TECHNICAL NOTES - RT3

Recorders are factory-configured with 220-Ohm pull-up resistors in the parallel port. This results in fast rising/falling times which are suitable for a wide variety of host computers.

With this termination, driving circuits in the host must be capable of sinking at least 24 mA (e.g. 74LS373 or 74LS374). If this is not the case, the termination can be increased to, for example, 470 Ohms when standard TTL gates are used to drive the recorder (16 mA sinking current capability), or to 1 Kohm when LS-TTL or HCMOS gates are used (8 and 6 mA sinking current respectively). It may also be necessary to increase the termination when series resistors are connected to the output of the drivers for protection (common practice in a number of PC-clones). Notice that increasing the termination may reduce somewhat the usable cable length; in any event, we recommend that the termination should not exceed 2.2 Kohms.

2.    With the recommended drivers, a regular flat cable with a maximum length of 6 ft. may be used. As many ground lines as possible should be connected (see Figure 4.1.2 in the User's Guide). Alternatively, shielded, twisted-pair cable may be used, with signal and ground lines wired in pairs (see Figure 1 below); this allows use of a longer cable, typically around 15 ft., even longer in some cases.

As a guide-line, Figure 2 illustrates the recommended parallel port cable to interface the recorder to an IBM-PC. Notice that, if twisted-pair cable is used, all ground wires of the signal/ground pairs must be connected to ground at both ends of the cable.

3.    The timing specifications illustrated in Figure 4.1.3 of the User's Guide must be satisfied. Note that "STROBE" is factory-configured for the 0-1 transition; this is adequate for most host interface circuits, including a variety of PC-compatible computers.