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manawyrm, 08/13/2022 01:11 AM


Using osmo-e1d's trunkdev functionality, osmo-e1d can provide OCTOI as a virtual E1 / PRI line to DAHDI on a Linux machine.
This can then be used in conjunction with a software PBX to switch between the virtual E1 line going to/from OCTOI and a physical ISDN interface, for example a more common S0 / BRI interface.

Experimental test setup - Hardware

Fujitsu Futro S900 thin client with CologneChip HFC-S PCI ISDN card (NT mode capable)
The HFC-S card needs numerous mods:
- External clock input
- 100 ohm termination
- NT/TE selection (aka RX/TX flip)

100 ohm termination could be achieved directly on the card by soldering 100 Ohm resistors to the 0603 footprints next to the RJ45 connector.
The card was permanently modded for NT mode by cutting all 4 traces going to the RJ45 line (but after the transformer) and flipping the RX/TX pairs with enameled copper wire.
This (as well as the termination) doesn't need to be done on the card, but it's very convenient.

This external clock input is terminated with a 50 ohm resistor (actually 51 ohm) in parallel, which is then fed through a 100n AC coupling ceramic capacitor into the CLKIN pin on the chip. It's accessible on one side of the crystal oscillator.
This mod loads the crystal oscillator so much, that the card will stop working without external clock input attached! Detaching the 100n cap from the crystal oscillator will make it start again.
A high-quality, low phase noise (low jitter), high precision time source with 12.288 MHz (12288000 Hz) is required. The "Mini Precision GPS Reference Clock" from LeoBodnar has shown to be pretty reliable so far.

Updated by manawyrm 4 months ago · 1 revisions

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