Homemade PABX - Page 1
Forum section: electronics / projects

author: Biduleohm
author_rank: Contributor
timestamp: August 02, 2023, 08:59:47 pm
content: Hi everyone,I'm currently designing my own PABX using plain 4xxx/74xxx logic (no MCU) and I have a question I can't seem to be able to find the answer to.In old school exchanges they used big inductors (multiple H...) to isolate the 48 VDC feed from the line audio (so it doesn't get shunted by the DC power supply). You can see an example here (first schematic):https://www.epanorama.net/circuits/teleinterface.htmlFor space and cost reasons I can't just put two 5 H inductors per line...Modern specialised ICs emulating such circuits typically feed the line with just resistors, no inductor at all. But they also can compensate for audio attenuation as they use amplifiers anyways.So my question is: can I only use resistors? or if I need inductors, will 0.12 H each be enough? (the highest value I could find which can handle 60 mA of DC current, has reasonable size SMD package of 13x13x6 mm and has a reasonable cost of less than 1 €).

author: Benta
author_rank: Super Contributor
timestamp: August 02, 2023, 09:06:00 pm
content: You need an IC called a SLIC (subscriber loop interface circuit).Intersil used to make some nice ones, Renesas still seem to have them... well, look around.

author: Biduleohm
author_rank: Contributor
timestamp: August 02, 2023, 09:10:37 pm
content: The whole point of using logic gates is to avoid using SLIC ICs or any specialised ICs (I only made a small exception for the DTMF decoder).My question is only about how to couple the 48 VDC to the line.

author: Benta
author_rank: Super Contributor
timestamp: August 02, 2023, 09:14:57 pm
content: Quote from: Biduleohm on August 02, 2023, 09:10:37 pmCheck how a SLIC works.

author: Biduleohm
author_rank: Contributor
timestamp: August 02, 2023, 09:21:06 pm
content: I already did, I done more than 3 hours of research and couldn't find a definitive answer, that's why I ask here (I never ask before I made extensive research).As I said in the first post SLIC ICs typically only use resistors (or more complex circuits using transistors, but in the end they are still purely resistive impedance wise) but they can compensate for any loss they create doing that as they have integrated amplifiers before the DAC/ADC.

author: Biduleohm
author_rank: Contributor
timestamp: August 02, 2023, 09:55:48 pm
content: I see, however if I can keep the component count to a minimum it would be nice.Basically the question is: is using only resistors ok?Edit: wait, @Benta where did your post about the constant current source go?

author: Benta
author_rank: Super Contributor
timestamp: August 02, 2023, 10:00:40 pm
content: Feeding -48 V throguh a current limiter before the line resistor to your line should work.

author: Biduleohm
author_rank: Contributor
timestamp: August 02, 2023, 10:30:29 pm
content: How do you keep the line balanced with that solution?NB: your deleted post was more detailed and clearer.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 03, 2023, 10:44:40 am
content: Inductor fed lines got dumped when EMXs died out. Seriously old school.The indcutor can be replaced with a constant current source. A TL783 is ideal.A current of 25mA being typical.Current sources reduce battery crosstalk issues too.A couple of lost volts are not an issue in phone circuits even in miles of wire.Its the loop current that matters.I worked for a railroad telco (transients from hell) back in the day and we used 783s by the thousand, they work a treat.Really good self protection. I never saw a dead one in 10years!A TPS7A43 or a LM317HV should work fine too.You can design a gyrator based synthetic inductor if you like, but that way madness lies.

author: Biduleohm
author_rank: Contributor
timestamp: August 03, 2023, 02:13:34 pm
content: Yep, I saw specs in the 20 to 50 mA range. I'll not have more than a few dozens of meters of line at most, so both voltage and current based circuits are fine here.I like the single chip + one resistor solution. They are on the expensive side tho, but I'm sure I can find a similar but less expensive part, especially given I don't need a high current one.But then my question about line balance remains, as the current source/sink will be on one side only. How do we solve this problem?edit: see my next post.

author: Benta
author_rank: Super Contributor
timestamp: August 03, 2023, 07:56:44 pm
content: TL783 is simple. This one is cheap (remove D1. The collector of Q1 is your current sink):

author: Biduleohm
author_rank: Contributor
timestamp: August 03, 2023, 08:02:19 pm
content: That's a 5 components solution, not really what I'm looking for.But I found one at less than 1/4 the price of the TL783:https://www.diodes.com/assets/Datasheets/ZXTR2005Z.pdfand if I want to make it more efficient there's this onehttps://www.mouser.fr/datasheet/2/268/MCHP_S_A0003550291_1-2576297.pdfmore expensive but only dropping 1.2 V instead of 5 V.But I still wonder about how you can keep a balanced line with a current source/sink?Edit: forget about the balance issue, it would be too complex to maintain balanced lines across the whole PABX anyways.

author: Benta
author_rank: Super Contributor
timestamp: August 03, 2023, 08:43:12 pm
content: Quote from: Biduleohm on August 03, 2023, 08:02:19 pmThat was not mentioned. 5 parts, <15 cent total.

author: Biduleohm
author_rank: Contributor
timestamp: August 03, 2023, 10:47:20 pm
content: Actually I mentioned it yesterday:Quote from: Biduleohm on August 02, 2023, 09:55:48 pm

author: Benta
author_rank: Super Contributor
timestamp: August 03, 2023, 10:52:36 pm
content: Good.I'll leave you to it.

author: jmelson
author_rank: Super Contributor
timestamp: August 04, 2023, 05:35:08 pm
content: I kind of had something like this some years ago. It was an NEC Electra PBX. It did have 8085's in the central controller, and in each CO line card and extension card. (Those cards served 4 lines each.) It finally died. I set up a mini-ITX system with Atom CPU and got the Asterisk software and FreePBX. It is capable of using VOIP lines, but my phone lines are both analog POTS lines. I got a 4-port telephony PCI card with 2 CO line modules. This system does automatic attendant / voice message / fax receive, and also intercom, distinctive ring and dial patterns, so I don't have to dial the area code every time on a local call. It also emails me the voice messages and FAXes. It can also do conference calls.I got some Snom VOIP extension phones on eBay for $5 each! These had all the features I needed (backlight goes dark after 20 seconds - good for the bedroom), distinctive ring, speaker phone and headset in addition to handset. Powered off PoE switch.I can't imagine going back to ordinary phone systems.Jon

author: jonpaul
author_rank: Super Contributor
timestamp: August 04, 2023, 07:19:24 pm
content: The real thing....Worked on these in 1970s...Bell 1A2 key systemWE #5 crossbarEnjoy!Jon