Modem circuit diagrams? - Page 1
Forum section: electronics / projects

author: metertech58761
author_rank: Frequent Contributor
timestamp: August 18, 2025, 12:35:27 pm
content: How would a modem have been implemented in the days before ASICs became available for this purpose?I have this picture of an old 300-baud modem card (I had the opportunity to have this exact card sent in - I declined because it was so far obsolete... I still kick myself for not having this relic sent in anyway as a museum piece).The connections were very straightforward - power (+/-12, +5, ground); 4 input lines for leased line or PSTN, and RS232 signals (TX/RX, RTS/CTS, DCD, DSR).

author: LaserSteve
author_rank: Super Contributor
timestamp: August 18, 2025, 06:56:11 pm
content: Modern Life began with the TI TCM3105 , I still have a tube of them someplace.Motorola had the ML145442 family.Prior to that Exar had FSK building block chips, Ie XR2202, XR2207, XR2211 etc.https://www.bitsavers.org/components/exar/_dataBooks/Exar_Applications_Data_Book_198106.pdfAt 150 and 300 baud, OPAMP filters were possibly in play...The slowest thing I ever had was the Bell 103...See:https://neuro.musashino-u.ac.jp/publications/pdf/modem.pdfhttps://www.wa4dsy.com/heatherington/hayes/downloads.htmlhttps://www.auvik.com/franklyit/blog/history-of-the-modem/Thanks for dredging up memories of being 15 thru 18, and the secret DPDT switch in the basement so Ohio Bell would not "see" the extra line loading, in order to make my Dad happy. He expected to be billed some day for a data rate line instead of home voice rate because of my Modem.Steve

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 18, 2025, 07:43:40 pm
content: Best I know, modems up to 56k6 baud have (also) been implemented by generic DSP's. And at around that same time, the whole computational thing started to be offloaded to the main CPU (80486 at that time), and the "modem" got reduced to a simple DAC with a few extra things to handle high voltage ringtones and galvanic isolation.

author: metertech58761
author_rank: Frequent Contributor
timestamp: August 20, 2025, 12:08:32 am
content: I should add, the RS232 lines fed into a MC6850 ACIA on the logic board...Also, it looked like a few of the chips had UA prefixes. I can't recall what IC manufacturer that was.

author: oPossum
author_rank: Super Contributor
timestamp: August 20, 2025, 12:18:50 am
content: UA is Fairchild

author: amyk
author_rank: Super Contributor
timestamp: August 20, 2025, 04:40:00 am
content: http://madrona.ca/e/LDSA/LDSModelASchematic.pdfDiscretes only, no ICs. How it works is left as an exercise for the reader.

author: bson
author_rank: Unknown
timestamp: August 20, 2025, 09:20:15 pm
content: Quote from: amyk on August 20, 2025, 04:40:00 amThis is the correct answer - modems started as handset coupled.A curious observation: the modulator works between ground and negative rail, and depends on the input having a negative swing, so can't accept e.g. 5V TTL. The demodulator sits between both rails.

author: metertech58761
author_rank: Frequent Contributor
timestamp: August 21, 2025, 03:57:51 am
content: A representative picture of what the modem would have gone into:Left side of the upper rack: Power supply, modem, two long-empty bays (likely for memory cards), the logic, and the receiver. Right side of upper rack: transmitter and backup. Lower rack is connections and coupling (empty bays are slots for additional circuits - most units had only one bay populated). Internal / External switch on power module refers to whether the RS-232 lines are tied to the modem (internal) or to the diagnostic port on the bottom of the unit (external). Modem shown is the 2nd generation card (300/1200) that used an ASIC that went EOL at one point.