1. CECOM LAR Operation Course
AN/TRC-170 Loopbacks
CECOM LAR Course
Logistics Assistance Division
Fort Monmouth, New Jersey
Students frequently have an imperfect understanding of COMSEC devices. This leads to problems in patching them in properly, attempts to use them incorrectly, and ineffective troubleshooting. A thorough understanding of the information in this chapter can resolve these problems.
Signal Flow

2. CECOM LAR Operation Course
Purpose and Types
Configuration Verification
Troubleshooting
Direction
Local
Distant
Types
User Channel
Group
Supergroup
Signal Flow

3. User Channel Loopbacks
J-1077 User Channel
Signal Entry Panel Channel
LGM AAU Card Switch

4. CECOM LAR Operation Course
J-1077 Channel
Direction: Distant
Installation: Place wire segments connecting the odd pair (Input/XMT Data) on the quad to the even pair (Output/RCV Data) as shown above. Recommend turning phone power OFF during loopback.
Effect: Loops back data/voice from the user at the distant end on the selected channel of the LGM back through all equipment in the shelters at both ends. The LGM channel should be “green”. End-to-end connection on the channel is lost.
Timing Considerations: None
Notes: Verifies the J-Box and 26 pair cable.
Companion Loopback: If the user on the quad is left hooked up to the quad, the loopback will be in both directions. Alternately, you can twist each WF-16 transmit wire to a receive wire and achieve the same effect.
4 Wire Loopback on Quad 1
Signal Flow

5. CECOM LAR Operation Course
Signal Entry Panel
Direction: Distant
Installation: Place wire segments connecting the A and B binding posts (Input/XMT Data) on the quad to posts C and D (Output/RCV Data)as shown above. Recommend turning phone power OFF.
Effect: Loops back data/voice from the user at the distant end on the selected channel of the LGM back through all equipment in the shelters at both ends. The LGM channel should be “green”. End-to-end channel connection is lost.
Timing Considerations: None
Notes: None
Companion Loopback: If the user is left hooked up to the binding post push-pins, the loopback will be in both directions. Alternately, you can twist each WF-16 transmit wire to a receive wire and achieve the same effect.
Signal Flow

6. CECOM LAR Operation Course
LGM AAU Card
Direction: Local
Installation: Move the AAU channel card loopback switch from “NORM” to “LOOP” for the desired channel.
Effect: Loops back data/voice from the local user back through the Signal Entry Panel or J-Box. End-to-end connection to the distant end on the channel is lost.
Timing Considerations: None
Notes: There is no similar loopback capability for the Loop Modem channel card.
Companion Loopback: None.
Signal Flow

7. Group Loopbacks LGM Composite BBPP TGM Port (Group) BBPP
TED 2 Black Side BBPP
TED 2 Red Side BBPP
MD-1026 NRZ BBPP
MD-1026 CDI Signal Entry Panel

8. Supergroup Tropo Modem BBPP TGM Supergroup BBPP TED 1 Black Side BBPP
TED 1 Red Side BBPP
IF Test Panel Tropo Modem IF

9. CECOM LAR Operation Course
LGM Composite
Direction: Local
Installation: Patch LGM x DATA IN to OUT and TMG IN to OUT for the desired LGM(s). Reconfigure for Station Clock, if necessary. Must loop back IFTP to give Tropo Modem BER and release Frame Squelch.
Effect: Loops the LGM composite group back on itself, having the effect of looping back all user channels on themselves. Traffic for all channels will be disrupted to the distant end.
Timing Considerations: If loop timing is being used from the TGM, it will be lost and the LGM will be somewhat unstable. If station clock is available, patch it and select station clock on the LGM.
Notes: Rarely used, except to test an LGM off-line. Must loop back IFTP and provide Station Clk to green up.
Companion Loopback: If the shelter is in system, normally a TGM Port (Group) Loopback will also be installed.
Signal Flow

10. CECOM LAR Operation Course
MD-1026 NRZ
Direction: Local
Installation: Patch GM GRP x Data IN to OUT and TMG IN to OUT for the desired group(s). Groups 1 & 2 are shown in loopback above.
Effect: Loops back the selected GM group(s) toward the local user. Only traffic on the selected group is affected.
Timing Considerations: May interrupt timing to the TM if group timing is being used (unlikely).
Notes: Very common loopback used to verify the local path to a group user, e.g. switch, RMC or LTU.
Companion Loopback: TGM Port Loopback or TED 2 Black Side Loopback, depending on configuration.
Signal Flow

11. CECOM LAR Operation Course
TGM Supergroup
Direction: Local
Installation: Patch TGM SGP (TM) DATA IN to OUT and TMG IN to OUT.
Effect: Loops back all local traffic through the TGM. As a result, all end-to-end traffic is disrupted.
Timing Considerations: You must use TO or RO timing from the Tropo Modem and Station Clock should be selected and patched to the TGM.
Notes: Checks TGM operation and configuration and all shelter DGM and Crypto equipment should be “green”.
Companion Loopback: Install either a Tropo Modem BBPP Loopback (preferred) or a TED 1 Red Side Loopback.
Signal Flow

12. CECOM LAR Operation Course
TED 1 Black Side
Direction: Local
Installation: Patch TED 1 RCT DATA to TCT and RCTC TMG to TCTC. TED 1 BSC (Black Station Clock) must be patched to either TO or RO. Leave TED 1 RSC and Resync patched to the TGM or LGM1.
Effect: Loops back the output of the TGM Supergroup or LGM Group from the black (encrypted) side of TED 1. All local users are in loopback and normal end-to-end traffic is disrupted.
Timing Considerations: None
Notes: Checks operation of TED 1. All DGM equipment in the system should be “green” and the TED should show Full Operate and Resync.
Companion Loopback: Install a Tropo Modem BBPP Loopback.
Signal Flow

13. CECOM LAR Operation Course
IF Test Panel
Direction: Local
Installation: Place one or more of the RCVR Switches in the LB (loopback) position. Set the test signal attenuators to achieve the desired AGC level on the Tropo Modem. If the antennas are connected, detune all receivers at least 50 MHz.
Effect: Connects a sample of the 70 MHz transmit IF signal from the Tropo Modem to the Tropo Modem IF input for the selected receiver channel. Receive data from the distant end for the selected RCVR is not disconnected, however since the loopback signal will also be present, traffic will be disrupted. Transmit data to the distant end is unaffected. Placing any switch in loopback will cause a “configuration” fault.
Timing Considerations: None
Notes: If you are in system, and don’t detune all Receivers at least 50 MHz, results will be unpredictable and equipment will probably not be “green”. With no input from outside the shelter, any receive channel can be set to loopback by itself to check that it is functioning properly and attenuation can be set to simulate a weak signal. The system should show “green” and all users should see the loopback.
Companion Loopback: None.
Signal Flow

14. CECOM LAR Operation Course
Tropo Modem
Direction: Distant
Installation: Patch Tropo Modem (TM) DATA IN to OUT and TMG IN to OUT.
Effect: Loops back the distant end’s supergroup through the and Baseband sides of the Tropo Modem. All local traffic is disrupted.
Timing Considerations: Will interrupt timing from a local source to the TM if group timing is being used (unlikely).
Notes: Checks the Tropo Modem settings and performance with the distant end.
Companion Loopback: Install either a TGM Supergroup loopback or a TED 1 Black Side loopback, depending on normal configuration and preference.
Signal Flow

15. CECOM LAR Operation Course
TED 1 Red Side
Direction: Distant
Installation: Patch TED 1 RPT (Receive Plain Text) data to TPT (Transmit Plain Text). Leave TED 1 BSC IN (Black Station Clock) patched to either TO or RO BSC. Remove the TED 1 RSY patch from either the TGM or LGM, depending on configuration.
Effect: Loops back the distant end’s supergroup through TED 1 on the red (Unencrypted) side. All user traffic is interrupted.
Timing Considerations: None likely.
Notes: Checks the Tropo Modem and TED 1 operation and keying. Unstable because no resync is available from the TGM/LGM. Normally used to verify a TED 1 key match with the distant end. Disrupts all user traffic.
Companion Loopback: TGM Supergroup Baseband Patch Panel Loopback.
Signal Flow

16. CECOM LAR Operation Course
TGM Port
Direction: Distant
Installation: Patch TGM GP x DATA IN to OUT and TMG IN to OUT for the desired group(s).
Effect: Loops the selected group(s) back through the TGM to the distant end. Local traffic on the selected group will be affected.
Timing Considerations: If group timing is being used, the local source may be lost, depending on configuration.
Notes: Common loopback used to verify the transmission path and TGM settings.
Companion Loopback: Depends on configuration. Usually a GM NRZ BBPP loop or LGM Composite BBPP loop is installed. TED 2 Red Side loops are not normally used.
Signal Flow

17. CECOM LAR Operation Course
MD-1026 CDI
Direction: Distant
Installation: Install a CDI Loopback Plug on the desired port(s).
Effect: Loops back the selected GM group(s) through all shelter equipment to the distant end. Only group traffic is affected.
Timing Considerations: Will interrupt timing from a local source to the TM if group timing is being used (unlikely).
Notes: Checks all local equipment through to the distant end.
Companion Loopback: Place another CDI Loopback Plug on the CX cable.
Signal Flow

18. CECOM LAR Operation Course
TED 2 Red Side
Direction: Local
Installation: Patch TED 2 TPT (Transmit Plain Text data) to RPT (Receive Plain Text data), you may leave RPTC (Receive Plain Text Clock) patch as is or remove it. Remove the resync patch.
Effect: Loops back the group being input to the TED. May disrupt all traffic through the TGM unless TGM port 1 is also looped.
Timing Considerations: Group Timing from the affected port missing.
Notes: Insure that the Tropo Modem “Group” Thumbwheel Switch is set to the data rate of the group being looped back. TGM Port 1 must be looped back or the TGM will not frame. There will be no resync from the TGM to the TED. This loopback is usually only installed to verify a key match with the local user.
Companion Loopback: TGM Port Loopback. Port 1 is the port normally associated with TED 2.
Signal Flow

19. CECOM LAR Operation Course
TED 2 Black Side
Direction: Distant
Installation: Patch TED 2 TCT (Transmit Cipher Text data) to RCT (Receive Cipher Text data), patch TCTC (Transmit Cipher Text Clock) to RCTC (Receive Cipher Text Clock).
Effect: Loops back the group from the distant end being input to the TED from the TGM (usually Port 1). Should only affect the group in loopback.
Timing Considerations: Any local user on the looped port will be cut off from Tropo Modem timing.
Notes: Insure that the Tropo Modem “Group” Thumbwheel Switch is set to the data rate of the group being looped back.
Companion Loopback: GM NRZ BBPP Loopback.
Signal Flow