Interface module for connecting collision detection lan user terminals to different access mode switching network

In an interface module (2) for connecting user terminals (1) of the carrier sense multiple access collision detection (CSMA/CD) local area network (LAN) to a different access mode switching (3) network, a transmit buffer (12) is connected at one end through a transmit data channel (4) to a user terminal and connected at the other end to a desired switching system. A receive buffer (13) is connected at one end to the switching system to store a receive packet supplied from it. An OR (14) gate is provided having a first input connected to the transmit data channel, a second input connected to the other end of the receive buffer and an output connected through a receive data channel (5) to the user terminal. A detector circuit (16) detects when a transmit packet is received from the user terminal. A control circuit (11) responds to the output of detector circuit to enable the transmit buffer to store the transmit packet from the user terminal and further enables the receive buffer to forward the stored receive packet to the OR gate in the absence of a transmit packet detected by the detector circuit. The control circuit detects a simultaneous occurence of a receive packet being forwarded from the receive buffer and a transmit packet being detected by the detector means and causes a collision-presence signal to be supplied to the user terminal through a signalling channel (6).