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40.1 GENERAL DESCRIPTION . . . . . . . . . . . . . . 40-1
40.2 FUNCTIONAL SPECIFICATION. . . . . . . . . . . . 40-1
40.3 CONSTRUCTION . . . . . . . . . . . . . . . . . . 40-2
40.3.1 Front Panel. . . . . . . . . . . . . . . . . . 40-2
40.3.2 Rear Panel. . . . . . . . . . . . . . . . . . 40-3
40.4 DRAWING PACKAGE NUMBER . . . . . . . . . . . . . 40-3
40.5 RESPONSIBLE ENGINEER . . . . . . . . . . . . . . 40-3 CHAPTER 40 REMOTE AMPLIFIER MONITOR (RAM)
40.1 GENERAL DESCRIPTION The RAM is a module for checking the performance of the amplifiers in the CATV cable used for SLCNET, etc. It measures the output signal levels for transmission in both directions. This is done by means of a low frequency "pilot" signal which is injected into the system at a constant level at the "tail" end of the cable towards the "head" end where it is up-converted and returns as a higher frequency signal. It also measures the ambient temperature near a cable amplifier. By looking at the pattern of measurements as a function of position along the cable, it should be possible to localize all simple amplifier failures. Other failures, such a cable breaks may also be localized, although the diagnosis may not always be unique. Many other types of network malfunction will not be detected by the RAM. It only checks a single frequency in each band and therefore will not detect "cable tilt" (variation of level across the band). Thus there may be marginal failures in channels distant from the pilot frequencies, even though the measured levels are correct. The RAM modules communicate with the VAX through a dedicated RS-422 telecommunication line, "daisy-chained" through each unit.
40.2 FUNCTIONAL SPECIFICATION. The RAM measures the amplitude of the low frequency upbound pilot signal as it leaves each amplifier and digitizes it to 8-bit precision. It does the same for the upconverted pilot signal in the opposite direction. The measurement range is 6 dBmV +/- 12 dB at the input to the RAM. This corresponds to a cable signal level of 36 dBmV +/- 12 dB. Since the system gain varies with temperature, a measurement is also made of the ambient temperature near the amplifier. This is done by means of a thermister composite and the output is digitized to 8-bits precision. The measurement range is 24 to 114 degrees F. Not all units are fitted with the temperature probe.
REMOTE AMPLIFIER MONITOR (RAM) Page 40-2 Each RAM is identified by an address which is used to denote its physical location in the network. On interrogation from the host, by the receipt of the ASCII string RXX, where XX is the RAM address, it responds with the following data stream: WXXAABBCCDD Where XX = RAM station ID No. AA = High-band amplifier output - mV (HO) BB = Low-band amplifier output - mV (LO) CC = Not assigned DD = Temperature - mV The values being in Hexadecimal. It is also possible to set thresholds in the RAM for the various parameters, so that, if if a value drops below a threshold level, it will automatically initiate a message to the host. To set the threshold levels, the host sends SXXAABBCCDD, where the symbols have the same significance as above. When a value drops below the threshold level, the RAM sends TXXAABBCCDD to the host. To implement the above, each RAM has two ports (UARTs). The logic is arranged so that if a message is received on one port, and if the first character is "R" or "S" and the second and third characters are the RAM's address, the module will respond on the same port that the command came in. If the address does not match, the RAM will relay the message to the other port, passing it down the chain. It will also do this if the first character is "P" or "Q". In the case of "Q", the RAM will add its address to the mesage, so that the host receives back a list of station addresses, in the order in which they are chained, and for "P" it adds both address and present data.
40.3 CONSTRUCTION The RAM is a rack-mounting unit, 5-1/4 in. high.
40.3.1 Front Panel. The front panel has two rows of 4 LEDs. These display the address and the threshold values set in the unit.
REMOTE AMPLIFIER MONITOR (RAM) Page 40-3
40.3.2 Rear Panel. The rear panel carries the following: o 2 Connectors type DB 25 P for the RS-422 links J1 Link towards terminator J2 Link towards host computer o Special socket, Switchcraft A3ML for temperature probe. o 2 connector type BNC for the connections from the Amplifier J4 High band J5 Low band o 117 V ac inlet.
40.4 DRAWING PACKAGE NUMBER 120-009
40.5 RESPONSIBLE ENGINEER W. Struven