These octal bus transceivers are designed for asynchronous two-way communication between data buses. These devices transmit data from the A bus to the B bus or from the B bus to the A bus, depending upon the level at the direction-control (DIR) input. The output-enable () input can be used to disable the device so that the buses are effectively isolated. The -1 version of the SN74ALS640B is identical to the standard version, except that the recommended maximum IOL for the -1 version is increased to 48 mA. There is no -1 version of the SN54ALS640B. The SN54ALS640B and SN54AS640 are characterized for operation over the full military temperature range of -55°C to 125°C. The SN74ALS640B and SN74AS640 are characterized for operation from 0°C to 70°C.