The dual-port COM-2S and single-port COM-1S cards can be used in ISA Bus computers. They support all of the popular communication protocols. RS-232 communication is fine for short distances and development work. RS-422 adds differential drivers and extends the distance. And RS-485 communication uses tri-state drivers to produce a multi-drop network capability for up to 32 ports on a single twisted pair.
The cards have type 16550 buffered UART’s and are compatible with MS-DOS and multitasking operating systems. I/O bus address is set up by DIP switches on the card and address is not limited to just COMx addresses. Continuous address selection is available anywhere in the I/O address range 100 to 3F8 (hex). Interrupt levels IRQ2 through IRQ14 (except IRQ8, 9, and 13) are supported. A crystal-controlled oscillator is located on the card. This oscillator permits precise selection of baud rate from 50 to 115,000 baud. Higher baud rates are optionally possible using a different UART and higher crystal frequency. The transceiver used, a type 75176, is capable of driving extremely long communication lines at high baud rates. It is capable of driving up to 60 mA on balanced lines and receiving inputs as low as 200 mV differential signal super imposed on common mode noise of maximum -7V to +12V. Also, in case of communication conflict, the transceivers feature thermal shutdown.
In addition to differential Transmit and Receive lines, single-ended, buffered RTS and CTS lines are provided at the I/O connector. Normally, CTS must be pulled up to +5 VDC for the card to operate but you can position a jumper so that the COM-2S will auto-detect the transition between transmit and receive operations. The COM-2S supports full duplex, half duplex and simplex operations selected by jumpers.
RS-485 multidrop communications requires the transmitter drivers to be enabled and disabled as needed to allow the ports to share the common communications lines. The COM-2S card has two methods to control the drivers: automatic (AUTO) and Request to Send (RTS) control. Under automatic control, the port is normally in the receive mode and the driver is enabled only when data is ready to be transmitted. The driver is then enabled and remains enabled throughout transmission plus the transmission time of one character after data transfer is complete. Then it is disabled. The COM-2S automatically adjusts its timing to the baud rate of the data. When operating with Windows programs, this AUTO mode is mandatory. With RTS control, your application software must enable/disable the driver.
A transmission line should be terminated at the receiving end in its characteristic impedance in order to eliminate “ringing.” You can install jumpers that apply a 120ohm and 0.1 µF capacitive load across the input for RS-422 mode and across the transmit/receive input/output for RS-485 operation. In RS-485 operations, where there are multiple terminals, only the RS-485 ports at each end of the network should have terminating resistors. Also, for RS-485 operation, there must be a bias on the RX+ and RX- lines. If the COM-2S card is to provide that bias, you can install jumpers at the two locations labeled BIAS.
- Tranzorbs to absorb potentially damaging voltage spikes occurring due to lightning or other transmitted electrical interference.
- High-speed crystal oscillators to achieve higher baud rates.