| DVB-SPI Interface |
Comtech SDM-2020D hookup |
Written by Dave Emery (thanks Dave!)
This documents how to connect a 25 pin ribbon cable with a female DB-25 connector on it to provide a homemade SDM-2020D LVDS/SPI interface built from a scrap SDM-2020D ECL/HSSI receive board. Comtech/EF Data/Adaptive Broadband SDM-2020D satellite modems were often originally used with the very common optional ECL/HSSI-RX receive interface board, but almost no HSSI to PCI or USB cards exist (or are affordable or supported by TSR).
The SDM-2020D ECL/HSSI configuration was standard for satellite ISP configurations that used HSSI to connect SDM-2020Ds to Cisco routers and SDM-2020Ds with this particular interface and 8PSK capability are currently listed on eBay as of 3/17/07.
A standard unmodified DTVWorks SPI-DTV to USB adapter plugs into the female DB-25 just like it would into any other SPI interface, no special modifications for TTL input or other changes are required for the SPI-DVB adapter or the TSR software.
The DB-25 cable needs to be long enough to read outside the SDM-2020D frame when attached to these points on the modified interface card, but ideally should not be much longer than a foot or two lest there be signal integrity issues (noise and errors). This is, however, a LVDS interface rather than a TTL interface and in a pinch the cable could be somewhat longer if good grade ribbon or twisted pair cable is used. The interface consists of 11 differential signals (two wires each which MUST be twisted pairs or ribbon cable pairs) and three grounds.
| Signal | Resistor | Pad Near | Pin on DB-25 | Backplane |
| Data Bit 0 + | R100 R | R101 | 10 | 4b |
| Data Bit 0 - | R100 L | U25 | 23 | 4c |
| Data Bit 1 + | R101 L | R100 | 9 | 5b |
| Data Bit 1 - | R101 R | C44 | 22 | 5c |
| Data Bit 2 + | R89 L | R88 | 8 | 6a |
| Data Bit 2 - | R89 R | C44 | 21 | 5a |
| Data Bit 3 + | R88 R | R89 | 7 | 6b |
| Data Bit 3 - | R88 L | U25 | 20 | 6c |
| Data Bit 4 + | R57 L | R56 | 6 | 7b |
| Data Bit 4 - | R57 R | TP1 | 19 | 7c |
| Data Bit 5 + | R56 R | R57 | 5 | 8a |
| Data Bit 5 - | R56 L | R55 | 18 | 7a |
| Data Bit 6 + | R79 R | R80 | 4 | 8b |
| Data Bit 6 - | R79 L | R78 | 17 | 8c |
| Data Bit 7 + | R80 L | R79 | 3 | 10a |
| Data Bit 7 - | R80 R | C25 | 16 | 9a |
| Clock + | R55 L | R54 | 1 | 10b |
| Clock - | R55 R | R56 | 14 | 10c |
| DVALID + | R78 L | R77 | 11 | 3b |
| DVALID - | R78 R | R79 | 24 | 3c |
| PSYNC + | R77 R | R78 | 12 | 2b |
| PSYNC - | R77 L | C32 | 25 | 21 |
| Signal Ground | TP1 | TP1-GND | 2 | |
| Signal Ground | TP1 | TP1-GND | 15 | |
| Signal Ground | TP1 | TP1-GND | 13 |
These pads on the ECL/HSSI receive board all have 100 ohm SMT terminating resistors on them (which ideally should be removed before soldering the ribbon cable to the pad, though things seem to work fine with reduced margin if they aren't). 100 Ohms is 101.
Note the LVDS drivers are a current mode - rather than voltage mode - device, so forcing them to drive 50 ohms from the non-removed termination resistors and those on the SPI-DVB board will not overload the drivers, but will reduce the voltage swing in half, reducing signaling margin.
Second modification to a ECL/HSSI receive board
In order to make the SDM-2020D think the board is a LVDS interface instead of a ECL/HSSI board - required so the clock mode menus are enabled - there also needs to be modifications in the resistors around U58.
R200 and R202 need to be removed and the pads closest to U58 those two resistors were attached to connected with jumper wires to the pad of R206 closest to U58. This pulls pin 4 and 5 of U58 to ground through the same 100 ohm resistor (R206) used for pin 7. In addition either a grounding jumper or ideally another 100 ohm chip resistor needs to be stuffed into R205, which is unstuffed for the ECL-RX card.
Once this is all accomplished the INTERFACE TYPE in the FUNCTION SELECT UTILITY -> INTERFACE should read LVDS (NOT ECL/HSSI RX) One needs to set INTF MODE SELECT to PARALLEL here.
In the CONFIGURATION menu under INTERFACE one needs to set the RX CLOCK PHASE to NORMAL, the RX DATA PHASE to NORMAL, the RX DATA FAULT to NONE and the DVB FRAMING TYPE to 188.
On the SPI-DVB adapter, the jumper than enables signal ground on 2 and 15 of the DB-25 MUST be connected else there will be lots of SYNC BYTE errors and possibly other corruption. This jumper is right near the DB25 adjacent C6. Apparently some SPI-DVB USB adapters were shipped without this jumper in place, but without it there are serious noise problems.