal setup to go south. When I "reseated" the boards, I had to turn everything off, thus giving the Hayes time to cool down and causing the problem to go away (temporarily). This red herring behavior led me astray until I read the recent net stuff. When just hooked in as a modem-to-terminal setup everything worked okay, so I have to conclude that the h-19, using newer technology (software programmable UART), is more tolerant of out-of-spec received baudrate than my "old" hardware-programmable-UART-based Godbout Interfacer (sans II (inside "joke"...)) Now the problem is to find a way to sufficiently cool down the Hayes without the silly muffin fan... Keep them informative messages coming, they've helped more than once, Russ Smith Date: Tuesday, 10 April 1984 From: hplabs!sdcrdcf!darrelj To: net.micro Re: Hayes 1200 Problems First note that the bit pattern in 'U' is 01010101. What can happen is that since the modem actually sends of four states for PAIRS of bits, noise can occasionally get one of the modems out of proper sync with the data stream. Another thing which can happen is that the Bell 212 protocol includes a way to put the far end modem into a loop-back self- test mode (and occasionally noise will mimic the sequence). In fact this just happened to me while reading about other 1200 bps problems (symptom: first, about 20 Us, then remote end started loopback, echoing exactly what I typed, notable in that RETURN comes back without the LINEFEED that unix adds), and I fixed it on my old Prentice 212 by turning the Remote-Digital-Loop control on and off, taking the remote modem out of loopback mode. Or course most of the new cheap 212 compatibles save some of the money by taking out all the switches (the prentice has 5 on front panel, ~40 internal, plus 10 LEDs and output level programmable with an external (TELCO provided) resistor). The cheap modems will work in 95% of applications and on 95% of phone lines instead of all applications and 99% of lines. Generally, an acceptible tradeoff. -- Darrel J. Van Buer, PhD System Development Corp. 2500 Colorado Ave Santa Monica, CA 90406 (213)820-4111 x5449 ...{allegra,burdvax,cbosgd,hplabs,ihnp4,sdccsu3,trw-unix}!sdcrdcf!darrelj Date: Monday, 9 April 1984 20:07-MST From: decvax!ittvax!dcdwest!sdcsvax!sdchema!bam To: net.micro Re: Hayes 1200 Problems The Hayes Smartmodem 1200 had a firmware problem where it would hang in a locked state under certain conditions. This could usually be remedied only by power on reset. If this is what is actually going on, call Hayes and they will fix it for you free of charge. Bret Marquis -- Bang World Communication Center - San Diego.