The RS-232 serial ports on most Symbol barcode scanners are RS-232TTL ports. RS-232TTL signal levels are lower than those of RS-232C ports, like the ones used in ProSoft solutions. Therefore, without some kind of adapter to boost the RS-232TTL signals up to RS-232C levels, these scanners will not send recognizable signals to your ProSoft ASCII module.
Symbol can provide an adapter cable that is supposed to boost the RS-232TTL scanner signals so that they can be seen properly by RS-232C devices. In addition other third-party providers offer RS-232TTL to RS-232C converters. If these offering are externally powered, and not port-powered, then they usually work.
Another possibility is to obtain an RS-232TTL to RS-485 converter. Since the RS-232TTL and RS-485 signal levels are do not usually require an external power source or port power to function properly. ProSoft ASCII solutions support RS-485, as well as RS-232C, this providing another possible solution to successfully get the data from the scanner to the ProSoft module.