it will be used; if not, the frequency indicated by the SLU is used ROLL CALL 3.– The first part of roll call 3 is the SLU exhale word 3. This consists of the HF frequency status data. The word is built in the same manner as the first two words. This word is then inhaled by the IRC and processed and displayed. The IRC then exhales a roll call 3 word in the same manner as the first two, and the cycle continues. The only exception is the UHF status words. The status display is shared by both UHF radio sets, but only the UHF set that is on-line will be displayed. ROLL CALL 4 THROUGH 12.– The digital control and input register sections of the SLU process the inhale and exhale words, making the same decisions as they did on the first three words. The buffers react differently, depending on the current word or situation. Another difference during UHF configuration changes occurs in the channel set/channel select and frequency buffers. If the UHF frequency word has bits 32 and 33 at binary zero, the word is sent directly to the UHF frequency buffer. There is a separate buffer for each of the radio sets. When the word arrives at the buffer, it is checked for validity. If the frequency is illegal, the frequency command sent to the UHF set is a fixed-logic 300-MHz word that comes from the random access memory (RAM) in the SLU. In either case, the frequency is sent out in BCD format from the SLU to the radio set. The status is sent to the IRC in the same manner as for the HF status. If the channel set bit (bit 32) is a binary one, the frequency and channel number is sent to the RAM for storage. There is storage for 22 channels and frequencies in the RAM. Each UHF radio set has its own RAM. The channel set data remains stored in the RAM until called by bit 33. When bit 33 is a binary one, the channel number attached to the word addresses the frequency associated with that channel in the RAM. If the channel and frequency have been previously stored in the RAM, the frequency code is sent to the UHF frequency buffer for processing. If they have not been set, the 300-MHz frequency code is sent to the UHF frequency buffer for processing. The IRC/SLU roll call exchange ends with the first part of roll call 8. The SLU/crew ICS panels roll call exchange includes the last part of roll call 8 through the first part of roll call 12. Roll call 8 is recognized by the IRC and the copilot’s crew ICS panel. For the IRC, it is a status word as established by the tag bit. The copilot crew ICS panel also recognizes the word and prepares to send an exhale word to the SLU. Further processing of the inhale word is inhibited by a transmit reset signal. A crew ICS panel responds to a single word; therefore, the digital control section can be greatly simplified. Instead of an inhale word register, it has inhale word logic. The address coming in is compared with the fixed identification logic. If the word is recognized, the inhale word logic is stored for 800 microseconds, and the transmit detector is set. At the end of 500 microseconds, the exhale word assemble register is enabled The exhale generator functions the same as the exhale word generators in the IRC. The exhale word generator shifts the data under clock control to the exhale word assembly register, where the housekeeping bits are added. When the word is clocked out of the parity circuit, the reset-transmit detectors set. Resetting the transmit detector enables the inhale word logic. The data word arriving at the SLU from the crew ICS panel is processed and sent to the ICS buffer. The ICS buffer processes the word and places the microphone and headset in a selected configuration. The bit change counter for TACAN, IFF, XMIT SEL, CONF 1, and CONF 2 switchlight selection is located in the ICS buffer. This counter counts the times that the bit associated with the function changes from binary one to binary zero. The times the counter changes determines which function has been selected by a multiple function selector switch. The status word from the SLU to the crew ICS panel is constructed and transmitted the same way as all other words. When the first part of roll call 9 is on the multiplex line, it is recognized by the TACCO crew ICS panel, but it is also present in the copilot crew ICS panel. It is there because the word logic is still held open by the address detector, which recognized roll call 8. Only the TACCO crew ICS panel will process the data. Processing of the roll calls continues in this fashion until the roll call rolls back to zero. At this point it starts all over again. VHF COMMUNICATIONS Learning Objective: Recognize components and operating principles of a VHF radio system. The AN/ARC-197 VHF communication system provides an aircraft with two-way, plain voice radio communications in the very high-frequency range. Its actual frequency range is from 116.000 to 151.975 MHz. 1-11