# TCM3105 modem modes please explain

1. ## TCM3105 modem modes please explain

Hello the TCM3105 can be configured for different speed modes.
I would like to ask a few things about these, assumming there are two identical modems that are connected to each other.

1. On 1200/1200 mode what happens if both ends try to send data/text at the same time?

2. What happens if the two end computers can only transmit and receive at 1200, but the TCM3105 are configured for lower speed modes?

3. The datasheet states full duplex on 1200 transmit and 75 receive. In this mode two TCM3105 cannot communicate full duplex, since the tcm3105 on the other end must reverse the speeds 75 transmit, 1200 receive and this mode is not full duplex. Am I right?

4. What is the maximum speed where full duplex can be transmitted in equal rates at both TX and RX?

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2. ## Re: TCM3105 modem modes please explain

1. It could work if you have perfect matching to the telephone lines in both ends, so nothing of the transmitted signal is reflected back to it's own receiver.
I reality, the receiver will receive too much from it's own transmitter for a reliable function.

2. The other end will receive garbage.

3. You can do 1200/75 full duplex between two TCM3105's.

4. There is no symmetrical full-duplex mode, but you can reduce the data rate in any mode, so 150/150 is possible in the 1200/150 modes.
If you have a 4-wire connection between the units, you can have 1200/1200 full duplex since the TCM3105 receiver will not see it's own transmit signal.

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3. ## Re: TCM3105 modem modes please explain

From the data sheet:
The variation of dc level with received frequency is presented to a comparator that slices at a level externally
fixed by the RXB bias-adjustment pin. This voltage depends on received bit rate and internal offsets. The
comparator output is then the received data at RXD.
which implies it has an analog frequency discriminator. That means it cannot distinguish signals between two simultaneous signals in the same frequency range so either the receive and transmit signal paths have to be isolated or they have to be in different filtered ranges. The TCM3105 does not have any concept of actual data rate, the reason 1200, 150 and 75 Bauds are specified is just because they are the standards used in V23 and Bell systems. There does have to be sufficient difference in frequency between high and low bit tones ( 1 and 0 in the data) for the comparator to detect them though.

Brian.

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