GUN DISPLAY UNIT
The gun display unit links the howitzer into the BCS. At each gun, the GDU displays firing data and fire commands from the BCS and transmits the status of the gun to the BCS throughout the fire mission. The GDU consists of a section chief's assembly (SCA), the case assembly (CA), and two gun assemblies. The section chief receives his fire commands on the SCA. At the same time, the deflection and quadrant elevation are displayed on the gunner's and assistant gunner's gun assemblies, respectively. Wire, the AN/PRC 68, or the AN/PRC 126 radio, is used for communication with the BCS.
a. The SCA (Figure L-1) gives the section chief a display of firing data and commands to fire. The SCA can be connected to the case assembly and to a headset.
b. The following data can be displayed on the SCA:
- Special instructions.
- Powder lot.
- Fuze setting.
- Fire commands.
- Quadrant elevation.
- FFE data.
c. The keyboard allows selection of piece data, sending status to the BCS, and self test. The controls, indicators, and connectors (Figure L-2) are discussed below.
(1) The panel control is used to vary the lighting intensity of keys and legends. It has ON and OFF positions.
(2) The display control is used to vary the brightness of the display window data.
(3) The display window shows red letters and numbers of firing data and command bars. DNL, AMC, and FIRE bars are not lit except during fire missions.
(4) The MSN MOF/1 key is pressed to show the section number and mission number in the display window.
(5) The SP INST/2 key is pressed to display special instructions of azimuth, high angle, or gunner's quadrant or to enter the section number in the display window.
(6) The SH/3 key is pressed to show the shell to be used in adjustment or to enter the section number in the display window.
(7) The LOT/4 key is pressed to show the projectile and powder lot or to enter the section number in the display window.
(8) The CHG/5 is pressed to show the charge to fire or to enter the section number in the display window.
(9) The FZ/6 key is pressed to show the fuze to be used in adjustment or to enter the section number in the display window.
(10) The FZ SET/7 key is pressed to show the fuze setting or to enter the section number in the display window.
(11) The DF/8 key is pressed to show the deflection or to enter the section number in the display window.
(12) The QE/9 key is pressed to show the quadrant elevation in adjustment or to enter the section number in the display window.
(13) The FFE SH key is pressed to show the FFE shell data in the display window.
(14) The FFE LOT/0 key is pressed to show the FFE lot data or to enter the section number in the display window.
(15) The FFE RDS key is pressed to show the number of FFE rounds in the display window.
(16) The FFE FZ key is pressed to show the FFE fuze data in the display window.
(17) The READY key is pressed to lock in the section number and is pressed when the piece is ready. The ready message is sent to the BCS.
(18) The SHOT/RC key is pressed once after the first round is fired. It causes SHOT to show in the display window and sends the SHOT message to the BCS. It is pressed again when the last round is fired. This causes RC (rounds complete) to show in the display window and sends the ROUNDS COMPLETE message to the BCS.
Note: In one round FFE missions and during the adjustment phase, the section chief must ensure that he presses the SHOT/RC key twice after firing or he may not receive data for the next adjustment/mission.
(19) The SELF TEST key is pressed to start the GDU self-diagnostic test.
(20) The CYCLE key is pressed to acknowledge to the FDC the receipt of a message. It silences the alarm and causes the gun number and mission number or updated firing data to show in the display window. The SCA window goes blank 15 seconds after the last key is pressed. Pressing the CYCLE key causes the display to reappear.
a. The case assembly (Figure L-3) provides circuits for data reception and transmission and for power conversion required for GDU operation.
b. In the battery compartment are the active battery and a spare battery.
c. The power supply unit provides power and data circuits.
d. There are connectors for the SCA, power, and data.
e. Binding posts allow for the connection of the following:
- Quadrant elevation gun assembly.
- Deflection gun assembly.
- LCU (labeled BCU on the case assembly).
- Auxiliary power.
f. The controls, indicators, and connections (Figure L-4) are discussed below.
(1) The POWER connector allows power connection between the case assembly and the radio.
(2) The DEF 1, DEF 2, and DEF 3 binding posts allow connection of field wire between the case assembly and the power, ground, and signal binding post, respectively, on the deflection gun assembly.
(3) The QE 1, QE 2, and QE 3 binding posts allow connection of field wire between the case assembly and the power, ground, and signal binding posts respectively on the quadrant elevation gun assembly.
(4) The AUDIO connector allows signal connection between the case assembly and the radio.
(5) The BCU binding posts allow signal connection between the case assembly and the BCS (through the wire line adapter).
(6) The 28V AUX POWER binding post allows connection to an external 28 volt power source.
(7) The GND connector allows connection to earth ground.
(8) The alarm gives an audible alert to the operator when the GDU is receiving a message.
(9) The ALARM control is used to vary the volume of the audible alarm. (To increase the volume, turn the control clockwise.)
(10) The POWER ON-OFF switch is used to give primary power to the case assembly.
(11) When placed in the NORMAL position, the NORMAL-BITE switch allows digital communications between the BCS and the GDU. In the BITE position, it completes the BITE circuit.
(12) The SCA connector allows signal connections between the case assembly and the SCA.
a. The gun assembly (Figure L-5) provides instant identification of required deflection to the gunner or elevation to the assistant gunner.
b. The display window shows quadrant elevation or deflection information. The tenths digit shows on the QE display only when the special instruction of GUNNER'S QUADRANT is received.
c. Three binding posts allow connection to the case assembly as follows:
(1) The TERM 1 binding post is used to connect field wire between the gun assembly and the power binding post on the case assembly.
(2) The TERM 2 binding post is used to connect field wire between the gun assembly and the ground binding post on the case assembly.
(3) The TERM 3 binding post is used to connect field wire between the gun assembly and the signal binding post on the case assembly.
d. When the RECALL switch is pressed the display reappears. The display normally stays in the display window for 15 seconds. The display also goes blank when the GDU receives EOM, CEASE LOAD, or CHECKFIRE; and it stays blank until data for the next mission is received.
e. The display brightness control is used to vary the brightness of the display window data.
The following cables are provided for connecting the GDU:
- SCA cable (W35) (15 feet long), which provides signal and power connection between the case assembly and the SCA.
- Cable assembly W33 (3 feet long) or W33A (15 feet long), which provides signal connection between the case assembly and the AN/PRC 68 or AN/PRC 126 radios.
- Cable assembly W34 (3 feet long) or W34A (15 feet long), provides power connection between the case assembly and the AN/PRC 68 or AN/PRC 126 radios.
a. When a unit is not operating with radio, wire is used to connect the GDU to the BCS.
b. The BCS - GDU digital wire link is sensitive and works best with unspliced wire in good condition. A single poor splice or multiple splices will hamper data transmission. Terminal strip TM-184 should be used when connecting wire in the BCS - GDU link. The MX-155 has not performed well. A good lithium battery (BA-5590) should be installed if a battery is required. Weak or dead batteries may result in lost data, display problems, and nonacknowledged messages.
c. The TA-312 field telephone and/or the AN/GRA-39 should always be used on separate wire lines from the BCS - GDU digital data link. Use of this equipment on the same wire line as the BCS - GDU wire link may hamper communications and damage the GDU. The ring voltage on the TA-312 and the AN/GRA-39 can damage the GDU circuitry.
The cable and field wire connections discussed below are necessary to apply power to and establish wire communications between the GDU, the chief of section, the gunner, the assistant gunner, and the BCS. Apply GDU power by one of the methods described in paragraph L-9.
1. Because of the unique characteristics of a digital wire circuit, its efficiency is seriously impaired by an electrical leakage through the wire insulation or a bad splice. Therefore, great care must be taken in laying the firing battery wire circuits to avoid damage to the insulation. Wire lines must not be laid in water. Splices must be carefully made and well insulated to avoid excessive leakage or possible shorts. Splices on wire lines must be kept to a minimum to reduce line resistance.
2. The use of terminal strip TM-184, instead of jack panel SB-16, is highly recommended. This will reduce circuit resistance due to the internal circuit design of the SB-16.
a. Operation With Terminal Strip TM-184. Terminal strip TM-184 (Figure L-6) is an expendable Class IX item (NSN 5940-00-238-8493, reference TC 24-20). It is the preferred device to use in connecting wire lines from the GDU to the BCS. If no TM-184 is available in the unit, recommend that the SB-16 be used. The digital wire system connecting the BCS through the TM-184 terminal strip to the GDU is shown in Figures L-6 and L-7.
b. Wire System Construction. The wire system is constructed by laying a wire line of WD-1/TT field wire between the GDU and the BCS. The modified reel unit RL-39 (A, Figure L-7) is used. Starting at the TM-184 (B, Figure L-7), attach the ends of a WD-1/TT wire (NSN 6145-00-226-8812), (C, Figure L-7) on the strip. Continue laying wire to the gun position, stake it off, and connect the wire to the binding posts labeled BCU (D, Figure L-7) on the GDU. For this connection, prepare both ends of a short piece of WD-1 wire as shown in A of Figure L-8. Connect one end to the connector labeled M-221 on the spool DR-8 (E, Figure L-7) and the other end to the binding posts labeled BCU on the GDU. Using wire from an RL-39 reel, insert the open end in the two binding posts on the TM-184 corresponding to the gun number as shown in F, Figure L-7. Prepare the wire ends according to A, Figure L-8, and ensure that the TM-184 is prepared as shown in Figure L-6. Connect wire circuits for the rest of the guns in the same manner.
c. Connector Wire Preparation. Prepare wire for connection as follows.
(1) Cut six pieces of field wire about 25 feet long. Strip about ½ inch of insulation from each end (A, Figure L-8), leaving a ½ inch strip of insulation on each end of the wire, and twist the two conductors (B, Figure L-8).
(2) Take three pairs of wire. Tag and mark the two ends of each pair QE 1, QE 2, and QE 3 (C, Figure L-8). Connect one end of the pair marked QE 1 to the binding post labeled QE 1 (G, Figure L-7) on the GDU and the other end to the left binding post labeled 1 on the QE gun assembly. Using the same procedure, connect the other two pairs to the QE 2 and QE 3 binding posts.
(3) Tag and mark the other three cables DEF 1, DEF 2, and DEF 3. Connect one end of the pair marked DEF 1 to the binding post labeled DEF 1 (H, Figure L-7) on the GDU and the other end to the left binding post labeled 1 on the deflection gun assembly. Using the same procedure, connect the other two cables to DEF 2 and DEF 3. Run the GDU self-test to check internal operation. Always use a separate wire line for voice (TA-312 communications). Then make a digital communications check using the BCS; RING and the GDU; RING.
The GDU is mounted in the cab of the M109-series of howitzers (Figure L-9). Be sure to leave enough slack in the wire leading outside the cab to the DR-8 to compensate for traversing.
The GDU connected by wire to the BCS can be operated from one of two power sources:
- Internal battery power.
- External power by power cable.
a. Internal Battery Power (BA-5590). Install the battery in the case assembly as follows (Figure L-10):
When putting the new battery in the case assembly, handle it very carefully. It is easily damaged by rough handling.
- Loosen the cover fasteners, and remove the cover.
- Put the new battery in the battery compartment, and push the battery to seat the battery connector with the case assembly connector.
- Install the spare battery in the battery compartment.
- Put the cover in place, and lock the fasteners.
Lithium organic batteries or cells are used in this equipment. They are potentially hazardous if misused or tampered with before, during, or after discharge. The following precautions must be strictly observed to prevent possible injury to personnel or damage to equipment:
b. External Power (W34 or W34A Power Cable).
(1) Use the W34 cable with the M109-series of self-propelled howitzers and all towed howitzers. Use the W34A cable only on the M110-series self-propelled howitzers.
(2) Install the BA-5590 battery in the case assembly.
Note: Use internal battery power when the W34 (W34A) power cable is connected to the OG-174 AN/VRC. The purpose of the internal battery is to provide backup power whenever external power drops below 12 volts direct current (DC). Power will not be drawn from the internal battery as long as the prime power source voltage is greater than the internal battery voltage.
(3) Remove the dust cap from the power connector of the case assembly.
(4) Join power cable W34 (NSN 7025-01-122-2984) or W34A (NSN 7025-01-122-2985) connector P2 to the power connector on the case assembly.
(5) Connect power cable W34 or W34A connector P1 to the Bendix connector on a 24-volt vehicle power cable. If you have the AN/PRC 68 or AN/PRC 126 SUT, connect the W34 (W34A) power cable to the OG-174 AN/VRC amplifier power supply group instead of directly to the vehicle power.
a. The following is a safe, accepted and proven method of remoting power from various vehicles to the GDU when it is used with a towed howitzer. The Communications - Electronics Command (CECOM) Equipment Safety Office agrees that it is a practical solution for the towed howitzer power problem. The required cable can be fabricated locally.
(1) The cable can be built by using the following Class IX parts:
- Connector, plug, electric, NSN 2910-00-567-0128.
- Adapter connector, NSN 5935-00-322-8959.
- Cable, power electric, NSN 6145-00-643-3482.
- Cable, assembly power, NSN 7025-01-122-2984.
(2) The power electric cable comes in 250-foot spools. From these, cables can be cut in varying lengths, according to the needs of the unit. For example, about five 50-foot cables can be made from one spool; so the cost can be divided by five.
(3) The assembly power cable is issued with each GDU but is only 3 feet long, so it has to be modified at no cost to the using unit. The connector plug on one end (the GDU end) is all that needs to be used on this cable.
(4) The total cost of each 50-foot cable will be about $55.09.
(5) Instructions for fabricating the cable are as follows:
- Remove the connector plug (GDU end) from the GDU power cable, and clean old solder from the connections.
- Cut the new power cable, NSN 6145-00-643-3482, into the required lengths, and strip the insulation from the ends.
- Connect both the black and white wires of the new cable to pin B (+) of the GDU cable connector. Connect the green, or ground, cable to pin A (-) of the same connector. One end of the new cable will be complete.
- Connect the other end of the new cable to connector plug, NSN 2910-00-567-0128. Make sure to observe the same polarity, black and white parallel and attached to the positive terminal and green wire, or ground wire, attached to the negative terminal.
- Use electrical ring (crimp-on) connectors to connect the cable to the terminals on the electric plug connector. Use a short piece of wire or cable (scrap) to fill in the extra hole in the electric plug connector. Finally, seal both holes with RTV (GE silicone caulk).
b. Location of the GDU on various howitzers is shown in Figures L-11 through L-14.
a. When vehicle power of the SP howitzers is used, firing data can be lost. This can happen when the 5-horsepower electrical motor is started; that is, when the loader rammer is being used, the vehicle is started, or the master switch for the cab is turned on. The result is that all segments of the SCA assembly are lit, causing a wagon wheel effect on the display window. This problem can be solved with the application of a MWO. All new GDUs will have this MWO already applied.
b. Until your GDUs are retrofitted, this problem can be avoided or minimized by announcing the fire commands before operating the loader-rammer.
c. Section chiefs should ensure that a good lithium battery (if available) is in every GDU during operations. Batteries must be changed every 5 to 7 days, depending on the intensity of firing.
d. If firing data are lost, the howitzer section must ensure
- Power is recycled on the gun display unit.
- The gun number is reestablished on the section chief's assembly.
- The firing data are retransmitted from the BCS.
If the GDU becomes inoperative, it should be turned in for repair. If communications cannot be established with the GDU NORMAL-BITE switch in the NORMAL position, try placing the switch in the BITE position. If this is successful, operation can continue until the GDU can be repaired.
The AN/PRC-68 or AN/PRC-126 provide an internal FM voice net. This net will be used for convoy movement, hasty displacements, emergency occupations, and during the initial occupation of a prepared position. Once wire communications are established, radio will no longer be used except for emergency notification of an enemy attack. Despite these radios short planning range, their signal can still be detected by ground and airborne RDF and/or intercept stations. This is another reason wire must be used for internal battery communications as soon as possible after occupation that-of a position. The radios use the BA-1588/U battery (NSN 6135-01-094-6536).
a. M577 Vehicle FDC Cabling. The system for the AM-1780 in the FDC is cabled as follows (refer to Figure L-15).
(1) Connect one end of power cable W32 A to the connector labeled J103 B on the OG-174 and the other end to the BCS power distribution unit plug J4.
(2) Connect one end of cable CX-4723 C to the connector labeled J102 D on the OG-174 and the other end to the connector labeled J505 E on the AM-1780.
(3) Connect one end of the second cable CX-4723 F to the connector labeled J101 G on the OG-174 and the other end to the connector labeled J501 H on the AM-1780.
(4) Connect one end of the interface cable I to the connector labeled J104 J on the OG-174 and the other end to the connector labeled AUDIO K on the AN/PRC 68 or AN/PRC 126.
(5) Connect one end of the antenna cable L to the connector labeled ANT M on the AN/PRC 68 or AN/PRC 126; connect the other end to the connector N on the antenna base.
(6) Connect one end of the data cable O to the connector labeled J105 P on the OG-174; connect the other end Q to the tactical communications interface module of the LCU (see Figure 9-4 and paragraph 9-5 of this publication).
b. Weapon Cabling. Radio sets AN/PRC-68 and AN/PRC-126 are cabled to the weapon as follows (refer to Figure L-16):
(1) Connect one end A of cable W33 or W34 B to the connector labeled RADIO on the GDU and the other end to the connector labeled GDU C on the OG-174.
(2) Connect the interface cable D between the OG-174 (J104) E and the connector labeled AUDIO F on the AN/PRC-68 or AN/PRC-126.
(3) Connect power cable CX-4720 G to the vehicle power system (Bendix connector) or the 24-volt DC connector on the OG-174 case assembly.
a. Solder all splices, and wrap them in insulating tape.
b. For WD-1/TT wire, ensure all seven strands are intact at the connections or splices.
c. Ensure all electrical equipment is protected from rain and moisture.
d. Inspect all cables and wires for cuts and rotting.
e. Ensure all connectors are clean with no bent pins or corrosion.
f. Clean the connectors on the GDU, LCU, wire line adapter, TM-184 terminal strip, and M-221 connector with cleaning compound FREON-TF (NSN 6850-00-3084) and a clean cloth.
g. Clean the female end of the connection on the modified reel RL-39 and the holes in the M-221 connector with cleaning compound FREON-TF and a Q-tip.
h. When splicing WD-1/TT wire to cord CX-231, use either a standard field wire splice (TC 24-20) or the expedient method. With either method, be sure to solder and tape the splice.
i. Ensure that personnel do not use solvent to clean the case assembly. This destroys plastic parts inside the pressure relief valve, allowing water to enter.
j. Ensure that personnel do not use high-pressure water hoses on the equipment. The pressure relief valve on the case assembly is designed to open at 3 to 4 pounds of pressure; thus, pressure from the hose will allow water to enter the case assembly.
k. Inspect the case assembly to ensure that there are no missing screws and that all screws are tightened securely.
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