Recycling Fibre Channel MIA as "RS422-like" ?

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Y888099
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Recycling Fibre Channel MIA as "RS422-like" ?

Unread postby Y888099 » Mon Apr 03, 2017 4:13 am

"MIA" stands for [M]edia [I]nterface [A]ssembly. it's a converter from db9 (copper) to optical fiber (laser/photo diode, on a wire glass).


Fibre Channel signal characteristics
Fibre channel electrical signals are sent over a duplex differential interface. This usually consists of twisted-pair cables with a nominal impedance of 150 ohms, in differential mode. This is a genuine differential signalling system so no ground reference is carried through the cable, except for the shield. Signalling is AC-coupled, with the series capacitors located at the transmitter end of the link.

I don't actually understand the definition of the Fibre Channel signalling voltage, since it's declared complex.

Image

On it's db9 (copper) connector, it offers the following signals

Code: Select all

Pin   Signal-name          Comments
-----------------------------------------------------
1     +OUT                 Fibre channel output
2     +5V                  Optional
3     Module Fault Detect  Optional
4     *                    Reserved
5     +IN                  Fibre channel input
6     -OUT                 Fibre channel output
7     Output Disable       Optional
8     GND                  Optional, return for pin 2
9     -IN                  Fibre channel input


Basically, it's a differential TX+/- and RX+/-, with the following characteristics

Tx PECL Signal Requirements
VI (max) 2000 mV
VI (min) 650 mV
Line Impedance ZO 150 ? ± 30 ?
Input Impedance RI 150 ? ± 10 ?
Det. Jitter (max) 0.09 UI p-p
Total Jitter (max) 0.24 UI p-p
Mark ratio 50 ± 10 %
Rise/fall time Max 350 ps Min 100 ps
Diff. Skew (max) 20 ps


Rx PECL Characteristics
VO (max) 2000 mV †
VO (min) 370 mV †
Line Impedance ZO 150 ? ± 30 ? †
Equivalent Output
Impedance | ZE |75 ? ± 25 ?
single-ended
Det. Jitter (max) 0.38 UI p-p
Total Jitter (max) 0.60 UI p-p
Diff. Skew (max) 205 ps



I wonder if I can recycle two of those MIAs as "RS422-like". Of course the analogical side needs an adapter from TTL to PECL Characteristics. Can it work?

Optical fiber is better than copper against noises when you have to connect things around motors. I could use BASE1000-SX, but ... I don't need the complexity of TCP/IP.

The coding system on Fibre Channel ensures an equal number of ones and zeros are always sent. This is useful as it allows very tight automatic gain control in the receiver (esp for optical)

I wouldn't expect it to like data the does't have a balanced number of ones and zeros, or it the runs of ones and zeros are too long (upsetting any AGC loop)


What do *you* think, guys?

robespierre
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Re: Recycling Fibre Channel MIA as "RS422-like" ?

Unread postby robespierre » Mon Apr 03, 2017 8:05 am

As you observed, FC uses 8b10b scrambling to eliminate DC bias, while RS422 does not; with serial the duration of constant output can be VERY LONG, e.g. a break signal. I have no idea if the MIA can cope with that.
You also have the issue of rise/fall time (which is like the bandwidth). 350 ps is 1 GHz bandwidth. It may have band-pass characteristics, where the signal needs to be within the band from 0.5 to 1 Ghz or be rejected
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Y888099
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Re: Recycling Fibre Channel MIA as "RS422-like" ?

Unread postby Y888099 » Mon Apr 03, 2017 12:17 pm

Optical RS422 does exist :D

e.g. the ATC-277MM is a modem on optical fiber, that allows the connection of RS232/422/485 devices through a cable in 850nm multimodal fiber. The device allows the connection up to two km of cables in fiber and supports the point-point mode or the ring mode, with speed of data transfer up to 250kbps, on both lines of transmission and reception, and it also offers the complete hardware control of the RS-422/485 drivers with automatic circuit for control of the data sent.

This means that ATC-277MM can replace interface converters and isolators when you connect remote devices, while preserving the EMI/RFI characteristics and the immunity to the transients of the optical fiber.

Unfortunately it's 10x expensive than MIA.

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Re: Recycling Fibre Channel MIA as "RS422-like" ?

Unread postby guardian452 » Mon Apr 03, 2017 5:52 pm

A number of BMS systems implement what is basically RS422 or RS485 protocol over fiber (for isolation). Which makes slightly less sense in the automotive world because the copper wiring is physically identical to CAN (120ohm twisted pair).

I guarantee there is a fairly inexpensive COTS transceiver chip they are using... but I don't know what it is.

Y888099
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Re: Recycling Fibre Channel MIA as "RS422-like" ?

Unread postby Y888099 » Mon Apr 03, 2017 11:37 pm

Never used, but I have designed a lot of boards with OrCad and EagleCad, including the CANbus interface.

I wonder how laser and photo detector, and their coding/encoding are physically handled in modems on optical fiber.
What's inside the ATC-277MM's box?

robespierre
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Re: Recycling Fibre Channel MIA as "RS422-like" ?

Unread postby robespierre » Tue Apr 04, 2017 3:22 am

If I had to guess, it has a translator to something SDLC-like (because synchronous is easier to encode) which might get scrambled after
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