Interfacing The CCS 7710A Asynchronous Serial Card

Sam Bassett
San Francisco, CA

The following is a list of the connections needed to set up the DTI27 to work with the California Computer Systems' 7710A Asynchronous Serial Interface Card for the Apple II:

The CCS Asynchronous Board is defined as a Data Communications Equipment (DCE) terminal, and the signals at its DB-25S (female) connector are defined in the same way that a modem's would be.

The CCS board transmits its outgoing signal on Pin 3 (RD). This must be connected to the DT-127's incoming signal connector on the CCS board — Pin 2 (TD).

So far so good — we have information signals being passed back and forth from the printer to the Apple. There is a possible problem, however — the printer can only print so fast, and the Apple can generate and send characters much faster than the printer can print them. Most printers run from 10 to 55 characters per second, which is equivalent to 100 to 550 baud. The Apple can transmit at well over 20,000 baud — 2,000 characters per second. The DTI 27 has a 625 character buffer built in (expandable to 16K), but if the Apple is sending characters faster than the NEC can print (55 cps), the buffer gets full, characters are lost, and weird things happen to the text that was to be printed.

All is not lost, however — the definition of RS-232 includes several hardware "handshaking" signals, and the CCS 7710A (unlike the Apple Inc. Serial and Intelligent Interface Boards) is set up to recognize and use these signals. When the Printer signals that it has enough characters in its buffer, the CCS board will stop sending characters until the printer sends an "OK" signal.

The DT-127 signals that it is OK to send characters to its input on Pin 2 (RD) by making Pin 6 (Data Terminal Ready — DTR) on the Molex connector high — + 3 to + 12 volts. The CCS board monitors Pin 4 (Request To Send — RTS) to see if the peripheral is ready to receive another character. If Pin 4 goes Low (-3 to -12 volts), the 7710A will not send another character until it goes High again.

The CCS 7710A board signals the peripheral that it can accept a character through its Pin 2 (TD) by making its Pin 5 (Clear To Send — CTS) High. The DT-127 watches its Pin 4 (CTS) to see if it is OK to send a message to the computer. If this signal goes Low, it will not send any characters until it goes High again.

Pins 1 & 7 on the RS-232 connector are connected to Ground, so Pin 8 of the Molex must be connected to one or both of them.

The last two signals are not absolutely necessary, but it is well to hook them up so that nothing is left hanging, or unconnected.

The DT-127's Pin 5 (Request To Send — RTS) is not implemented — it should be High at all times. It should be connected to the CCS board's Pin 20 (Data Terminal Ready — DTR), which tells the Apple: "Yeah, boss, there is somebody out there."

The CCS board's Pin 8 is also permanently at + 12 volts, so it should be left unconnected, so that it does not accidentally short to ground and shut down the Apple's power. Pins 9 to 25 (except 20) are not implemented on the CCS board, so they should not be connected to anything.

The CCS board can transmit at any baud rate from 50 to 19,200, and matches the DT-127 perfectly — be sure to check Page 2-1 in the CCS manual for the correct Baud Rate Selector Switch settings, and Page 5 of the DT-127 manual, so that the baud rates being used by both your Apple and your Sellum are the same.