Is CAT5 cable good enough for RS-485 vs "true" RS-485 cable

While often shielded, CAT5 can be of the UTP type, meaning unshielded twisted pair. The "true" RS-485 cable you link to has two twisted pairs and a shield. If I remember right, CAT5e (and above) has a shield, at least most cables I have seen have - the exact standard can vary. I guess those would do the job just nice. Just make sure you use the twisted pair like in the original RS-485 cable (your data sheet looks like [1 White/Orange Stripe] and [2 Orange/White Stripe] are a twisted pair, and [3 Blue/White Stripe] is a third, untwisted wire.)

There seems to be a small impedance mismatch (100 Ohms for CAT5, 120 Ohms for RS-485). This will cause reflections at the driver and at the receiver, but I am pretty sure your application will still work. While you may read that 120 Ohms is typical for RS-485, the termination network uses 120 Ohms between the differential pair and 2 * 680 Ohms to VCC and GND.

Source

Thus, the value of the termination that the cable "looks into" is smaller than 120 Ohms anyway: (120||(680+680)) Ohms = 110 Ohms.

If you have the chance to test, you could go for it. I would probably not even hesitate and use STP CAT5(e). If your contract says you pay money for every minute your installation fails, you probably want to use cables with the proper specification. (The latter will still not make sure nothing ever fails, but you're probably in a better position if you can blame it on the "true" cable instead of having someone else blaming your "wrong" cable. But you see how this last paragraph has nothing to do with physics...)

Generally speaking CAT5 s fine for RS485. IME the first limit you hit is the series resistance driving a termination over a long cable. I've run 250kbaud over 100m reliably. Things started getting shaky at around 200-300m.

Per reference to Maxim APPLICATION NOTE 3884 How Far and How Fast Can You Go with RS-485? from Jul 25, 2006 (cited 2104-05-28):

mentions rs485 and cat5 with measurements.

The performance of a Maxim driver (the MAX3469 in this case) and an equivalent >driver from another manufacturer are presented

What Factors Limit the RS-485 Data Rate?

The following factors affect how far one can reliably transmit at a given data rate:

• Cable length: At a given frequency, the signal is attenuated by the cable as a function of length.
• Cable construction: Cat5 24AWG twisted pair is a very common cable type used for RS-485 systems. Adding shielding to the cable enhances noise immunity, and thereby increases the data rate for a given distance.
• Cable characteristic impedance: Distributed capacitance and inductance slows edges, reducing noise margin and compromising the 'eye pattern'. Distributed resistance attenuates the signal level directly.
• Driver output impedance: If too high, this limits drive capability. Receiver input impedance: If too low, this limits the number of receivers that the driver can handle.
• Termination: A long cable can act like a transmission line. Terminating the cable with its characteristic impedance reduces reflections and increases the achievable data rate.
• Noise margin: Bigger is better. Slew rate of driver: Slower edges (lower slew rates) enable transmission over longer cable lengths.

System designers often choose a driver and receiver from two competing manufacturers, but most designers are primarily interested in how far and how fast the RS-485 driver can drive a signal. The performance of a Maxim driver (the MAX3469 in this case) and an equivalent driver from another manufacturer are presented

Signal integrity is tested by observing the driver's differential output. Set the oscilloscope to look for trigger points between the 80mV and -400mV thresholds. (These thresholds are chosen because receivers have an input range of 20mV to -200mV, plus a noise margin.) Then, when pulses (bits) begin to 'run together', use eye patterns to determine the overall contributions of distortion, noise, and attenuation to the parameter called intersymbol interference (ISI).

ISI forces you to reduce the bit rate to a level that allows an adequate distinction between pulses. Tests of the Figure 1 circuit show a consistent and clear correlation between trigger points and eye patterns. The eye patterns exhibit 50% jitter, measured using methods documented in National Semiconductor's application note 977[4]. Measuring jitter at 0V differential and ±100mV differential yields the data shown in Figures 4 and 5.

...
Depending on drivers you get different result

At 39Mbps and 340 feet of Cat5 cable, the driver output of Figure 2 exhibits an eye pattern in which signals cross in the middle of the eye—a condition indicating possible bit errors. The Maxim device at the same data rate, however, (Figure 3) shows no such condition. The Maxim transceiver offers better performance due to symmetrical output edges and lower input capacitance.